M57 bore diameter, .312?

[lazarus]

So before I thought my M57 was .312 bore diameter (32). I just want to check. I think my morning google fu is failing me. I am just having some second thoughts loading up 32 FMJs. Last range trip I loaded up some .313 diameter coated lead in both 78 and 100gr and they worked great. Accuracy was much improved and closer to point of aim, vs commerical Tok ammo that is generally a pie plate low at 12yds. I am doubtlessly over thinking it, pretty sure I don't want my M57 to go boom with the harder jacketing on an FMJ if I am wrong (but would it even go boom if I was off?).

Anyway, I just slugged the bore again using copper plated berry's 71gr .313 diameter bullets that are pretty soft, as well as 100gr .313 coated lead. I do have some mild concerns as it seems the Berrys tore petals off the bullet engaging the lands then again soft lead with a copper plate. Not sure if that is normal (I shoved it from crown to chamber with the bullet reversed, so that might have contributed to it).

Anyway, I am seeing about .302" diameter at the lands and about .313 diameter at the grooves. That feels like a 32 to me and not a .308. I also rammed a .309 coated bullet (Xtreme 110gr for a M1 carbine) and an FMJ 147gr down the bore. The Carbine bullet came out also at basically .302" at the lands and .313" at the grooves (went in at .309). The 147gr .308 came out at .302 at the lands but only around .309 at the grooves. I am guessing the hard jacket there kept it from swaging out to the diameter of the grooves from the force of pushing it in to the lands. From what I can find is a .308 should be .308 at the grooves and .300 at the lands. And a few precision barrel makers, make them at .306 at the lands and .298 at the grooves.

Thoughts? Seems like a "32" barrel. Seems like yes, I should properly be using .312/.313 bullets in it. Also, even if it really was a .308 barrel, swagging the bullet 4 thousandths of an inch as it travels into the barrel isn't going to do anything worse than increase the pressure a bit, not something that is scary bad (but also, seems to be a 32 barrel)

This feels like a 32 barrel (well, really a .313 groove diameter). And it feels like it is safe to shoot .312" FMJ and .313" coated/plated lead down it. From what I can find on 32s, they run a VERY wide gamut as small as .307 up to .315".

Also, I am thinking and wondering if that is why I am seeing the accuracy gains I have with reloading. If this is a .313 groove diameter, and a .308 diameter bullet is going in, it is getting engagement since the lands are .302, but it isn't engaging nearly as well with an FMJ, as it isn't fully obturating the bore and the bullet is staying at .308 at the lands, allowing some gas blow by and maybe not stabilizing the bullet as well. Possibly also de-stabilizing some if the bullet is flying a lot of blow-by? No idea about that later if that would impact anything. I did see with loading 110gr .309s before I had better accuracy, and slugging with one looks like it swages from the lands to fully occupy the grooves (well, more or less) pretty much obturating the bore, or at least obturating it better. Aiding accuracy/stability? And lastly on the .313" diameter coated lead I've shot, I am assuming there, its full obturating the bore and actually swaging down at the lands and elongating the bullet that fraction of an inch (the lead has to go somewhere that is being displaced by the lands).

I appreciate any feedback anyone can give. My confidence level is actually pretty high that I am "doing things right". But things going boom close to my hand makes me nervous...

 

[GuitarmanNick]

I have not slugged my M-57 barrels, which is an oversight on my part. Been loading 110 gr. FMJ in .308 and getting decent results.

Great information, sir. Watching!

 

[lazarus]

Well, I guess I'll find out of my M57 blows up when I test my 100gr FMJ RNFP .312" loads in the next week or two.

I am mildly confident enough to order another 2k (for now. If this tests out, I'll probably order another 2-3k).

A tangent now.

I also loaded up a ladder load of 7.65 browning. I finally learned/figured something out about 32/7.65 and my Cz50 specifically.

32 and 7.65 is really not the same. The difference is in the extractor groove. I'd read about it a number of times, but I finally stumbled across this picture and THEN it hit me. I kept thinking a rim difference and I kept looking at and measuring rims on my 32/7.65 and I could just not find any difference even though it was RIGHT in front of me.

Anyway, I haven't seen an extraction difference between 32 and 7.65 (or really US/Asian vs European cases, though the several head stamps I've looked at, all 32acp marked has had an extractor grove like the Remington case in the picture shows, all 7.65 browning has a grove like the Geco does). What I have noticed is a difference in rimlock and most importantly for my Cz50, is how the rounds stack in the magazine.

7.65 shows a bit more propensity for rimlock if not careful. Not sure why, but I've managed rimlock in a Cz50 magazine once. I've done it twice in my Yugo model 70 magazines (only two 32s I own). With 32s. With 7.65, testing dummies in both magazines I managed to get rimlock once in my Cz50 and twice in my model 70 magazines in about half an hour of playing with them. Load the magazines carefully and fine. Don't and the top round skips over the rim of the round below it and lock.

Anyway, the 7.65 stacks flatter than 32 does. It is especially noticeable in my Cz50 magazines. I've been having issues with reloads in my Cz50 with the top two rounds slamming into the feed ramp. I've improved that by reprofiling the feed ramp a little and polishing it mirror smooth. Its running about 100% with FMJ 71gr RN. But my 100gr FPRN it isn't quite there for the top two rounds and anything lead/coated it jams up on the top round or two as the nose deforms with 32s. Which is a shame because I have a few hundreds 32 star cases...

But I also have a BUNCH of GFI/Geco 7.65 browning. About 500 rounds commercial loaded still and around 200-300 cases. Plus another maybe 300-400 7.65 cases from other manufacturers (and about 100 rounds of Fiocchi commercial loads still). No commercial 32acp loaded ammo, but about 400 star 32 cases and another 300-400 32 range brass.

Anyway, I can work with the 32 cases, I'll just have to load the 71gr FMJ, which I fortunately have around 1000 bullets of. I've got around 800 plated that'll need to go in to the 7.65 cases and I've got a bunch and more coming of the 100gr RNFP FMJ that seems like it could go in either case, but prefers 7.65 cases.

Oh, but my last "issue" is because of the shape of the 100s, they have to be loaded very short unfortunately. Which is NOT leaving much powder room. At longer than .967" they'll rub in my Cz50 magazines and hang up if loaded to more than 5 rounds. At .965" they run perfectly in the magazines themselves. Except my Yugo Model 70 has a short throat. I've found that with 71s already needing to load around .970", where as my Cz50 can take rounds at .985" just fine with coated lead and even longer with FMJ (Model 70 I think will run to around .980 with RN FMJ). The FP with the fat ogive has to be loaded to .955" or less or the bullet runs in to the lands.

Ugh.

Anyway, near as I can tell, the powder is not compacted, but it isn't a lot of volume, even at 1.7gr max load I am testing. I am also testing 1.6gr. When I loaded them at .970" (so I had to only fire 4-5 rounds in a mag at a time to test), I was seeing around 540fps with 1.5gr and 590fps with 1.6gr.

I've found some claims on the interwebz that guys have down flat point lead 100gr at 1.6-1.7gr of Bulleseye which is what I am running. Except of course FMJ and just a fraction shorter COAL. At least steel frame autos, so if it is running a little over pressure it should be okay. I am hoping the 1.7gr works okay and can get me up around 650fps with the 100s. Not looking for screamers, just hoping to get good function and not too much wear. But I'd also like it if I can hit the target at 25yds without having to aim over the target to hit it.

Back on the M57, I am testing the 100s loaded on the longer side (I don't have my COAL in front of me), but testing at 4.8, 5 and 5.2gr of Bullseye. Yes, I know 5.2gr is max for an 85gr lead. Testing with 5.2gr on coated load had an ejection pattern similar to commercial loads as well as primer swipes that looked similar to commercial loads and no issues with flattened primers or any over pressure issues visible. 4.8 or 5gr is likely where I'll end up with bullseye.

After that, it'll be figure out a CFE Pistol load. I've yet to test it in 7.62x25 due to lack of published load data, but I did run across someone who published a VERY detailed youtube video of their load development using 90gr bullets. So it gives me a good idea on a comfortable charge to start with and a do not exceed for 100s. IIRC they started at 5gr and worked up to something like 7.6 (7.8?) with 90gr Hornady XTP before they stopped. I figure 100s 5gr is also an okay starting load (they didn't get their M57 to cycle until closer to 6gr) and a do not exceed would probably be 7gr with the heavier bullet. I don't need to try to push these as fast as possible, but I'd LIKE to try to get a 100gr pushed to around 1400fps if I can if it still seems safe (Bullseye I figure I am probably only going to get around 1250fps as it is "too fast" with the bottleneck cartridge to get it going super-fast and still keep the pressure in check. But i also have a lot of BE and a little goes a long way, which is also a perk)