Base Expansion?

[planeflyer21]

Howdy All!

Looking at the design of a FMJ round, I was wondering if the jacket expands and seals the bore like a Minie round would in a muzzle loader?

Seems like it would.

Jon

[blue today]

The FMJ that I have seen did not. Most are boat tails and does not have a cavity. I have shot some minies that were not accurate. All depends on the gun.

[planeflyer21]

Sorry, Blue, I was specifically referring to the pistol FMJ bullets.

They basically look like a copper cup, with lead squished inside.

What got me wondering is the fact that so many pistol-caliber guns need lead bullets a few thousandths of an inch over bore size, but they shoot jacketed bullets okay and some the FMJ rounds even better.

Just things that make you go hhmmmm....

Jon

[35remington]

Which cartridge?

Some of the lower pressure rounds likely don't have enough pressure to expand the bullet base much, even given that most FMJ's have relatively soft lead cores.

[Gohon]

Lead bullets whether pistol or rifle are usually a couple thousands over bore so they will seal the bore and prevent escaping gasses from vaporizing the lead which would lead the barrel. The lead being soft enough to bump up for the speed driving is another factor. FMJ bullets are not affected in the same manner by hot gas blow by so they are not over sized and they wouldn't bump anyway. I can't even imagine what the pressure would be like for a over sized FMJ or one that expanded inside the bore. Having said that I can't speak for plated bullets but then that would also depend on the hardness or softness of the plated bullet. Since it is a very thin plating I guess there is a possibility that plated bullets may bump up.

[DWB]

Regarding pistol (specifically .45 ACP, 230 grain) bullets, I have two in my desk drawer. They are the same make, fired on the same day, same load, etc. and are both the open base with exposed lead cores. Neither of them expanded to seal the bore like a softer (swaged) lead bullet would. The resulting rifling engraving on both is uniform for the full length of the bearing surface.

Like 35remington said, which cartridge? It would be interesting to see how something with more pressure looks.

[planeflyer21]

Well, I have some fired .45acp's and a few other fired bullets too, from .380, 9mm Luger/P, and .40S&W. There may be a couple of .38s too.

The rifling on the jackets is plenty deep.

All the lead I've found is super squished, so can't really get a good idea of how the rifling on that looks.

Why can copper be right at or a little larger than the bore, but lead works better if forced in like a cork?

Just wondering out loud.

Jon

[35remington]

Lead bullets don't work better if "forced in like a cork" either. A little larger in diameter applies to the throat or chamber dimension, rather than the barrel dimension. If allowed to fit the throat, and the throat is concentric, the bullet will be reduced concentrically as it enters the barrel, and no harm is done, so it shoots well. If the lead bullet does not fit the throat, or is undersized for the barrel, insufficient stabilization may result, and the softer lead bullet may have its center of form displaced from its center of gravity as an undersized dimension may not allow the bullet to get a straight start.....and a straight start is critical for a lead bullet as it's less forgiving.

The jacket is the difference, and slightly undersized jacketed bullets resist gascutting better than lead bullets do. Slightly undersized jacketed bullets still grip the rifling well enough while resisting erosion. Lead bullets do not.

A better statement about lead bullets would be that they should fit the throat if applicable, and in any event should be never smaller than the diameter of the barrel.

"Forcing the bullet in like a cork" is a recipe for disaster with a lead bullet. If it's hard to chamber, then you're forcing the bullet in like a cork. If it chambers easily and is larger than the barrel, you're fitting it correctly. This is simply an opportunity to start the lead bullet straight with the rifling, and isn't "corking it."

[planeflyer21]

Thanks for the correction 35rem!

I find it quite interesting that there is such a big difference between the way copper and lead react to being forced down a barrel.

Anyone got a good book on the subject to recommend?

Jon