Quote Originally Posted by Ned Christiansen View Post
Update on the high-mileage Colt I first spoke about in post # 167.

This long-term, daily-use Colt got a going-over yesterday ... included a slide change due to the slide being cracked, it has fired ~ 50K rounds. Looks like it to me. For years now we have felt certain the gun has ~ 250K on it. One wants to stay conservative in the absence of carefully-maintained data but in the last five-ten years, tracking has improved and the rate of consumption has gone up. To peg it at this point at 250K is safe and conservative.

Note the “topography” starting to show up on the breech face. After thousands of rounds you start to see an imprint of the case head. The ring around the primer where there is no contact on firing, that is, the area to include whatever chamfer / radius is in the case’s primer pocket and then whatever radius is on the rear face for the primer at the junction of that surface and the outer diameter of the primer, shows less wear. This area is becoming a raised ring that at some point can actually start to present a bump that the up-feeding case has to overcome. On this one it can be seen that the “ring” has a bit of a flat on the bottom of it—that is wear from up-feeding rounds. Eventually that flat becomes a little more radiused, matching the diameter of the case rim. No feeding issues yet on this one but it would get there ere long. It will get stoned flat. Total depth of the topography is probably only about .003-.004.

... breech face pitting or cutting that I believe comes from occasional primer “leakage”, where some of the primer flash manages to get through to the breech face. If I’m right here it’s like a micro particle beam. Primer compounds as I understand it are actually “brisant”. I hear the word “detonate” used often to describe a cartridge going off. A primer may detonate but as reloaders know, or should I say as students of reloading are told, cartridges expel a bullet not be detonating, but by expanding gas from the powder burning progressively. Real explosions happen at 18-25,000 FPS. Gunpowders burn at various rates, we've all seen the burn rate comparison chart but compared to something like det cord, they are slow.

The frame is due for its third or fourth weld job; it was last welded in July of 2010 so it did last many tens of thousands of rounds.I have seen many of these that crack and get welded and then don’t crack again but of course that depends on the volume of usage. The usage meter on this one is pegged at 11. That last weld job might not have been the best ever. I never liked how the welded area went “grainy” after bluing.
Ned, we've never met -- but love ya, man (no homo). There are lots of IPSC gunsmiths out there who have built guns, but few who can document the wear and tear on specific customer weapons. This thread has and continues to be awesome.

Bolt face (slide) pitting around the primer doesn't necessarily mean high pressures, rather the primer pockets from dozens of reload cycles may be loosening enough to start leaking. Here are a couple of photos showing the brisance of different primer mixtures. Magnum and military primers have a larger "Mixing" effect to ensure powder starts to burn at extreme low (sub-arctic) temperatures:

Remington 7.5 Small Rifle Primer:



CCI BR4 (Benchrest) Small Rifle Primer: