[Unclenick]

Losacco,

The potential pressure danger Jim243 warned about is real. You really do need to keep the case mouth from getting into the throat of the barrel and interfering with bullet release. But that is the only absolute rule at work here, and there is more than one way to skin that cat.

One is simply to make the cartridges as John Browning intended. That means a crimp may be applied to the case mouth only to the point the outside diameter of the mouth is no smaller than the minimum dimension specification for the loaded cartridge. With .45 ACP this is 0.467" at the mouth, and with 9 mm it is 0.373" at the mouth.

You won't be able to taper crimp a jacketed bullet much without excessive pressure, and you don't need to, as copper gilding metal and brass have plenty of friction between them to keep the bullet in place. But with lead bullets, because of their softness and lubrication, they can sometimes be set back in the case during feeding, which can also create dangerous pressure. Putting a slight crimp in the lead (we're just talking thousandths here) creates a very small step that can help prevent that setback. With jacketed bullets I can identify no advantage to a crimp beyond removing the expander flare, but with lead I think that small bite has a small safety benefit. This is especially true using cases that have been reloaded many times and that have begun to become springy and no longer resize down as much in diameter to grip the bullet as well as they once did.

Many modern reloaders will be dismayed to hear that most all the old time top bull's eye match shooters used to roll crimp a good bite into .45 ACP lead bullets. They claimed it gave them best accuracy because the resulting higher start pressure makes the powder ignite and burn more consistently. You can go look up pictures of their loaded rounds in old publications and see it clearly, and where they've been interviewed on their loading technique they describe doing just that. The trick is doing it without letting the case mouth get into the throat of the barrel where Jim's pressure warnings apply, and at least two things can assure that: headspacing on the extractor hook or headspacing on the bullet.

Mr. Browning's original design intended rimless pistol cartridges to be headspaced (stopped from going further forward into the chamber¹) by the mouth of the case stopping against the shoulder in the chamber where the end if the case is supposed to be (the ridge Jim243 described). Often, this does not actually happen, though. Often chamber tolerances are loose enough that the case is actually stopped from going further forward by the rim contacting the extractor hook before the case mouth reaches that shoulder. This is called headspacing on the extractor hook. I once saw an estimate that up to 70% of new 1911's do this. I don't know if that's accurate, but it is the most common source of breakage of cheap or improperly heat treated extractors. If you have a gun that is doing this (and it has a good quality extractor), then no amount of crimping you do to the cartridge will put you in danger of the case mouth going into the throat.

Checking for headspacing on the extractor can be done several ways. One way is to drop a case that is fully resized and maximum length (0.898" for .45 Auto, and 0.754" for 9 mm Luger) into your barrel and measure how much distance is left to the back end of the barrel (if your case is shorter than maximum, just subtract the difference from your measurement; we want the measurement to cover worst case, and doing that will get it there). Next, add the minimum rim thickness spec for your cartridge to that result (0.039" for .45 Auto, and 0.040" for 9 mm). Write the final number down. Next, measure the distance from the breech face of your slide to the forward inside edge of the extractor hook. If the result is smaller than the first number you calculated, your gun is headspacing on the extractor hook.

That is a good thing to know about your gun, but because of the potential for extractor breakage, it's not the thing I depend on. Also, though it is functional with jacketed bullets and a good grade extractor, headspacing on the extractor interferes with accuracy with lead bullets because it lets them fire at a slight angle so the chamber shoulder scrapes lead off them and unbalances them. So I depend on headspacing lead loads on the bullet, which keeps the case head near enough to the breech face to prevent the leading edge of the rim from reaching the extractor hook before the bullet is already properly started into the throat (no angle or scraping).

Below is an illustration of bullets in different positions in a barrel that is being used as the gauge. Drop an empty case into the chamber of your barrel and note the distance to the back of the barrel that is left. That's where that case would be if it were headspacing on the case mouth and if the extractor hook didn't stop it first. Adjust your bullet seating depth out until the bullet stops it first. Bit if you simply seat the bullet out far enough that when you drop a loaded round in the barrel the case head is flush with the back of the barrel, as shown in the third position from the left, you will be headspcing on the bullet or at least getting the bullet properly started into the throat.

Headspacing on lead bullets maximizes accuracy as well as reducing leading by stopping shaving on the chamber shoulder. I've had several people on this board and others verify this result. In my own wad guns with light loads, the improvement in group size has been as much as 40%, so it's not a trivial improvement. The only hitch is if your chamber is so long the bullets seated this way won't fit in your magazines or won't feed properly. It normally takes a pretty long chamber to make that happen, but you should test. The standard 200 grain semi-wadcutter designs don't normally cause a problem with it.




¹This is not a correct definition of headspace, but is the practical purpose it serves. Back when the head of a cartridge was defined by its rim (still true in modern rimfire cartridges) and most cases were straight and many bullets were heeled, the amount of room in the recess at the back end of the chamber for the rim to fit into was the only thing that stopped the cartridge going too far into the chamber. Since the rim was the head, this rim room came to be called headspace. When later rimless and belted cartridges came along which were limited in their forward entry into the chamber by their shoulder, belt, or case mouth instead of some part of the case head, the old terminology nonetheless was carried over. Thus we now declare headspace by what the case stops against. The case is said to headspace on the rim, belt, shoulder, or mouth, depending on the design.