gas blow by

[mainiac]

just watched a show called "time warp" on discovery channel. They was shooting body armor with pistols and rifles. When they shot the rifle (they called it a .306!!!!!) they showed it in very slow motion, and there is a large amount of gas that excapes the muzzle before the bullet. This is with a jacketed bullet. Just how are you supposed to SEAL the bore with a cast boolit, if a copper jacket bullet @ 52000 p.s.i. cant do it?

[Russel Nash]

Methinks if you did have a perfect seal with any bullet you're almost there to turning your gun into a bomb.

with rifling thrown into the mix, you're never gonna get a perfect seal.

In my opinion.

[runfiverun]

that is the why we oversize them.
and gas check them
and make them softer
and harder
and mix alloys
and water drop them
and cook them twice.
is to get as much of a perfect bore seal as possible.

[clodhopper]

I have seen the same thing in slow motion. Wondered if that could be very excited air leaving the barrel pushed by the bullet.

[bobk]

That was my thought.
Bob K

[trk]

It seems to me that the FIT of the bullet to the throat is one of the critical things to accuracy and to not leading - obduration.

[felix]

Obdurate = inflexible (personality) .... Obturate = to block (an opening) ... felix

[DLCTEX]

+1 0n air, and oils, lubes, being pushed ahead of the bullet. One of the stores I do work in has a plastic curtain hung in a large back room to divide it. It's really amazing to see how much air you move as you walk alongside it. That bullet is moving air very forcefully.

[mainiac]

I was hoping that i could get a thought process going here, about this subject! Could you block it off completly,without blowing up the gun? Maybe this gas cutting the base of boolits is just theory, maybe its got nothing to do with heat,and just incredible aair pressure leaking around the boolit? Ive recovered boolits that had exact grain shapes of 3031, and 4064, on there bases. Now surely the sticks of powder closest to the base didnt burn, therefore the base aint getting exposed to heat!

[missionary5155]

Good morning and MERRY CHRISTMAS
John Browning used that escaping air pushed by a fired boolit to construct an automaitc self feeding winchester rifle. It fired a whole tube of pistol caliber ammo(44-40 ?) in about 2-3 seconds. I read the article some years ago in American Rifleman and may have that article in a file somewhere.. But that was the basic design for the Colt Auto Machine Gun ( potato digger) .

[Larry Gibson]

Most bullets that fit correctly can seal the bore. If the bullet does not seal the bore then obviously there will be some blow by. However there is a llarge volume of air in front of the bullet in the barrel that must be pushed out by the bullet. It gets compressed and obviously exits at the same velocity that the bullet does. The air in front of the bullet being pushed out of the barrel in front of a bullet is called the "precurser wave".

Larry Gibson

[runfiverun]

i use a water baloon over my bbl when hunting, it just blows off no big deal, i have put electric tape over the bbl before too,it gets pushed off also.

[trk]

Obdurate = inflexible (personality) .... Obturate = to block (an opening) ... felix

Dang! I KNEW that too!

Thanks Felix, I stand corrected. It's good to keep things like spelling checked - especially when it makes such an obvious difference.


Three kinds of people in the world - those that can count and those that can't.

[beagle]

There will always be some gas escape by the bullet base before it obturates. This is where we get leading.

I think this is one reason the old folks seated the bullet and then inserted the cartridge with powder back in the Pope days. That also insured that you had the bullet started in the rifling and there was no mistake that it disn't tale the rifling straight.

I've see slow motion IR photos of the muzzle at firing and it's definitely ignited powder gas and not displaced air/residue from the bore./beagle

[Ricochet]

That was mostly muzzle blast operating Browning's muzzle sleeve or flap.

A lot of the smoke exiting the muzzle ahead of the bullet got past while the bullet was passing through the chamer throat, befor entering the rifling. It doesn't make a perfect seal, in any case.

Even if I could get a perfect seal, I doubt I could afford the fish for it.

[MtGun44]

Many rifles (most?) have bores that are oversized for the size of
jacketed bullets used. This works fine in most cases, if the oversize
(in groove diam) is only .001 or .002". I think the manufacturers do
this intentionally, so that the pressures will be lower when they drift
a bit on manufacturing tolerances, rather than too high. You were
probably seeing some of the gases leaking past in the grooves with
a .001" smaller bullet. I don't think most jacketed bullets obturate
very much.

This is why we frequently have to shoot .309, .310 or .311 diameter cast
boolits in ".308 caliber" rifles like .30-06, .308 Win, .30-30, etc.

Bill

[Buckshot]

..............I believe the gas you see expelled before the bullet is called "Early Gas". As Ricochet mentioned, it's gas that has escaped around the bullet. This happens from the moment of ignition until it is fully engraved and has sealed the barrel, IF in fact it is large enough to do so. An example of one NOT so is the M31 Hungarian 8x56R cartridge, used in the 1895 model Austrian Straight pulls.

The jacketed bullet in this round is .329" in OD while the grooves commonly encountered are up to .333" or maybe even a thousandth more, but .332 & .333" seem common. Add to that the fact that (at least my 2 examples) these rifles will handle a .338" bullet in the throat. A few thousandths of an inch isn't much room, unless you're a gas molecule with a couple thousand pounds psi of enthusiasm to work with, then it becomes a veritable freeway.

................Buckshot

[shotman]

Lets look at it another way. On a one cock pellet rifle you compress about 8in x 1/2 dia of air. That released will drive a pellet at 1000+ fps. You take a bullet that is compressing 3/8 in by 22in +/- at 2700fps . that is going to be a good size muzzle blast before the bullet leaves.

[S.R.Custom]

..............I believe the gas you see expelled before the bullet is called "Early Gas". As Ricochet mentioned, it's gas that has escaped around the bullet. This happens from the moment of ignition until it is fully engraved and has sealed the barrel, IF in fact it is large enough to do so...


What he (they) said.

[Ricochet]

Lets look at it another way. On a one cock pellet rifle you compress about 8in x 1/2 dia of air. That released will drive a pellet at 1000+ fps. You take a bullet that is compressing 3/8 in by 22in +/- at 2700fps . that is going to be a good size muzzle blast before the bullet leaves.
Very true. And the muzzle blast coming after the bullet leaves will be quite a bit larger.

[Larry Gibson]

"early gas" is called the precurser wave in scientific ballistic jargon. "Early gas" sounds like that which wakes the wife up and brings tears to the dogs eyes.

Larry Gibson

[longbow]

I always say "It was the dog!"

[Echo]

And when one compresses air, then releases it, the moisture condenses, so it MAY be steam that we see coming out before the projectile.

Or it may be phlogiston...

And we don't dog-sit anymore, so I don't have that excuse any more - dang...