Mosin Nagant inaccuracy and chamber dimension differences


PDA






hang fire
June 14, 2012, 02:49 PM
I was given a box of hand loads done in new Norma brass for 7.62x54, of which five had been fired. The guy told me his Soviet MN 91/30 was so inaccurate with hand loads he had gotten rid of it.

As I don’t shoot other people’s reloads, I pulled the bullets. The J-word’s miked at .308”, the inside diameter of the fired case necks was .314, so small wonder his MN was inaccurate with the hand loads.

I decided to use the five fired cartridges for a little dry fire practice, but that didn’t pan out. The chamber difference between the rifle they were fired in and my two MN was so great, that no way could they be chambered. (On my M39 Finn, the groove diameter slugged at .311”, on the Tula carbine at .3145”)

To double check, I neck sized a couple of the fired cases, but result was the same, no go. So set up side by side on a flat metal plate and even to the naked eye, the fired case’s shoulders were seen to be stretched much more forward than on unfired cases and some empties fired in my two MN.

If you enjoyed reading about "Mosin Nagant inaccuracy and chamber dimension differences" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
docsleepy
June 14, 2012, 08:10 PM
So, what if he neck sized only, so his reloaded cases would still chamber, and then loaded .312 bullets. Would he likely have gotten better accuracy, assuming a slugging of his barrel confirmed. .312 as safe?

hang fire
June 14, 2012, 08:45 PM
It was new brass, so ammo was hand loaded, not reloaded.

With the fired case neck inside diameters of .314" the throat was even larger, for brass has some spring back, .312" diameter bullets would have helped accuracy some. But as too how much?

Russian made MN rifles are notorious for often having oversized chambers and bores. So long as cartridges can still be chambered with oversized cast boolits, they often prove to be accurate in such rifles.

In my Tula MN carbine which has grooves slugging at .3145", I shoot .316" diameter cast boolits that are very accurate. To achieve this, I size down 8mm (.324") cast boolits to get the .316" diameter.

But with jacketed bullets limited to just a few diameters for a given cartridge, hand and reloaders of them do not have the luxury of getting most correct fit.

Cosmoline
June 14, 2012, 08:55 PM
I've had similar issues of cases fire-formed in one Mosin being totally unable to chamber in another. Full length resizing is needed.

hang fire
June 14, 2012, 09:04 PM
Think I will just chuck the five fired ones, little leery of reworking brass cases stretched that much.

docsleepy
June 15, 2012, 12:31 PM
I don't think that would bother me. I make 7mm08 cases bu taking fired 308 range brass and full length sizing in a 7mm08 die-- that is much more sizing If you want you can anneal the neck then they should be fine

hang fire
June 15, 2012, 02:02 PM
This brass in shoulder expansion area shows frosted crackling effect, looks like stressed metal just before it cracks or breaks.

If you enjoyed reading about "Mosin Nagant inaccuracy and chamber dimension differences" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!