Pressure rule...

[Martin Luber]

I vaguely recall a rule of thumb for bullet hardness and either pressure or velocity...something like 15000/ x = bhn

Struggling to recall, anyone else remember what it was and where it came from?

Thanks

[Rcmaveric]

That's going to be in the Lee Reloading manual. It's good base line to start experiments and work ups I normally that pressures and velocity are much higher once tuned.

Sent from my SM-G965U using Tapatalk

[JBinMN]

Approximate "Maximum" Chamber Pressure For Lead Alloys (PSI)


Plumbers lead, stick on WW 13,000 - (Black Powder Only)


Wheel weights / clip-on 25,000 - Non-Magnum handgunloads, Rifles to 1,900 fps



Lyman # 2 (alloy varies in Lyman cast bullet books) 35,000 - Magnum handgun & rifles to 2,000 fps



Quench-cast WW (dropped from mould into cool water) 48,000 - Magnum handgun & rifles to 2,200 fps



Oven heat treated WW 55,000 - Jacketed velocities in handguns and rifles with quality bore & balanced load


Bullet BHN / "Minimum" Chamber Pressure For Lead Alloys (PSI)

The formula (from the pages of HandLoader Magazine) to determine at what pressure an alloy of given BHN will obturate the base of the bullet and seal the bore. If the bullet is too hard to obturate, gas cutting usually occurs on the base band on the non-driving side of the rifling and barrel leading is likely. Simply multiply the alloy BHN by 1,422.
Example: Alloy BHN of 12 multiplied by 1422 = 17,064. An alloy of 12 BHN should be used with a load that develops a "minimum" of 17,000 psi. Need more info on minimum / maximum alloy BHN? These Glen E. Fryxell articles explain alloy BHN in easy to understand language.

1) http://www.lasc.us/FryxellCBAlloyObturation.htm

2) http://www.lasc.us/FryxellCommentsCBAlloys.htm

3) http://www.lasc.us/FryxellLubeCastBullets.htm

-----------------------------------------------------------

Source for above:http://www.lasc.us/castbulletnotes.htm ( about 1/3 down page on the Right side with Common Bullet Alloy to the Left side)

Hope that helps a bit.

[Larry Gibson]

That formula doesn't really "hold water" or more bluntly; that dog doesn't hunt......

I shoot COWWs + 2 % tin and #2 alloy at 2700 to 3000+ fps at measured 50,000+ psi and maintain 1 t0 2 moa accuracy to 500 and 600 yards. (all info has been posted in numerous threads here).

[OS OK]

That formula doesn't really "hold water" or more bluntly; that dog doesn't hunt......

I shoot COWWs + 2 % tin and #2 alloy at 2700 to 3000+ fps at measured 50,000+ psi and maintain 1 t0 2 moa accuracy to 500 and 600 yards. (all info has been posted in numerous threads here).

Mr. Gibson....geez, I hate to ask this...but...would you please give us a link to the most comprehensive post you have made regarding this?

[kokomokid]

Would there be any downside to shooting lino alloy with gas check at reduced velocity?

[Rich/WIS]

Other than a more expensive alloy than needed, no.

[Rcmaveric]

Would there be any downside to shooting lino alloy with gas check at reduced velocity?

The bullet might not obturate and cause leading

Sent from my SM-G965U using Tapatalk

[swheeler]

I think you will find the 1422xBHN= MINIMUM PRESSURE NEEDED TO OBTURATE THE BULLET BASE is correct.
Now as for rules with cast bullet velocities and pressure, there are none written in stone simply by BHN, just like the RPM limit theory! It only matters when it does.

[Larry Gibson]

Mr. Gibson....geez, I hate to ask this...but...would you please give us a link to the most comprehensive post you have made regarding this?

OS OK......geez, where have you been? Assuming you can use the search engine on this forum (?) but in case you can't there are many posts in the following threads readily found on this forum;

http://castboolits.gunloads.com/show...-XCB-600-yards

http://castboolits.gunloads.com/show...h-three-twists

http://castboolits.gunloads.com/show...ists-Chapter-2

http://castboolits.gunloads.com/show...old-discussion

http://castboolits.gunloads.com/show...nt-HV-Accuracy


If you've a more specific question I'd be glad to respond?

[JBinMN]

That formula doesn't really "hold water" or more bluntly; that dog doesn't hunt......

I shoot COWWs + 2 % tin and #2 alloy at 2700 to 3000+ fps at measured 50,000+ psi and maintain 1 t0 2 moa accuracy to 500 and 600 yards. (all info has been posted in numerous threads here).

----

Talk to Glen Fryxell then, since he is the one who wrote the articles. If I am not mistaken, he is also a member here.
( Perhaps you could PM him & tell him that he is wrong & that he can look around the forum to find info that makes his articles less than correct. He might ask for more links to the info that contradicts him & his info though. Just like some of us are asking)


I was just sharing info that has been around for quite some time & as of when I posted, still there for others to read.

"Don't kill the messenger for the message", comes to mind, Thank you.

If you have links for folks, please be so kind as to post them up. I will certainly be one to take such info & "run with it" if it applies. I like researching for more knowledge. It helps though when someone who contradicts one source offers other sources for folks to go check out.

I am an "equal opportunity provider" of what info I am aware of. I try to give sources for folks to check.

Thanks for your giving me something to go find myself, ifyou are not able to offer other sources for your info.

I am learning.

Perhaps I should not provide any info to others, but to tell them to go look for themselves in the CB.GL forum archive, or in a search engine.

IOW, "pass the buck", give no attempt to help out, & make them do their own homework, rather than their asking others here their "opinion" or where they might go to find more info...

But then it would not be as good a forum as I thought...

Thanks again for the info provided...

ETA -
Note: Apparently I was typing this post when Post # 10 was being posted & as I did not refresh the page and see Post # 10 until I posted this post. My reply above is to post #4 as quoted at the top.

In regard to post # 10:

Thanks for taking the time to reference the sources for your point that Mr Fryxells info is incorrect. I reckon I will go check out those sources and see if I have reason to change " at least" my mind, of what I am reading & have been working with so far based on Mr Fryxells info.

[Larry Gibson]

I think you will find the 1422xBHN= MINIMUM PRESSURE NEEDED TO OBTURATE THE BULLET BASE is correct.
Now as for rules with cast bullet velocities and pressure, there are none written in stone simply by BHN, just like the RPM limit theory! It only matters when it does.

The "constant" in that formula is based on the "compressive strength" (Lee's terminology) of lead. The constant should change with binary and ternary alloys. I see by your emoji that your "blowing smoke" which is usually the case when you bring up RPM. You've been told, along with others, the RPM Threshold is not a "limit" and that fact has been demonstrated to you and the others numerous times.

Read the above threads, particularly the first, where you will see an NOE 30 XCB cast bullet of #2 alloy pushed at 2900 fps at 50,000 psi (actual measured psi in that rifle) going into an 11 shot sub moa group at 600 yards. That in and of itself disproves the formula. Lee's big mistake, and others, with that formula is the reliance on just velocity vs pressure to prove that theory. They disregard RPM which is why their "formula" fails. Not only I have disproven that formula but many others here on this forum also have.

For your edification here is the 11 shot group at 600 yards.

Attachment 250429

[OS OK]

OS OK......geez, where have you been? Assuming you can use the search engine on this forum (?) but in case you can't there are many posts in the following threads readily found on this forum;

Mr. Gibson...I have and continue to use the search function.
In my post you'll notice...I ask...
"geez, I hate to ask this...but...would you please give us a link to (the<meaning 'singular') most comprehensive post you have made regarding this?"
I had thought you might have posted one link in particular that is the most comprehensive post you have made regarding this...if so it would have saved me time in this discovery.

http://castboolits.gunloads.com/show...-XCB-600-yards

http://castboolits.gunloads.com/show...h-three-twists

http://castboolits.gunloads.com/show...ists-Chapter-2

http://castboolits.gunloads.com/show...old-discussion

http://castboolits.gunloads.com/show...nt-HV-Accuracy


If you've a more specific question I'd be glad to respond?

I thought I had asked a specific question.

[Larry Gibson]

"would you please give us a link to (the<meaning 'singular') most comprehensive post you have made regarding this?"

Still not sure what you mean by "this" as "this is not very specific.

Somewhere in those threads and possibly/probably others is this post; ""Yield strength" of an alloy is not as cut and dry as it would seem in Lee's manual. As the alloy yields during acceleration the nose slumps back or to one side depending on where the alloy yields first, the lube grooves collapse depending on how big they are and their design, the alloy on the lands can chip off if too brittle, etc. What is wanted is a bullet design that minimizes these adverse potential "yields" and an alloy that is malleable and yields evenly around the circumference of the bullet. In other words we can cast a perfectly shaped and balanced bullet but during acceleration we want to keep the changing shape (the "yield") as close to balanced as possible.

Given a good bullet cast 311291 of Lyman #2 alloy and used in a 10" twist '06 for instance; If we load to yield strength using a fast powder such as Unique the yield point will be much lower than 140,000 RPM, same if we use the "slow" fast powders such as 2400, 4227, 4759, 4198 and 5744. Using a medium powder such as 4895 we will find the yield strength right around 140,000 RPM. And using a slower powder such as 4831 we find the yield point to be around 145,000 RPM. The psi for all the above loads may be close to the same but by slowing the acceleration rate with a slower powder we increase the velocity before the yield point is reached, i.e.; the pressure at yield for all the powders is close to the same yet the rate of acceleration based on the burning rate of the powder changes at what velocity/RPM that "yield" will occur.

At or above this "yield point" with regular cast bullets are where the centrifugal force of the imbalanced bullet over comes the rotational stability of the bullet and it begins to spiral around its line of flight in a non linear manner. This is not saying the bullet becomes "unstable" as it does not, it is still flying forward.

If we are going to push through the RPM threshold and attain accuracy with cast bullets at higher velocity/RPM then we must use a bullet of such design that minimal setback/obturation can occur. We must use an alloy that will give even set back/obturation around the circumference of the bullet when/if it occurs. We must use a cartridge of sufficient capacity allowing a slower burning powder that keeps the psi under the “yield" strength of the alloy at the projected velocity/RPM. The powder must be easy to ignite and must ignite and burn consistently at the lower psi's of these cast bullet loads. In my .308 Palma with a 14” twist I am pushing a 311466 cast of 80/20 linotype/lead bullet with a BHN of 18-20 at 2600+ fps using AA4350 with excellent accuracy. The psi is over 40,000 which is above the “yield” for that alloy using all the current formulas. The RPM however is 134,000 which is under the threshold. That bullet design, even though it is exceeding the so called “yield” point still gives excellent accuracy (1.5 moa average for 10 shot groups at 100, 200 and 300 yards) . I believe the reason is that the alloy sets back/obturates evenly which keeps the bullet as balanced as possible and the RPM is kept under the RPM threshold so the centrifugal force has not over come the rotational stability.

Which, in context, is a discussion of the above mentioned formula. That formula tells us that the below should not be possible. Now I don't need to find Fryxell or Lee to discuss it with them. They, if they should want to, need to revise their theory or discuss it with me.

If the formula is correct then please explain (or perhaps Fryxell or Lee could?) the below 20 shots in 1.607" (includes fouler) at 2907 fps (15' from muzzle) at 50,000+ fps? I have posted many such groups on this forum. I have also posted many Oehler pressure test data sheets showing the actual pressure measurement along with the velocity as measured with the Oehler M43 PBL.

Attachment 250434

[Larry Gibson]

Perhaps this post will also help explain(?).

A New Age In HV Cast Bullet Shooting

I have gone to great lengths in numerous threads on several different forums over the last 8 to 10 years learning about, researching, understanding and explaining the cause of and why there is an RPM Threshold. Just what do we mean by “RPM threshold”:

The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a non-linear helical spiral in flight or its flight path goes off on a non-linear tangent from the line of flight.

Exceeding the RPM Threshold does not adversely affect bullet stability except in extreme cases. Most often, even though over the RPM threshold and accuracy is decreasing both in increasing group size and in non-linear group expansion as range increases, the bullet will still be stable in flight.

It is best noted when working up a load as velocity increases flyers begin to happen. Then as velocity is further increased the total group size increases sometimes to the point some bullets fly so far off they miss the target. A further indication the cast bullets at or over the RPM threshold is (or some of them in a load that is on the edge of the RPM threshold) the non linear dispersion of the group size as range increases.

Let us keep in mind the RPM threshold most often falls in the 120,000 to 140,000 RPM range with regular lube groove cast bullets. Exactly where the RPM threshold will be in fps depends on numerous factors; alloy, bullet design, fit, sizing, lube, GC’d and seated square, powder burning rate and the length of the barrel, etc. The RPM threshold may be lower than 120,000 RPM by careless casting and loading techniques or when using very soft alloys with very fast burning powders. Conversely, the RPM threshold can be above 140,000 by careful casting and bullet selection and preparation along with careful accuracy enhancing loading techniques, especially those for cast bullets at high velocity such as using slow burning powders that ignite easily and burn efficiently at lower pressures. The trick is to get the cast bullet to exit the muzzle as balanced as possible with as little deformation to it during acceleration. The more balanced the bullet is and the closer the axis of rotation coincides with the center of mass on exit from the muzzle and during flight the more accurate the bullet will be and thus, the higher the RPM threshold will be.

The RPM threshold is not a set “limit” of RPM or velocity. Best accuracy will be just under the RPM threshold or lower. Useable accuracy can be had above the RPM threshold if the ranges are not long and the accuracy requirement is not small.

To further understand why we want to control the RPM to be able to shoot cast bullets at higher velocity accurately (preferably to the same velocity level as the cartridge will shoot an equal weight jacketed bullet) we need to understand exactly what it is that causes the loss of accuracy when such a span of RPM is exceeded.

In this post, my first post in this thread, I want to give you another ballistician’s explanation. I have mentioned his name before and at his request I will quote him chapter and verse from his very fine book; APPLIED BALLISTICS FOR LONG-RANGE SHOOTING, 2nd ed.

So let’s take a look at what Bryan Litz says in that very fine book that relates to the RPM Threshold:

Chapter 10, Bullet Stability, page 144;

“The relationship between stability and precision [what we generally refer to as “accuracy” or group size] is probably where the bulk of stability misconceptions exist. Gyroscopic stability can affect precision in two ways. First if the bullet is not adequately stabilized, it will emerge from the muzzle and fly with some significant amount of yaw until it stabilizes (goes to sleep).This situation is bad for precision and adds significantly to the bullets drag. This problem can be simply solved by choosing a proper twist rate.
It’s also possible for precision to be compromised if the bullet is spinning faster than it has to be for adequate stability. When a bullet emerges from the muzzle of a rifle, it’s spinning very fast. Any imperfection in the shape, balance or alignment of the bullet will cause it to disperse away from the bore line when it exits the muzzle. The amount of dispersion is related to how severe the imperfections in the bullet are, and also how fast the bullet is spinning. Higher spin rates produce more dispersion. This situation can create the illusion that the bullet’s dispersion is caused by excessive stability, but that’s not the reality. The actual stability level of the bullet is not what causes the dispersion! The imperfections in the bullet cause dispersion, and the dispersion is increased the faster the bullet is spinning. The more balanced the bullet the bullets are, the less dispersion will result from spinning them faster. One of the reasons why short range Benchrest shooters choose to shoot short, blunt, flat based bullets is because they require such a slow twist to stabilize. The slower twist barrels aggravate the imperfections of the bullets much less than faster twist barrels and smaller groups result.

The relationship between spin rate and dispersion is stronger for low quality bullets. This is why British Commonwealth ‘Target Rifle’ shooters long used 1:14” twist barrels as until recently the discipline’s rules required the use of ‘as issued’ 7.62mm NATO military ammunition which was loaded with relatively poor quality 145 grain FMJBT bullets. As precision bullet manufacturing technology matures and the resulting products move ever closer to having perfect balance, the relationship between spin rate and dispersion is diminishing.

There is much debate about what barrel twist rate or bullet RPM is optimal for precision shooting. The answer isn’t as complicated as it’s made out to be. The right way to choose the proper twist rate for a particular bullet is to select a twist that for a particular bullet is to select a twist that results in a stability factor (Sg) of at least 1.4 [Miller Stability Formula] at your intended velocity. If the stability factor is a little higher, that’s ok. The effects of over spinning a bullet are not nearly as bad as under spinning it. If the Sg is up near 2.0, that’s not necessarily bad. As long as you’re shooting good bullets there will be no detectable difference in precision. If you get a batch of bad bullets, a slower twist barrel may be more forgiving, but the difference would be hard to resolve.”

Whew….sure is a lot of information in there! However, notice any similarity to the definition of the RPM Threshold? Now to be fair to Bryan Litz his very fine book is not about cast bullets. As a matter of fact he does not mention cast bullets at all. His book focuses on the best of match grade jacketed bullets made by several manufacturers. None the less, let us understand his discussion of jacketed bullets does not mean the same Laws of Physics and Ballistics do not apply to cast bullets in flight. They indeed do apply to cast bullets just the same as jacketed bullets. It’s just harder for us to cast, let alone launch, cast bullets with anywhere near the quality of the “good bullets” that he refers to as Bergers, Sierra’s and Hornady’s are his primary choice of bullets.

So what does it mean? Mr. Litz says we should choose the correct twist for the bullet and velocity we want keeping the Miller Formula Sg Factor above 1.4 (stability) and under 2.0.Let’s say we want to use the 30 XCB bullet with a weight of 165 gr and a length of .985” at 2500 fps minimum. Looking at the Sg factors for the following barrel twists we find they are;

10” twist; 4.31 Sg/180,000 RPM
12” twist; 2.99 Sg/150,000 RPM
13” twist; 2.55 Sg/138,461 RPM
14” twist; 2.20 Sg/128,571 RPM
15” twist; 1.92 Sg/120,000 RPM
16” twist; 1.68 Sg/112,500 RPM
17” twist; 1.79 Sg/105,882 RPM
18” twist; 1.33 Sg/100,000 RPM

From those Sg/RPM figures we can see why it is extremely difficult to attain accuracy at 2500 fps from the 10 and 12” twist barrels. There is an obvious difference in the RPM at a given velocity of 2500 fps the faster the barrel twist is. Spinning at such high RPM rates can create a lot more centrifugal force acting on any imbalances in the bullet. That is the basis of the RPM Threshold. With the 12” twist if we do everything just right and can get a fairly decent cast bullet of correct design launched with few imperfections we can get 1 – 1.5 moa at 2500 fps. You can see here with this test of the 30 XCB bullet in a target .308W rifle.

(Note; according to the "formula" under discussion in this thread even the 2500+fps at 31,900 psi as shown in the below Oehler data sheet shouldn't be possible....yet it is with a 12" twist barrel)

Attachment 250441

The 13” twist is fairly common now as there are numerous Palma rifles and barrels of that twist available. The 13.5” twist seems to also be gaining in favor for Palma rifles. Both should do well with the 30 XCB bullet in the 2500 – 2600 fps range as we see the Sg/RPM at 2500 fps is lowering under the top end of the RPM Threshold.

We now have a lot of experience with the 14” twist barrels in several 30 XCB rifles and with my own Palma .308W. We have proven the concept that given a longer barrel, slower burning powders and quality cast 30 XCB bullets we are seeing excellent and repeatable accuracy in the 2600 – 2700 fps range. We also see the 14”twist’s Sg Factor is getting down where Mr. Litz says it needs to be. A lot of practical shooting at 100, 200 and300 yards with 10 shot groups is proving him correct with those14” twist barrels using the 30 XCB cast bullet.

The 15” twist is looking better with the 1.92 SG (under the level Mr. Litz recommends) and it should do well but I’ve no knowledge of anyone who has one of one in the pipeline.

My building my HV cast bullet rifle I selected to use a 31” Broughton 3 groove barrel with a 16”twist. Given my extensive experience with the 14” twist in my 27.6”barreled .308W Palma rifle I felt there just wasn’t enough case capacity in the .308W or the 30 XCB cartridge (30x57) to reach the initial prophesized goal of 2800 – 2900 fps with the 30 XCB bullet. Thus I had a former member who is an excellent gunsmith install the Broughton barrel on a very fine BRNO VZ24 action and chamber it to the 30x60 XCB cartridge which has about 5 – 6 gr of powder larger capacity than the 30 XCB (30x57 which has essentially the same capacity as the .308W).

Attachment 250442

The 16” twist has an Sg Factor of 1.68 which is very good according to Mr. Litz. Again practical shooting and a lot of testing has proven him correct. I have put over 1500 rounds testing various alloys, powders, primers, lubes, different bullets and even recovering 2900 fps 30 XCB bullets at 300 yards. I have pushed velocities to 3100+ fps to the point accuracy was lost with the bullets plasticizing and the GCs coming off because the alloy they were in contact with melted. In essence I believe we’ve discovered the true velocity limit to naked ternary cast bullets while maintain any real semblance of accuracy.

After attaining all that I backed off a bit to find a “standard load” that would provide excellent accuracy at as high a velocity as possible to maintain on “repeat on demand” scale. I had predicted about 8 years or so ago that probably 2800 – 2900+ fps should be attainable while maintaining 2 moa or less linear accuracy out to 300 or 400 yards with 10 shot groups. I’ve been very pleased to find my prediction was correct. My standard load hovers right between 2890 and 2910 fps. Thus far I’ve maintained linear moa accuracy to 300 yards. Here is a recent linear group expansion test I ran because an individual on another forum doesn’t believe cast bullet group expansion can be linear as the range increases said; “odds are you can do it again”. Not only can I do “it” again but that day I ran another test at 300 yards and shot seven additional 10 shot groups that ran from 3.1” to 3.35”. The 100 yard group has 12 shots in it; 2 sighter/foulers. The 1st fouler is the high one out of the group. The 2nd fouler went into the middle of the group. The 10 shots for record measure 1.1”. The 200 yard 10 shot group measures 2.08”. The 300 yard group measures 3.35”. BTW, the stamped diamond measures .75” on a side.correct. My standard load hovers right between 2890 and 2910 fps. Thus far I’ve maintained linear moa accuracy to 300 yards. Here is a recent linear group expansion test I ran because an individual on another forum doesn’t believe cast bullet group expansion can be linear as the range increases said; “odds are you can do it again”. Not only can I do “it” again but that day I ran another test at 300 yards and shot seven additional 10 shot groups that ran from 3.1” to 3.35”. The 100 yard group has 12 shots in it; 2 sighter/foulers. The 1st fouler is the high one out of the group. The 2nd fouler went into the middle of the group. The 10 shots for record measure 1.1”. The 200 yard 10 shot group measures 2.08”. The 300 yard group measures 3.35”. BTW, the stamped diamond measures .75” on a side.

Attachment 250443


So again, if the psi/alloy formula is correct then how is all the above possible? Bottom line is the psi/alloy formula is not correct because it does no take into account the RPM of the bullet and thus "that dog don't hunt"......

[OS OK]

L.G.> Still not sure what you mean by "this" as "this is not very specific.

You nailed it (this). <Referring to the topic of this thread.

This is exactly what I wanted to read and I wanted to read it without going through a dozen or more links from a general search term(s).
I appreciate your comprehensive answer and will copy/paste it where I can find it anytime I wish.

"Mr Gibson you have, should you accept it...my humble apology for not being specific enough in my request . . . (would you please give us a link to the most comprehensive post you have made regarding this?)"

charlie irby

[JBinMN]

Mr. Gibson,

I have been searching thru those links you offered & not found something that I was looking for, in them.

Are these COWW + 2% tin, & Lyman#2 alloys 30XCB boolits being used for these tests you have done to check for accuracy at 2890-2910fps being water dropped or heat quenched in any manner, or just coming out of the mold with no more heat treatment(air cooled)?

[BNE]

Mr Gibson,
Thank you for the work you have done for our hobby. I wish we were neighbors, I suspect you have lots of info to share!

I always appreciate your detailed input and frankly, your bluntness.

BNE.

[Traffer]

Wow, this is turning out to be a great thread for an accelerated education for a noob such as myself.
I would like to join in the fun of trying to produce precision lead bullets by swaging (just lead no jackets). I have a lot to learn. One perhaps very stupid question...There are several people who are now using powder coated then sized cast bullets for somewhat higher speed and longer range rifle shooting. Are there any general rules of thumb concerning the shift from using lubed bullets to grooveless PC'ed bullets for rifle shooting? Is there any parallel to paper patching?
Yes I am cheating by trying to get answers to questions that I could research myself but if you will excuse me that with some general answers I would be obliging.

[Larry Gibson]

OS OK

No apology necessary. Glad what I assumed was what you meant was correct. Happy to have provided the correct answer.