[Unclenick]

jal5,

The trimming is just about getting uniform crimp, precisely to avoid the situation you describe where a short case makes a light crimp. This is something that only matters with roll crimps in revolver loads that either have enough recoil to back bullets out until they protrude from the cylinder and jam the gun, or with magnum loads of the hard-to-ignite slower powders, and H110/296 powder, in particular gives irregular performance if you ignore this.

For target loads, you really don't need to worry about it. I often just use a taper crimp on wadcutter loads in my .38 and .357 and .44 revolvers. There isn't enough recoil to back the bullets out, and the taper crimp gives longer case life where you can use it.


Islander,

You're correct that a minimum COL is sometimes given for a specific bullet. That doesn't change the SAAMI COL which is a maximum. That minimum that is specific to the bullet still needs to fall within the SAAMI maximum if you are going to insure the round will chamber in all guns and fit all magazines designed for it.

I think the best way to clarify the matter would be for the manuals to specify a maximum seating depth for all bullets of a given weight and construction (i.e., lead or jacketed) rather than separate COLs for each bullet. Seating depth determines how much volume the powder starts burning in, and that is the major factor in determining peak pressure. Seating depth is easily figured. Sum the case and bullet lengths and subtract the COL of a published good load, and you've got it. You can then take any other same-construction, same-weight bullet and give it that same seating depth with that same load, regardless of the resulting COL (except where the resulting COL would jam you into the rifling, which raises pressure). My old Hornady Second Edition loading manual used seating depth rather than COL and I wish they would return to it or add it in, because it is more useful information in many cases.


Teddy,

Trimming case length for case stretch is different from uniforming case length for uniform roll crimp tension. Almost no cartridge fired below about 30,000 psi will stretch. It doesn't see enough pressure to stick the brass to the chamber wall, which is required for stretching the head back to the breech and forming a pressure ring. At .45 ACP and .38 Special pressures it just doesn't happen. Indeed, those cases often shorten with each load cycle because the pressure backs them up like a piston in the chamber, then seals the chamber walls by expanding them which shortens and fattens them. Resizing tends to squirt the length back forward, but it never quite returns all the way to what it was before the last loading cycle. Instead, a tiny portion of the brass gets massaged back toward the head during sizing and stays there.

I once tracked a batch of a 1000 Winchester cases I used with light Bullseye loads in 2700 matches through 50 reloading cycles, applying the calipers to a dozen of the them every 10 reloadings or so. They actually shortened an average of half a thousandth per load cycle, and were 0.025" short by the time I retired the ones that hadn't been sacrificed to the range gods. The nice thing about taper crimping those was that the taper is gradual enough to deal with mixed brass lengths and keep functioning after all that.