Mosin M39 B / cartridge OAL question (lengthy)

[_N4Z_]

I have two M39 Mosins, one is a 1942 Sako, and the other is a 1942 B. I acquired the B in unissued condition with a new bore. My Sako has been well used. I have been cutting my reloading teeth on these two rifles. I have had exeptional results from both, but also a good share of not so great results.

I've tried multiple bullet brands and weights, and a couple different powders. All the same brass and primers. Also use a Lee Factory Crimp die with my latest loadings.
The B barrel has gotten the most attention, and as I said earlier it has printed some outstanding groups. But consistancy is lacking. I can get a lot of rounds churned out that all have the same weights and OAL's, go to the range and shoot, and get just head shaking results. First group may print sub 1", and the next may bound up close to 3" or more...:banghead: It is frustrating. I know how to shoot and have been at it a long time, so my variations to the equation should be minimal.

So the other day I picked up a bit about checking out where your COAL should be by individual rifle. Seat a round long in a fired case, and then chamber it in the given firearm. The rifle should push the bullet back further in, and when you extract it.. WAH-LAH! You've got your chamber length. Pull it back .015 or so, make sure it still fits the magazine, and giddy up. Now I have been using manual callouts, and commercial round measurements to set my OALs. Was I in for a surprise.

I checked the Sako first and most of the rounds I use in it could handle an OAL slightly longer then where I had been seating them. Good deal.

Then I checked the B barrel. WOW, does this thing have a huge chamber! Most x54r loadings call for OAL's from 2.8something inches for softpoints, to around 3.01something inches for fmj's and hpbts. After measuring and remeasuring it looks like my B will take softpoint Sierra ProHunters (180gr) out to 3.08 inches. And the MatchKings can seat out past 3.1 inches!

Are there any people here with B barrels, or any other Finn Mosin, that have such large chambers, or do I have an oddball?

 

[NCsmitty]

I don't believe that the chamber is huge per se, rather the leade into the rifling is tapered differently than your other Mosin. This is not a bad thing as it gives more powder space in the case with the bullet seated out. I do not know if your chamber is the norm for your B barrel as I only have two M44 Mosins that I have not checked myself.
Along the same line, I have a custom 98 in 6.5x55 that the books say should be seated at 3.050 for that caliber. The Remington 140gr core-lokts touch the rifling@ 3.240 and I have used 3.200 as my COL to start. They fit with room to spare in the VZ-24 action magazine. So you see, It will vary with brand of bullets and the leade cut and your magazine's length to come up with something that works.
My thoughts are that if you seat your bullets .020 off the rifling, you will see more consistency in your grouping. Whether they fit your mag is another matter. That also means that you may need to keep your ammo separated for each rifle or just use one of them.

NCsmitty

 

[rcmodel]

Before you do anything else, you need to slug the bores on both rifles and measure them to find out what size bullets to use in them.

Some of the Finnish M-N rifles had .3075" to .308" bores.

The Russian ones usually run .312" to .314".

No amount of tinkering with seating depth is going to help much if you are using the wrong size bullets.

rcmodel

 

[_N4Z_]

Before you do anything else, you need to slug the bores on both rifles and measure them to find out what size bullets to use in them.

Checked that awhile back. Don't think that is the issue now.

The B has a .3115 groove dia.
The Sako a .312 groove dia.

B has been getting Sierra and Remington .311 bullets.
Sako gets Hornady .312's, though the .311 Sierra ProHunters do well in it also.