Gas Check Boolits without Gas checks ???

[MT Gianni]

This is a good read. Bret comes closest to my view on this. I am new to cb and reloading in general, but did metal failure analysis in extreme environments and motion for 30 years. All of these issues show up elsewhere in machinery failure, manufacture or assembly.

Please forgive my nerd-speak and understand that I am just excited with this really fun topic. I do not intend to imply that ed-ja-ma-cation is in anyway a substitute for experience, especially in a topic like this one. These are musings, written like I write for fun, and I prolly sound like a pedant or some other horrid ting.


With the lead bullet, the situation is so much more complicated. The lead is already very close to its melting point so addition of heat from gas and rifling friction interact with the above-yield stress state at the rifling to push the total lattice energy above the melt enthalpy.So the contact point lead leaves the area, allowing cocking.

It is so unpredictable because all these exponential effects are combining to eat away at the tiny remaining lattice stability of the lead. And when pushing a rod from the back it is always hard to predict when it will go sideways. The driving force, the powder explosion, is an instability in itself, and varies a lot from shot to shot, day to day, powder to powder. Finally, while we get to hear about the caliber, length and rifling of our barrel, we usually get no information on the surface roughness or actual dimensions of the land and non-land geometry. The non-lands are the flow paths for the superheated gas and grit to rush past the bullet surface adding heat and stress.

The good thing is that all of these factors do not involve chaos, relativity, atmospheric corrosion, or radiation. That aside, mixed phase flow, contact stress and friction, dynamic thermal material property change, phase change, three dimensional stress state, and deformation dependent loading are all some of the most challenging stress analysis modelling that can be accomplished, one at a time, let alone combined.


Just my 2 cents on the really cool issue.

smw

Very interesting explanation.I have examined many hundreds of range pick up bullets, my own, others and commercial. I have never seen evidence of a melted base. The milliseconds that the temperature is there is not enough to melt them or most commercial lubes. Nor will the powder flame melt lead fouling.
Thanks for your insight on the equations of a booits stress.

[stephen m weiss]

I would expect you are right that it would not look like what we think as 'melting'. What would be occuring is the lead metal at the tail of the bullet that is squished by the rifling lands is at the highest stress. The heavier the squeeze, the larger this zone of material that is above yield stress. As the edges of this zone heat from the superheated gas, primarily at the back of the bullet, the rifling indents at the back would simply deepen as the bullet became more cocked. This would increase the stress concentration to just the corner, and deflection would increase. So, its not so much melting away globally, as losing the last little bit of strength in an overstressed contact point allowing worse cocking.

If the lead is smeared on the gun barrel, the barrel with act as a heat sink and protect it. Loose sharp edge shavings may melt in flight a bit. I have recovered melt globs of wax lube in from my barrel looking to see if there was leading. I guess the wax doesnt stick to the powder residue very well.

thx again!

smw

It is like shearing off the hot sprue just after pouring the mold is easier than waiting for the mold to cool completely. It's still shearing a solid metal not wiping away molten metal, its just easy and moves with little effort because there is little strain energy left in the metal, being so close to melt.

Thanks for adding your insights. Observations make the theory match much better.

I have recovered some heavy slow bullets that showed what I thought might be tail skidding (not melting) but since I shot into soft oil-sand, it's a bit hard to be sure what was done in the barrel and what was done in the sand. But I know I shot the heavy bullet kinda hot with no packing and it was starting to get inaccurate at a very short range (18 yrds) so I had trouble believing a low in flight stability could push it off course so far so fast. It seemed more like it never was going truly in the right direction. Then i shot some 223 50 grains at 2200 fps with no packing and many just plain came apart, and were 2 foot off course. This was obviously in-barrel damage. I expect those little bits were pieces of the tail that the rifling had filed away and spit out.

[stephen m weiss]

Oh, more anciliary evidence that tail rifling heating is important. If it were not, then it would take a tail gas check and a nose band to increase accuracy. But as we see, only .03 or so of rear gas check on an engagement length of .625 is enough to get very close to fmj speeds. Right? A gas check 30 cal cb can go 2600 fps where the mil spec fmjs of that weight are 2800 fps or so? Sure that fmj can go even faster, but full magnum bullets need a little tougher construction get smokin to 4000ish fps right?

I bet if you made up some copper wire spring rings and stuck em in grooves fore and aft on a cb, you could get all the way to fmj speed. Kind of like an ic engine piston. If i had a bullet sizer, i would make up some on a regular cb and size em into grooves shortly after casting and before oven water drop hardening. Its easy to make soft copper springs by winding around a drill bit shank 1/32 smaller than the bullet. Wind them long then nip to 2 turns or so. Turn in the nipped ends a bit to grip the lead so they cant spin.

Ha a test for a different day!

smw

[popper]

S.W. - I had thoughts of a 'plastic ring' for the base or wrapping the base in wire. Then I realized the hard ring (in your case Cu) would act just like the barrel. Gas cuts between ring & boolit. Time in barrel is microseconds, not milliseconds. No way you can get thermally applied heat to the lead. Pressure generated energy that is converted to heat, yes. I've shot 308W to > 1800 sans checks (PC so no leading) accuracy suffers from (IMHO) shank riviting. Same boolit > 2400 with accuracy when checked. GC also prevents base 'twist' damage. Lots of torsional force along the length of the boolit when pushed to twist. Once the GC is in the grooves, torsional force on the boolit is changed to compressive force.

[stephen m weiss]

Hmm, metal yield gas seals are some of the best. The copper will seal to the lead up to the compressive strength of the lead, and there may be some self energizing component. Even if there is leak because the gas pressure is so high, the flow rate will be very low because the gap is near zero. The gaps in the riflings are much larger. It's true the gc has the gas pressure to energize it to stick to the back of the bullet, at least until the bullet leaves the barrel.

[stephen m weiss]

mm time in the barrel is about twice of 2'/2000fps =.002 sec = 2 millisec. Doesnt really matter, until i dig out my time constant material notes, I dont remember off the top of my head what the time constant for little bits of stuff would be. Its in that order of time though. I think the corners of the steel rifling would heat first because steel is lower density, the corners stick out, and high strength steel has **** for thermal conductivity. Unfortuneately, those corners are yield strenght pressed into the lead bullet right at its highest stress location. So that would put the heat right into the lead.

I really need to test some of this. Unfortuneately, my bullet casting skill is still low, with lots of flaws. I dont have temp control or bottom pour, and there are visible flaws which cant help. LOl I am still using paper towels as flux. Worked good till the smoke alarm went off. haah

I am still doing most of the calculations in my head and havent developed my own spreadsheets yet. I developed simple hardness testing techniques in industry, but havent applied the better stuff to bullets yet, no jigs or fixtures yet.

Good to see you thought of the bullet rings also. Merits a test at least then. A bullet sizer is just one more Amazon purchase away! lol

smw

[MT Gianni]

Great thoughts. I had an old gun digest where someone modified a .375" mold to take 0.100" copper bands cut from 3/8 tubing. I don't know how fast you could drive them even in an H&H. That is an interesting concept.
I like the explanation of heat stressing the bullets rear rifling marks. Always something more to understand from our boolits.

[popper]

S. W. correct on barrel time, I incorrectly stated that. Time for high pressure is generally 1/2 mSec, depending on the load & powder speed. High pressure/small nozzle = high velocity which is the culprit for gas cutting, not the heat. Like a water cutter/plasma cutter. Mass & velocity do the work.

[stephen m weiss]

Y'up Popper. mass and velocity means energy aka work so agreed energy means moving material. The more I read of your views and expanations, the more i get the feeling we are using different words for the same concept. Just as gas solid liquid have no use at teh eutectic point, there is not much point in debating material failure so close to the strain-melt point. It is an energy controlled process.. so cutting off the energy causing failure is one method, ie packing while another is raising the energy required to fail, ie fmj is another. A method that does both at once is golden, ie gas checks.

This of course does not mean that there are not more cost efficient methods. Also different methods have secondary advantages. Toilet paper versus Dacron (polyester) packing is one such issue. They are nearly identical materials. However, since polyester is basically chemically immune, toilet paper packing will be gone from my land many years before teh poly-fuzz would be.

I cant wait to test copper rings. I have a bunch of 18 ga wire.....

smw

[popper]

post the results, I'm interested. I use dacron, the TP is a fire hazard here in drought stricken country.

[rsrocket1]

Very good estimation smw,
Looks like total muzzle time is around 1.8 milliseconds:

[Metroxfi]

I had a thought on leading due to the bullet skidding/smearing on the rifling... A while ago I had read about guys making soft nosed cast bullets for hunting by casting with wheelweight alloy and water dropping, then put the bullets nose up in a pan and fill the pan with water until it came up to the nose section. They would then take a torch and heat up the noses to undo the heat treating from the water dropping on the nose section only, so you have a hard base/driving bands and a softer/expanding nose section. What if you did it the other way around and heat treated the bullet then softened the rear driving band? That way you'd have a solid front band that wouldn't skid as much and you could also benefit from obturation, a sort of 'best of both worlds'. Or is that too much trouble for little to no reward?

[stephen m weiss]

I think the rear driving band, just where the gas check would sit, is the most critical for keeping the bullet stable in the barrel, and you want that hard and slippery, like copper.

[rsrocket1]

Here is a picture of my last outing with cast 30 cal boolits. 36 shot and a clean shiny barrel. I have to admit that 10 shots were with a gas check (the 800X loads) so that may have cleared any leading, but the last 12 shots were at subsonic velocities with the 200g bare based bullet with 6.5g Red Dot. Max velocities was in the mid 1600's with the 113g bare based bullets.

I have a bunch more loaded with Dacron tufts to see if MV's change. 10g Unique seems to be pretty position insensitive however. After that I will work up to 2k fps to see how the 113g bullets hold up with bare bases.

[stephen m weiss]

Very nicely done rsrocket! So, if I am seeing correctly, you are getting 2-4 moa with bare base bullets using unique and 800x.

I shot over the weekend and my cast bullets were hitting 8 moa at about the same speeds using TP packing and wax lube with 2495 powder. The bullets are not sized and have a lot of visible flaws because my process is still pretty bad. But higher than 1600 fps and the accuracy began to drop off sharply. A flawed bullet will of course be much more prone to canting in the barrel from eccentric center of gravity and missing material at the driving band.

I would be quite happy with 2-4 moa by comparison! So, I think I need a bullet sizer and single press, bottom pour temperature regulated pot, proper flux, and need to water drop from the mold.

Thx again, smw

[stephen m weiss]

Oh, when I did not use TP packing, i got very aberant speeds, like 0-20%, and some muzzle flashes.

[stephen m weiss]

I finally got a round to calc'ing the time constant for lead. I ran this years ago for the condition of a plate beginning heating at time zero on one side, what does the temp on the other side do? For a cylinder its gonna be much faster, or about the same if heated from one end like a bullet with perfect seal in the barrel. The equation is:

Time Constant = (4 x specific heat x density x thickness^2)/(pi^2 x thermal conductivity) = .18 sec/m^2 for lead.

The values can be found in wiki for common elements. Alloying changes the value. 6% Sb is about 16% slower, or larger. Time constant means the time to change 63% toward the surface heating temperature. Note that this may well be limited in practice by the convection doing the heating, especially in gas heating systems, like just-cast bullets cooling on the bench.

Some example thicknesses> for a 223 heating on the diameter, with no packing, time constant is about 2 msec. That means the 2 msec in the barrel combined with the thousands of degrees of gas temperature means that some surface lead heating at the rifling definately occurs. More justification for using good packing.

For a 308, that number is about 5 msec, so much less subject to in-barrel heating.

This also shows that cooling during water dropping is convection limited, or else water dropping would never work, the bullet would be cool before it hit the water. In this case, it is surface area to mass ratio that limits cooling, so a 223 will cool faster by 308/223 = 38%. So water dropping is gonna work, ya just gotta have them hit the water quicker by 38%.

If anyone wants to geek out on how much cooling occurs in the air, cooling power = 7 Watts/m^2/C for still air. You can get the density and specific heat from wiki. The surface area of bullets is pretty simple.

[popper]

convection limited Nope, conduction. Convection is due to moving heat absorber. Thermal resistance of air is much greater than water. 308 needs to be dropped into colder water (I'll accept your 38% number) to get same effect as 223, delta F controls the heat removal RATE, which is what makes W.D. work. I will agree, get then from the mould or oven as quickly as you can. Takes me ~ 5 sec max to dump the tray from the oven.

[stephen m weiss]

I guess i spoke unclearly. It seems we agree, just not on the wording.

In order for water dropping to help, the bullets need to be still hot when they hit the water. If the cooling in air was conduction limited, air quenching would be faster than the time to get them to the bucket. I was just speaking of the air phase. Clearly in the water, at 1000x the convection rate, conduction is doing the limiting. If the bullets were small enough, they would air quench just fine. If they were big enough, even the water would be unable to quench them fast enough. It would only require the 2nd or 3rd root of 1000 times bigger, so maybe 3 inches diameter. In that case, only surface hardening would occur.

Let me dig out the time constant equation for convective cooling. I can determine roughly how long is ok to take from teh oven to the water. They actually solved that problem in my book and I did not have to derive it myself. ahahah

Say popper, I have been using my 10x steroscopic microscope to inspect my 223 bullets. What size in cubic mills visible flaws do you reject for? I am just sort of using a general 70% rejection at this point. My 223 mold has lots of issues, like slipping alignment pins, wave washers dont hold down the sprue plate well, wood handles slip off the metal part...It's like herding cats to get good casts. lol

[popper]

I have never seen evidence of a melted base

That means the 2 msec in the barrel combined with the thousands of degrees of gas temperature means that some surface lead heating at the rifling definitely occurs

Not much. Joule content of a 150 gr boolit is ~1J @ 60F. Joule gain from a 2000F source for 1/2 millisec is ~6.8e-5 (sorry, bad number 1.7e-11). Multiply by 4 for 2 millisec time still is VERY SMALL. Temp rise of boolit & barrel is mostly pressure related. Boolit is compressed and barrel is expanded. There is a frictional component.
I inspect for any 'smiley face' wrinkles, sharp corners (fillout) & pits. Any visible(bifocals) surface defect. I did some weight sampling, most shot the same.