Estimating Barrel Friction in Pounds

[barnbwt]

Is there a way to do this for various calibers/bullets?

I am trying to nail down how much force the barrel experiences for a rifle design, and the closest thing I can find for the frictional component is a study (whose methodology seems somewhat questionable) of 223 that found values around the 150lb range. However, my design is in 7.62x25, so I expect things to be slightly different due to both the bullet diameter (surface area) as well as the contact-area length of the stubby bullets.

Right now, it looks like I have enough design headroom for 200lbs or so of barrel friction (in addition to barrel thrust), which is probably at least the right order of magnitude. But is that a realistic, or at least conservative figure for a small pistol caliber?

TCB

 

[taliv]

could you measure the difference in velocity for different barrel lengths and compute from that?

 

[Berger.Fan222]

The attached figure is from this paper:

http://www.dtic.mil/dtic/tr/fulltext/u2/a431357.pdf

which, together with this paper:

http://www.dtic.mil/dtic/tr/fulltext/u2/a469801.pdf

probably include most of the related work on bore resistance in 7.62 mm done by the DoD.

If I needed a good estimate, I would find the bullet construction closest to the one I was interested in, convert the units in the figure to the units I wanted, and then scale by the difference in bearing surface lengths between the bullet in the tests and the bullet I wanted an estimate for.

It's probably not news that solid copper bullets have a lot more friction than jacketed lead. The finding that the resistance force of pushing a solid copper bullet through a 7.62 mm barrel is 5000 N (> 1100 lbs) was more surprising to me.

From perusing the papers, it seems like 200 lbs is on the low side of what one might expect from a 7.62 mm rifle bullet.

It is also of note that QuickLOAD estimates a resistance force of 270 lbs for a 7.62mm rifle bullet (jacketed lead), but only 166 lbs for 7.62mm pistol bullets, likely due to the shorter bearing surface.

 

[rcmodel]

IMO: No.

If there were a way to compute it, or even a formula for it?
There would be no need for ballastics labs, or pressure test guns, or load testing.

But they are still in universal use to measure pressure with different bullet weight, construction, and powder burn speed.

rc

 

[barnbwt]

"The attached figure is from this paper..."

Well that's certainly counter-intuitive --you'd think friction would drop as the bullet is fully engraved, and level off at some point, but it looks like friction rose as the bullet conformed to the barrel at the forcing cone area, then rose until it fell out the end of the barrel (steep dropoff)

I do take some issue with their test methodology, since their testing was not done ballistically; the slugs were pushed through the bore. That means no effects of blow by or obturation at the rear of the bullet (admittedly the latter is probably negligible in FMJ or solid copper ammo, but blow by could have a lubricating effect for all we know). 1000lbs seems awful high, at least for Tokarev; that's about 1/4 the bolt thrust of 7.62x25.

Further down in the report they recorded standard M80 at 1000N (224lbs ), and moly-coated copper at 2000N. The really high 6000N figures are for uncoated solid copper, and is at least double the figure for every other condition tested. The report did sell me on a 1.5degree forcing cone, though, since it seems it consistently halved the peak swaging force in all cases compared to a 2.5degree cone

If 308 NATO FMJ is ~225lbs or so, and my projectile is the same diameter but both shorter and softer in construction (and all other figures are the same since I'm using a take-off 308WIN barrel), and I'm already using some pretty conservative assumptions*, I feel at least 'okay' at this point with my design.

Thanks a ton for the link to that report; that was excellent. The report I found earlier was trying to correlate different powder charges to velocity, and using the linear relationship between them to back out the 'constant' of barrel friction (which I found a dubious assumption since pressure affects blow by which likely affects bullet friction)

TCB

*I'm using an 'operating' pressure of 40ksi, both since I want to load the Tokarev round hot for this platform (locked breech carbine), and to cover my butt. The proof load is 60ksi, which is kinda low (explaining why proof is hardly ever the design-drive so far), and an insanely high rupture/ultimate design load of 90ksi. The barrel failure mode I'm investigating at present is the contact or bearing stess against the barrel trunnion; a stress figure that is notoriously difficult to nail down numerically --I'm using tensile failure stress numbers for these calculations, simply because I know they are lower than for bearing, but not by how much.

 

[barnbwt]

...In any case I'll be shooting proof loads through it with a string and following the headspace for a bit before putting it up against my face

TCB

 

[briansmithwins]

Friction is force X coefficient of friction, area doesn't enter into it.

http://en.m.wikipedia.org/wiki/Friction

BSW

 

[Berger.Fan222]

I do take some issue with their test methodology, since their testing was not done ballistically; the slugs were pushed through the bore. That means no effects of blow by or obturation at the rear of the bullet (admittedly the latter is probably negligible in FMJ or solid copper ammo, but blow by could have a lubricating effect for all we know). 1000lbs seems awful high, at least for Tokarev; that's about 1/4 the bolt thrust of 7.62x25.

One wonders if there will ever exist a perfect method for measuring bore resistance of a bullet in a barrel. It's a hard problem. Any method one thinks of will likely have some possible criticisms. A question one should ask is how well different methods agree with each other in cases where multiple methods are available.

Further down in the report they recorded standard M80 at 1000N (224lbs ), and moly-coated copper at 2000N. The really high 6000N figures are for uncoated solid copper, and is at least double the figure for every other condition tested. The report did sell me on a 1.5degree forcing cone, though, since it seems it consistently halved the peak swaging force in all cases compared to a 2.5degree cone

Yeah, I caught that too. It seems counter intuitive that the moly-coating increased bore resistance, but it makes sense that the smaller angle forcing cone decreased it.

If 308 NATO FMJ is ~225lbs or so, and my projectile is the same diameter but both shorter and softer in construction (and all other figures are the same since I'm using a take-off 308WIN barrel), and I'm already using some pretty conservative assumptions*, I feel at least 'okay' at this point with my design.

Take care not to apply your assumptions to the .309-.312" diameter bullets that might be loaded. These higher diameter bullets will take much more force to squeeze down a .308" barrel, depending on their construction. A thin jacketed, pure lead bullet will likely be easier to push through than a solid copper bullet, or a thicker jacketed bullet with a harder core. However, if obturation is a real contributor to friction, at higher accelerations, the pure lead bullet may be worse. Once the lead is acting like a liquid, a pressure gradient will develop between the front and back of the bullet, so the greatest normal force (thus highest friction) will be at the back.

Thanks a ton for the link to that report; that was excellent. The report I found earlier was trying to correlate different powder charges to velocity, and using the linear relationship between them to back out the 'constant' of barrel friction (which I found a dubious assumption since pressure affects blow by which likely affects bullet friction)

I can't imagine a friction measurement method that would be perfect (no objections). But if two methods that were greatly different gave comparable results and also produced accurate velocity predictions in models like QuickLOAD (and more realistic modeling efforts), I think one should grudgingly admit they'd at least be in the right ballpark.

 

[barnbwt]

"Friction is force X coefficient of friction, area doesn't enter into it."
Tell that to racecar drivers with those really, really fat tires without treads

I'll bet some strain gauges gauges on a revolver frame & barrel could back out the data more accurately, but are so prone to noise themselves that it might not resolve clearly. Check the difference between the peak recorded bolt thrust and barrel thrust by mounting the breechblock and barrel on separate load cells, and you would at least be able to calculate how friction changes for different bullets/loads, but you'd be assuming case friction remains essentially constant (which it likely does given a set pressure level). I do agree a set figure would be hard to determine, since the device firing the round will also have an effect on the variables you'd be trying to isolate (by affecting recoil, deflection and practically any other figure you'd like to measure)

FWIW, I plan to load the bullets that would be running hotter; my design load is at 40ksi, which is 10-20% higher than most milsurp 7.62x25. My chamber will also have a very roomy neck over a normal Tok chamber, so I can use 223 brass trimmed/necked down without having to neck turn cases to the nominal thickness.

I did find a way to get some better numbers, at least, and I now have two smaller locking surfaces restraining the barrel, which add up to a bit more surface area than I had initially. Again, I was failing in bearing at ultimate load, not tension or shear, so any plastic deformation of metal due to overload would be very unlikely to cause catastrophic failure (at least on the first shot). The worry is more about accelerated wear affecting shooting life (lame)

Yes, my mindset is backwards of what it should be (hopeful rather than skeptical ) but I am not convinced from what I've seen so far that my design is unsafe, and am leaning in the same direction regarding its durability. I think it's simply that my barrel seat will be the weakest part of the design, but still strong enough for even an extreme overpressure situation. My conclusion is "it's worth a shot"

TCB

Internal ballistics really is the most futile form of navel-gazing, isn't it? They found the Higgs-Boson, but they can't seem to hash this out...:banghead: