OK, I
posted this info over on the P64 forum but I figured it was useful stuff so I'm summarizing it here too, just so that it exists in more than one place.
The gist of it is that Brown Bear brand, steel cased, 9x18 Makarov ammo is very slightly over-long in the case length.
It doesn't seem to affect most guns because it is such a small over-length but it does seem to affect P64's with some fair regularity. Here's what happened.
I experienced the dreaded Limp Trigger. I went out shooting, only had time to run two mags through, but had at least one limp trigger with each. I worked the trigger through a few times and it eventually "caught" and fired right but it really bugged me. I took the P64 out of my carry rotation.
I cleaned it well, inspected everything closely but didn't see any issues, and turned the screw on my Marschal grips back a rotation or so just to make sure nothing was dragging. I figured it might have been just that it was dirty in the chamber or there at the trigger disconnect.
Then I took it back out shooting next time I had a chance. I was still experiencing at least one limp trigger every two or three mags. Sometimes I could bump the slide and and it would be fine. Really started to upset me. I like this little gun and I can keep really tight groups with it (for me anyway - I wonder what I could have done with this pistol back when I didn't have to wear reading glasses just to see the front sight?).
So I started digging through the forums again. Then I
found it. I recently started running Brown Bear ammo in it. I've run at least three different brands of ammo through it with no problem, then, when I started running Brown Bear, I just suddenly start having trigger disconnect issues? Riiiight...
My CZ-82 eats it happily but, apparently, it gives my P64 indigestion.
A few days later, I put my calipers on the cases to find out.
Here's what I found. First, the conclusion. Then, the methodology. Simply put, the Brown Bear 95 gr. steel cases that I measured were all over-length. Sometimes only ever-so-slightly, but nonetheless over-length. The specific measurement of 9x18 Makarov case length is supposed to be 18.10mm. The Brown bear were frequently over that.
Methodology:
First I don't have a bullet puller and I didn't want to try to use once fired cases because that would deform them. So I ran the caliper up the edge until it dropped over the mouth then backed it back down. It lacks a wee bit of accuracy but not usually more than 0.01mm.
One important thing I noted was that no case was exactly the same length for the entire circumference. For most cases, there was about 0.01-0.04mm variation.
The brass cased rounds I measured were Sellier & Bellot 95 gr. FMJ, Hornady 95gr. JHP, and Master Cartridge 95gr. JHP.
- Sellier & Bellot had the most variation with cases ranging from as little 17.9mm on the shorter side to as much as 18.1mm on the long side.
- Hornady cases ranged from as little as 17.8mm on the short side to 18.00mm on the long side.
- The Master Cartridge cases tended to be the shortest of all cases I measured, either steel or brass, with as little as 17.8mm on the short side and as much as 17.86mm on the long.
The steel cased rounds I measured were Brown Bear 94gr. FMJ, Wolf 95gr. FMJ, and Wolf 109gr. "Truncated Cone" FMJ.
- Wolf 95gr. FMJ measured as little as 17.88mm on the short side to as much as 18.01mm on the long side.
- Wolf 109 gr. "Truncated Cone" FMJ measured as little as 17.92mm on the short side to as much as 18.01mm on the long side.
- Brown Bear 95 gr. FMJ measured as little as 18.02mm on the short side to as much as 18.17mm on the long side. Nearly every case measured had one side which measured greater than 18.14mm.
I have to say, 6 thousands of a millimeter isn't much to be over-length, but I guess it can (sometimes) be enough if any of a number of circumstances come to pass. Possibilities include very exact tolerances on the chamber length, fouling in the chamber reducing over-all length, heat expansion changing the dimensions of the chamber, or some other possibility I having thought of. It's worth noting, again, that I fired exactly the same lot of Brown Bear ammo, during the same range session, from my CZ-82 and it ate it without any hiccups.
I also suspect that some of the rounds load in with the "short end" of the case aligned so that it isn't enough to engage the disconnector.
I can see how bumping the back of the slide or shaving the trigger disconnect by one tenth of a millimeter would fix this issue.
I have to admit that I'm really surprised that there was so much variance in the case lengths of the other ammo and particularly that the U.S. tended to be slightly under-length. But I guess it's was always within 1/10th mm of the specified 18.10 so it works.
Peace favor your sword,
Kirk
