Guns Magazine: To trim or not to trim…that is the question

Not too long ago, I received a letter from a reader who was concerned about his .45 ACP cases. He knew that he was supposed to trim them. But he couldn't find any that were as long as the trim-to length shown in the loading manual.

"What am I doing wrong," he asked?

The answer, of course, is absolutely nothing. When in the throes of terminal boredom -- with positively nothing else to do -- I have measured the length of several .45 ACP cases. I cannot recall a single instance of finding one that was too long. That surely doesn't mean that there isn't one out there. But finding one is probably less likely than hitting blackjack in Vegas. It can happen, but not very often.

Let me tell you why. Dimensions of cartridge cases and chambers are established by the Small Arms and Ammunition Manufacturers Institute for every caliber loaded in the United States. SAAMI is a voluntary organization to which most -- but not all -- of the players who make guns and ammunition belong. Members agree to abide by established industry-wide standards for things such as cartridge and chamber dimensions and the pressures to which ammunition is loaded -- important stuff.

To establish a new standard requires consensus among the members. If a concensus cannot be reached, there is no standard. A great example is the pressure for +P+ ammunition. We know it's Out there and that some SAAMI members load ammo to such levels. But the point at which something becomes +P+ is an individual decision. It is also of questionable value in any real-world context. But since some law enforcement agencies want it, some of the manufacturers will make it for them. Us ordinary folks are far better served by forgetting that it exists.

But SAAMI specifications for ammo dimensions are well-established and agreed upon. However, we are dealing with two separate components here, cartridge-case length and chamber depth. A maximum-length cartridge must fit into a minimum-length chamber. That makes sense.

But, of course, all manufacturing has some tolerance. So we'll see a range. In SAAMI, the length shown is the maximum allowed with a minus tolerance. So for the .45 ACP, the length is 0.898 inch, with a tolerance of -0.010 inch. The chamber is the other way around. Here we have a specified minimum length of 0.898 inch, with a tolerance of +0.004 inch. It is clear that the twain will rarely meet. Manufacturers control their case process to avoid producing brass close to the maximum length. So if we measure a bunch of new cases, we'll find most of them close to -- or sometimes even a little below -- the minimum 0.888-inch length allowed.

On the other side of the street, the guys who cut chambers don't want to flirt with making them too shallow. So they'll probably target something in the middle of the length tolerance. It's clear that it would be rare indeed for a case to ever touch the chamber shoulder in a .45 ACP, or an automatic pistol in general. Some of you have noticed that I've just committed heresy and contradicted doctrine. Everyone knows that most autopistol cartridges headspace on the case mouth... it says so in the book. And so it might, in a perfect world. But in our real one, it ain't that way very often. Actually, for a case mouth to touch the chamber shoulder would require a combination of worst-case dimensions. In most autopistols, headspace is determined by the fit of the extractor. Doubt me? Go inspect your autopistol and think about it.

We know that brass grows longer with repeated loadings. But if we start with a case that needs to be 0.010-inch longer to even think about touching the shoulder, it is unrealistic to expect this to happen often. Why? Because we either lose the brass; or it wears out before it gets that long. Like so many problems in shooting, this one is far more theoretical than real. Some people need something to worry about. This ain't it.

Well, what about revolvers? In the .44 Magnum, for example, we have a maximum case of 1.285 inches, with a tolerance of -0.020 inch. The chamber dimension is 1.303 inches, with a +0.004inch tolerance. The case rim is the determinant here.

The amount a case grows in length is based on a combination of factors, including the dimensions of chamber and die. But the intensity of the load is the greatest factor. Cases that are subjected to maximum pressures all the time are affected most. That is especially true with the magnums. But even then it is rare for a case to get to the point where it needs trimming to go into the chamber. Still, there is sometimes a reason to trim the magnum cases. With hot loads, a uniform and firm crimp is essential for good powder combustion. If the case length varies, so will the crimp. Both accuracy and velocity may suffer. So with magnum revolver cases, it pays to keep brass together in the same lot. Then, if the length does change enough to affect the crimp, the cases can all be trimmed to a uniform length and eliminate the need to continuously adjust the crimp die.

Sometimes we do need to trim handgun cases. But more often than not, we don't.