Does anyone know which of the slower powders are less sensitive to initial chamber pressure i.e. burn better at lower pressures?
As in using a slower powder for a gentler launch without suffering the penalty of inconsistent burn or ignition?
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Does anyone know which of the slower powders are less sensitive to initial chamber pressure i.e. burn better at lower pressures?
As in using a slower powder for a gentler launch without suffering the penalty of inconsistent burn or ignition?
Im interested here. More i learn about caressing the base of the boolit, the more i like the slower powders. And the more i try them the more i like them. But like you ive yet to find one reach its potential at anything less than a nearly-full load.
Since you are a pretty filler-happy guy i assume you are looking for a load that doesnt need one. So am i.
Ive been considering requesting a chart which lists optimal pressure for each powder. Most ive found seem to have a window of excellence, some powders give a broader window than others.
I dont test or use nearly enough powders to come up with this chart, but ill bet there is plenty of collective wisdom here to pull it off
All I know is that the modern Schuetzen guys seem to have been attracted to AA #9, AA1680, and Alliant MP300. Not especially slow, but they do give very consistent SD in 15K psi loads for plain base boolits. Some of these guys can shoot 1/2 MOA groups at 200 yards, so there must be something in what they say.
Me, I can only vouch for #9, but have managed to obtain MP300 and 1680 for the coming test season.
I use Varget by choice for full-power (40K+ psi) loads in 5.56 and .30-30 with J-warts, but I've also read a report here that some guys are getting it to work very well at under 20,000 psi, so maybe that's a possibility? Hard for me to test - I'm down to my last half-pound, and can't find any to buy anywhere.
Phil
There is more to it then just a optimum burning pressure. A gun is basically a simple piston engine, bore verses stroke = volume.
The larger caliber guns drop pressure at a faster rate then a smaller caliber. You also have to take into consideration the sectional density and bearing surface of the bullet on the rifling.
How about a 22-250 Remington that has a greater case capacity then a 30-30 Winchester, or take the 30-30 Winchester and neck it up to .375 diameter and see what it does to the time pressure curve.
Lots and lots of variables come into play in how the pressure rises and then falls affecting pressure.
Exactly! I have what could be termed a classic example. My 22 hornet - I loaded it with Lil'Gun which is reputed to require a mild primer (pistol) and a stiff crimp to get consistent ignition and burn. A hot primer pushes the bullet out before full ignition resulting in irregular pressure. I used an unsized case and neck that required a paper cup to take up the clearance with the bullet. I overcame the burn problem by using heavier bullets (55gr) and a compressed powder charge. I was effectively using the powder in its compressed state plus a heavy bullet to create sufficient back-pressure to burn well. And it burned very well indeed. 2740fps from a hornet with 55gr j-words! Pressures were a little higher than a hornet is supposed to have but the action was designed for a 222 so I was only concerned about the cases holding out and they did. So, a progressive powder being made to work by using tricks. Pretty much what corn filler does (I think). But the idea is to find a powder that burns better than the rest at lower pressure but still gives a gentle launch and high velocity at low peak pressure. A tall order but no one powder is the same as another so one of them out there is going to be better than all the rest for this application. I'm trying W780 and W748. I have tried H4350 and it did work but it kinda sorta peened the cr*p outta the boolit bases.
Attachment 66058
Those kernels are lying in the impressions made by them. There some visible indentations without kernels lying in them too. I did try a card wad which did help but I found Dacron with W748 the best of those I'd tried at that time.
I like Varget. I even found it works perfect in 10" barrel 7R and 7BR with light 120 gr bullets ("J" words) after Hodgdon told me it was too slow. It is the ONLY powder that gave me super accuracy from the fast twists with the light bullet. Both guns were made for heavy.
It is true what Doc said. Then most slow powder needs high powder amounts and can't be reduced much. That increases all pressures from start to finish.
Might be an idea to look at load charts and find the powders that show more reduction ranges.
If you want to play with options, open yer wallet and buy a copy of Quickload. I feel like a man born blind that has been given sight now that I have it.
The trick of using pistol primers in the Hornet holds for all powders. The best (lowest pressure) primer is the Remington 1 1/2, by a significant margin. Every so often somebody builds a test rig to measure the explosive pressure of primers, and every time I read a report the Rem 1 1/2 is the lowest.
The earliest such test I know of that was popularly reported was documented in a 1930s American rifleman. The test was to put a closefitting brass rod in the bore of a rifle, and launch it vertically with the primer. (No powder, obviously). A second operator eyeballed the height against a scale pasted to the side of the house. Redneck engineering at its' finest!
To address the original question: In general, you'd concentrate on single-base powders with little or no ignition suppressant. The complication is that a slow burn rate is often a slow ignition rate, because the way that powder engineers achieved a slower burn was to layer on more and more ignition suppressants. The harder it was to light, the longer the interval before all the powder was lit - and therefore the "slower" the burn time.
Today, powders are much more sophisticated. They can be made in layers with varying chemical properties in each layer to control the ignition and burn time of each kernel. That's essentially how modern spherical powders are constructed - and why such new ones as LilGun and AA#9 can be made to ignite easily but burn slower than they otherwise would if they were monolithic.
What have I said forever about the .44 doing better with standard primers? 303 even mentioned it. It is true, never push out a boolit with primer pressure first before good ignition.
I never made much funny tests, just shot to find accuracy. Primers are so important that everyone needs to test and never just read stuff.
Some cases are just too small for primer pressure. Some are too large for primer heat. Please experiment.
I hope you guys are right about Varget. I bought 16 lbs before the crazies hit to work up loads for a 200 gr .308. This is an interesting thread. A well respected gentlemen on this site has provided some feedback and he thinks I will need a slower powder (4831SC). It did not surprise me as it seems there is a lot more success with using slower powders with cast bullets in rifles. But I cannot find any 4831SC powder anywhere. I have one more place to visit next week to see if I can snag some to experiment with.
Like many casters, I started with fast powders in cast "target" loads and our history can work against us. Regrettably, the old Lyman loads for rifles also tended more to "reduced" loads with smallish charges of shotgun type powders. More history to overcome.
One option may be duplex loads but I am pretty sure I do not want to go there. They are probably OK but probably is still too risky for me. I will settle for less velocity.
One of the things I did was obtain a rifle with a 26" barrel. For a "target" gun made to shoot cast this will allow more time for the burn and better use of slower powders. I realize pressure is not be linear as the bullet moves down the barrel but it makes sense that if 30,000 PSI acts on a bullet for 26" of travel, it will be traveling faster than if it accelerated over only 20".
One thing to bear in mind is that a powders (like Varget for example) have a wide range of published pressure use in cartridges other than the high intensity rounds we may be using. For example Varget is listed for loads in the .30/30 at pressures as low as the high 20's and in the .45/70 with loads generating as low as 16,000 PSI. But it gets most of its "press" with higher pressure loads in the .223/5.56 and .308. This tells me it can be loaded down with some safety if case volume is not reduced too much.
We are saddled with published data that tries for maximum velocity with jacketed bullets. There is no marketing drive to test loads at the low pressures that are more suitable for cast bullets in rifles like 7mm/08, .30 cals, 8mm etc. Yet we may still be able to shoot those powders safely at below "optimum" loads just as 44man proved with his experience when he ignored the factory recommendation that "it was too slow". It was not too slow for his loads.
Hodgdon:
But it gets betterQuote:
110 GR. SPR FP Hodgdon Varget .308" 2.415" 34.5 2365 27,200 CUP 38.0C 2572 31,900 CUP
Quote:
110 GR. SPR FP Hodgdon BL-C(2) .308" 2.415" 36.0 2351 24,500 CUP 39.0 2526 25,400 CUP
More interesting load data from Hodgdon.
303 Brit
Quote:
180 GR. SIE SP Hodgdon Varget .311" 3.075" 37.0 2282 38,200 CUP 41.0 2440 43,400 CUP
I couldn't find any BL-C(2) so I got some W748. It's OK but doesn't produce the velocities with the low pressure of BL-C(2).Quote:
180 GR. SIE SP Hodgdon BL-C(2) .311" 3.075" 41.0 2395 37,100 CUP 45.0 2563 43,000 CUP
45-70
Quote:
300 GR. CAST LFP Hodgdon Varget .458" 2.465" 45.0 1599 17,800 CUP 55.0 1880 20,600 CUP
Quote:
300 GR. CAST LFP Hodgdon H4895 .458" 2.465" 45.0 1572 14,400 CUP 51.0 1703 15,500 CUP
H4895 looks like a winner so far.Quote:
385 GR. CAST LFP Hodgdon H4895 .458" 2.505" 35.0 1280 11,900 CUP 42.0 1526 23,100 CUP
Thanks for all the great tips!
I'm going to go play with primers now. :D:)
I also like 4759 for many guns but ran into a failure to ignite in my 45-70 BFR. I went to a heavier boolit and reduced the load 1/2 gr because I had no load info. Boolit and powder just went into the bore. Increasing the charge cured it and it is accurate.
Now this is a very easy to ignite powder yet it failed.
I also used Dacron filler in the loads but it did not help.
There is so much I don't understand or can solve because stuff happens we don't expect. Who in the world would expect a small reduction of 4759 to fail? Surely not me!
I don't suppose you would have tried it again after the failure to see if it always fails like that (I know I would not have)?
I keep coming across two powders that behave differently in different cartridges and at different load levels also with different bullets. BL-C(2) and W748 are two I've compared. CFE 223 is a new powder that is comparable to BL-C(2) in that it produces high velocity at lower pressures in a wide range of cartridges.
So I'm thinking of finding an available new or modern powder to try out. Sometimes BL-C(2) is available here but H4895 would do too but I'm not sure that these are different enough from W748. So far I haven't found a listing of W748 showing how low in pressure it can go and for me to test it I would need to chrono it. I have taken it as low as 23grs in my Brit but that was with filler and a heavy boolit and only into the 'test tube' so it doesn't mean much. Powders are not cheap here down under so to buy a can then not need it is silly but hey, one can fins an excuse to use it, right?
I noticed that W748 is only indicated for heavier bullets but is similar in performance to Varget, BL-C(2) and H4895. Harder to light up maybe?
I don't much care for blc-2 as it doesn't burn clean for me unless I really boost the load. Varget and H335 are a couple of my favorites.
I don't suppose you would have tried it again after the failure to see if it always fails like that (I know I would not have)?
QUOTE;
Not for a second and you are right. Once is enough and scares me silly. Just what if it would have lit off up in the barrel?
I have limits and it took a month before touching the trigger on my BFR 45-70 but I was still on my own. Rifle loads were out of the question for a short barrel. Nothing but flash at the muzzle and unburned powder. I actually poured unburned powder from the brass.
If anyone thinks all powder is consumed at peak needs beat with a large stick.
Some want a .500 S&W with a 2" barrel thinking it has more power just because it is big. They say using slow powder is still best. What planet are they from? It is recoil and muzzle blast only that they think is power.
So much powder energy is wasted in the air.
I was amazed at how I could see a secondary powder burn/pressure spike using a strain gage with AA5744 in my 308 Winchester. I will have to figure out how to post a picture of it from my lap top.
I fired a Federal large rifle primer with an empty case and a fairly loose patched boolit. The boolit stayed inside the throat but bumped out quite easily. There are leade marks on the patch but no impressions. There was virtually no grip on the boolit.
If you go to the threads on the S.E.E. that have been here on CB, there's finally a rational explanation that centers around the primer explosion pushing the j-wart into the throat, and a millisecond or so later the charge goes off, with the j-wart acting as an obstruction. The clue was a double peak in the pressure curve observed by a lab testing some 6.5mm round. Worst with reduced loads of a hard-to-light powder, and a magnum primer has been used to get it to light.
ON THE OTHER HAND - Warren Page wrote in the '70s that, when using slow-for-case-volume powders in benchrest rifles, jamming the j-wart into the leade was actually the right thing to do, to make it stay put while the powder burn built pressure. Just to opposite of what we've been told by umpteen experts for umpteen-and-fifty years. But maybe it's OK when the j-wart is a lightly built HP benchrest bullet, and not OK when the bullet is a long skinny 6.5mm cruise missile?
Finally - I've often slugged a throat by launching a boolit into it using just the primer. In large cases like the 7.5x55 I've been messing with recently I had to add 0.3 grains of Red Dot, but in small cases like .357 Max, a magnum primer did it just fine. No crimp of course.
Varget and H4895 are both stick powders, which are said to light easier due to have less aggressive deterrent coatings. If that means anything. And 4895 has always been one of the canonical cast-bullet powders in the .30-06, going back to the '50s, if what I read in the old sources is accurate. It's a good deal slower than Varget.
The comments about light loads of Varget were on a thread here regarding the 7.5x55, my remembery is trying to tell me.
H4895 is faster than Varget.Quote:
H4895 powder was chosen because it is the slowest burning propellant that ignites uniformly in reduced charges. For years
H4895 has been the top choice of cast bullet shooters.
You are right. I should make sure brain is engaged before putting fingers in gear. It's right next to Varget in the Quickload database, but it does come first. As punishment I shall let that post stand.
What burn there is will occur in the first few inches of bullet travel no matter how long the barrel as long as peak pressures get high enough to ignite the powder. The higher velocities of longer barrels is from their greater expansion ratio not their greater propellant burn.
Most powders have about the same energy per weight and produce similar amounts of gas. Use of a larger case will allow a greater volume of powder (more gas) to be used at a given peak pressure. For example if one charge burns 20 grains at peak pressure of 30 kpsi and another charge burns 40 grains at the same pressure the latter will produce more gas volume and will be able to maintain the gas pressure longer. The longer the pressure can be maintained the longer the bullet continues to accelerate.
Barrel pressure does not stay anywhere near constant over any distance of bullet travel much less 30 kpsi to 20 inches our 26 inches.
A QL check using a 30-30 with a 170 gr GC Lyman RN bullet and 30.3 gr IMR4895 shows 2000 fps @ 20" and 2100 @ 26 ". WOW. A whopping 100 fps difference. The increase in powder burned 20" vs 26" was 3 %. WOW again. Peak pressure 30 kpsi occured at 1.4 inches and decreased to 20kpsi (5 inches), 12 kpsi (10 inches), 9 kpsi (15 inches), 6.2 kpsi (20 inches, 4.65 kpsi (26 inches). If we can not maintain 30,000 psi from the peak at 1.4 inches out to 5 inches how do we expect to maintain it to 20 or 26 inches?
The advantage of slower powders is not longer burning times. It is the ability to burn greater quantities of powder, producing greater quantities of gases, without exceeding peak pressure limits. Their importance is that they are 'slower' in terms of slower pressure rise rate. Once peak pressure has been reached that advantage disappears. In fact a powder that has a faster burning rate after peak pressure has been reached would yield better performance since that gas would have longer to act on the base of the bullet. Better to have that energy released after 5 inches of bullet travel than after 15 inches.
Doc makes several very good points; having measured and observed the time/pressure curves of numerous powders in numerous cartridges the last few years I have to certainly agree with: primarily the expansion ratio of the cartridge has a lot to do with how well a slower powder burns at what psi levels.
Assuming the "slower burning powders" is refering to rifles I have found the following medium to slow burners to work well (read that ignite and burn reletively evenly) at low psi (down to the 20,000 psi level); 4895 (I prefer H4895), Varget, 4064, RL19, 4350 (I prefer AA4350), RL19, H4831 (I prefer H4831SC) and RL19. The exact bottom end of that level is very dependent on the exapnsion ratio, bullet weight and case volume (load density). The use of a dacron filler when load density is less than 80 - 85% greatly improves the ignition and efficiency of the burn.
Like Doc I also sometimes have secondary spikes with some cartridges with some powders, even faster burning ones. It's not the gun used or a defect in the gauge mounting as other loads give smooth time pressure curves. I've not yet figured it out......probably time to call Dr. Oehler?
I'm sure there are others out the but I've not tested all of them so can't say. The above listed powders are generally readily availble, work well in a wide range of cartridges with medium heavy to feavy cast bullets so I've not had a need for a lot of other testing so far.
Larry Gibson
Actually, with rifle powders the burn continues past the peak pressure point of the curve and slow powders burn quite a ways down the barrel. Maybe not all the way down and maybe not even half the way down but further than just the peak. It has been shown that a longer barrel will burn all the powder while a shorter barrel with the same loading will leave unburned kernels (that's with a specific load and powder, not any or all loads and powders). Increasing the powder charge with the specific loading will often burn all the powder. Point is some powders behave differently to others.Quote:
The advantage of slower powders is not longer burning times. It is the ability to burn greater quantities of powder, producing greater quantities of gases, without exceeding peak pressure limits.
My uncle was an armourer in the dessert in WWII and said that after a 50 Browning machine gun shoot at the range they would sweep up heaps of partially burned powder from in front of the guns.
That secondary pressure spike has been lab tested and it's not barrel harmonics or some such. They went so far as to pack the barrel with sand bags to dampen vibration and that spike was still there.Quote:
Like Doc I also sometimes have secondary spikes with some cartridges with some powders, even faster burning ones.
Quote:
A conclusion by some was that PressureTrace picked up barrel harmonics. In an attempt to prove this theory one shooter even hung a bowling ball off the end of his barrel, but of course there was no change.
Quote:
Public debate over whether these secondary spikes are real was finally put to bed when Charley Sisk at Sisk Rifles blew the end off two barrels. We have also verified changes to the rate of acceleration just prior to, and after these events. Case Closed, it is real!
I never did buy off on the "barrel harmonics" reason for the secondary spikes either.
Larry Gibson
This is correct. Powder continues to burn after peak.
If 200 gr of powder burned in the first inch of a .50 BMG, you have a BOMB.
It has been the printed word that says all is gone at peak. Attributed to gun writers. I did find that article about expansion ratio. It is false.
I talked to the powder companies and they just told me "internet idiots."
Progressive burn escapes the natural world.
Why not use dynamite? It really goes off all at once.
Maybe I am stupid but I know all powder is not consumed so fast. Smokeless is not classed as an explosive, it is a flammable. It does take barrel length to burn all of it.
Then someone said the muzzle flash from a short barrel was from the gas reaching oxygen in front of the barrel. No common sense shown because powder produces it's own oxygen or it would not work. Just maybe it is powder out there?
Someone is not thinking at all. Compare a rifle with 50,000 PSI using 35 gr of powder with a mag at 50,000 psi with double the powder charge. Did the double charge go off all at once? That would mean 100,000 PSI. Or, Heaven forbid, more distance to burn was needed.
Expansion ratio just must be more from a higher charge with the same peak pressure---RIGHT???? OH, OH, maybe you are wrong.
Put it in perspective. Why does a .50 BMG need 200 gr of powder? Just shorten the case to 1" and it will peak the same pressure and depend on expansion ratio.
Why do the guns on a destroyer need bags and bags of powder behind the projectile?
A few read gun rags for information, others think.
Expansion ratio just must be more from a higher charge with the same peak pressure---RIGHT???? OH, OH, maybe you are wrong.
The "expansion ratio" I was refering to is; "The ratio of the volume of the bore, measured from the base of the seated bullet to the muzzle, to the volume of the cartridge case. An expansion ratio of 9:1 means the bore volume is 9 times the case volume." (Firearms Encyclopedia).
A .38 SPL revolver with a 6" barrel actually has a greater expansion ratio than many rifles with 20" barrels. There is a maximum expansion ratio at which any given cartridge/load combination will produce maximum velocity. The .243W for example would require nearly 30" of barrel to produce an optimum expansion ratio and velocity. Many of todays "high intensity" cartridges would require barrels of 30 - 40" long to achieve the expansion ratio and efficiency of the 30-30 cartridge.
Larry Gibson
By definition the expansion ratio is the ratio of the volume of the case to the base of the boolit to the total volume. Total volume is volume of case plus bore volume.
A revolver is going to have a bit of a complicated expansion ratio because one has to take into account the expansion out the cylinder to forcing cone gap.
Lets say for argument there is only one caliber in the world and it is a 308 Winchester with a 24” barrel, and there is only one bullet in the world a 175 grain Sierra Match king.
Now with this one combination we still have all the powders we have, and you made a powder burn rate chart from fastest to slowest at a chamber pressure of 20,000 P.S.I. and you now have a perfect list.
Now you change your mind and you want a chamber pressure of 30,000 P.S.I., your perfect 20,000 P.S.I. list now has a few changes as to burning rate.
Now do this again at chamber pressures of 40,000, 50,000, and 60,000 P.S.I. and your perfect list is again changed with each higher level of chamber pressure.
These changes are with just one gun and bullet but different chamber pressures. Think of the additional changes if we changed just the bullet weight as to the perfect burn rate chart at each pressure level.
With all the combinations of different calibers, case capacities, expansion ratios, bullet weights and bearing surface, the possibilities are endless.
Powder burning rate charts are RELATIVE AT BEST! Add to that the lot to lot variables to burning rate of +/- 10% and it can make it seem over whelming.
The secondary pressure spike is caused by not having enough moment force/ start pressure allowing not fully burning/consumed powder to follow the bullet down the barrel. As we know powder needs pressure to make it burn and with a higher pressure it burns faster. The bullet as it moves down the barrel increases the volume behind it, lowering the pressure of the expanding gases due to a larger area/volume that they now displace. Because the rate of acceleration of the bullet slows some, the not fully consumed powder catches up with the bullet and the powder now has the resistance to accelerate it’s burning speed resulting in the secondary pressure spike. As the caliber goes up the pressure drops faster for every inch the bullet moves down the barrel/bore due to bore volume. A faster powder or a heavier bullet with more bearing surface will help out, and with fillers both reducing the powder capacity within the cartridge case and helping to position it for the primer to ignite it.
What's come out of it so far is the idea to look at largest powder charge for the lowest pressure at some average velocity. That's the powder that will have a slower pressure rise with the longest burn. Look at the same powder at the starting loads to complete the assessment. Then one has to discover how well that powder lights up and burns at lower pressure. BL-C(2) appears to be ideal for cast but others have said it burns dirty at lower loadings. Varget on the other hand burns well at low levels (that's from my own observations). IMR 4007 SSC is a powder that is listed with the lowest pressures and fills the case of the Brit.
303Guy
The definition I quoted has been around for many years and is the understood defintion of "expansion ratio". Is it wise we rewrite and change definitions? When we do that doesn't confusion generally reign? Or shouldn't we just accept and work with the accepted definition? When we do this isn't there a consistency and continuity to the thought process and discussion over time?
The slowest powder with the highest load density is an over simplification. The powder still must ignite and burn uniformly for accuracy. Many slow burning pwders won't do that at the low end psi's for many cast bullet loads.
Larry Gibson
No, not re-defining anything - Robert Boyle in 1662 and Jacques Charles in the 1800's did that. Charles was inspired by hot air balloons which were the rage at the time. Remember the gas laws?Quote:
Is it wise we rewrite and change definitions?
Attachment 66545
Expansion or compression ratio is defined as;
Attachment 66544
(Initial volume over final volume).
Anyway Larry, you defined case volume to bore volume and perhaps that is what is spoken of in gun circles which is fine - I stand corrected (but I call it case to bore volume ratio). However, barrel lengths vary and anyway, when we are considering pressure we are really only interested in boolit sectional density. An example would be the 308 and the 243. Both cartridges would use a similar powder charge range (not too similar) yet have a very different case to bore volume ratio. They do however have a similar boolit sectional density range. So to me, bore volume is a bit of a mute point. It's the expansion rate versus case volume that I'd be interested in. That would take into account velocity (more like acceleration rate) and boolit sectional density. Not all that simple as different burn rate powders are indicated and cartridge efficiencies are different with different bore and a different case to bore volume ratio.
Quote:
McGraw-Hill Science & Technology Dictionary:
expansion ratio
Top
Home > Library > Science > Sci-Tech Dictionary
(ik′spanˇshən ′rāˇshō)
(fluid mechanics) For the calculation of the mass flow of a gas out of a nozzle or other expanding duct, the ratio of the nozzle exit section area to the nozzle throat area, or the ratio of final to initial volume.
(mechanical engineering) In a reciprocating piston engine, the ratio of cylinder volume with piston at bottom dead center to cylinder volume with piston at top dead center.
Read more: http://www.answers.com/topic/expansi...#ixzz2PfVE6LsQ
That is correct Larry. It is something that changes.
What I referred to was using the term to say more powder going off with a slower powder still going off in the same distance as a fast powder will add more expansion ratio without increasing pressure. It has been used to show a lot of powder just adds more gas at the start and only that gas pushes the boolit.
We know that can't be done without destroying the gun.
It is the definition of an explosion.
If it were not true, we could just double the charge of Bullseye to double the gas and so called "expansion ratio." Imagine a .50 BMG case full of Bullseye!
It started as a gun writers pipe dream and still continues today with the thought that all powder is consumed in an inch or so.
It is why some think all powder is burned in a 2" .500 S&W, you really need to fill that huge case with Bullseye. Gun parts really reach velocity that way. (DANGER)
Larry, I don't now where that thinking came from but it is still repeated.
By definition the expansion ratio is the ratio of the volume of the case to the base of the boolit to the total volume. Total volume is volume of case plus bore volume.
303Guy, this is my understanding of expansion ratio also.
303guy, Doc
The expansion ratio of gases as expressed by Boyle is correct as it relates to the total volume the gas has to expand into. However, the "expansion ratio" in reference to firearms expresses the differences in case capacities to bore capacities (volumes). That is the difference; the result means two different things. My quote was not my definition but the standard definition taken from the Firearms Encyclopedia book on definitions of firearm terms for "expansion ratio" as it relates to firearms, not to gases in general. It is also used in other books/articles discussing the subject for many years. I believe it also is the definition used by Powley for his computations as he describes "expansion ratio as "total volume inside the case plus barrel exposed to gass, divided by the case capacity.." Thus again we see as related to firearms/cartridges we have a ratio between case capacity and the total volume for the gas to expand into. A bit different from the simple total volume are of expansion as expressed in Boyles gas laws.
That being as it may we are essentially saying the same thing. Earlier I posted "The exact bottom end of that level is very dependent on the exapnsion ratio, bullet weight and case volume (load density).". Your use of "sectional density" equates to "bullet weight" as bullets of equal SD in .30 and .243 caliber are comparatively the same in relation to the expansion ration of the .308W vs the .243. Same thing just different terms.....still the same page of the hymm book.....as it relates to your original question.
Powley also states that for a good working pressure the only 2 factors involved are the expansion ratio and the powder charge to bullet weight. That is what we are basically discussing except that at the lower psi's of cast bullet loads the psi level at which the powder itself will ignite and burn efficiently also must be considered. That is important because regardless of "burn rate" every powder has it's own psi level based on composition and retardent used for efficient ignition and burn.
Larry Gibson
Yes, my observations parallels yours in that Varget burns well and it does better than IMR 4064 for me; Varget is also more consistent throughout the year as ambient temperatures change. I thought about trying H4895 but chose to stay with Varget to have a bit slower powder to fill the case more.
44man
Another quote from Powley if I may which adds to your premiss; "a gun with low Expansion Ratio and a high ratio of Powder Charge to Bullet Weight will be affected much more by barrel shortening than will a gun of both high Expansion Ratio and low atio of Powder Charge to Bullet Weight.
Larry Gibson