Massive Headspace???

[Buckskin]

I recently got a "new" $100 toy, a Mosin Nagant from Cabela's.  After reading up on them and slugging it, finding out it was a bit loose in the bore and that I would need to go with .312 ammo. I decided against the spam can of corrosive primer ammo and that I would reload for it.  So I bought a 100 rounds of reloadable PVRI ammo.  To make a long story short, it shot pretty good with that ammo, much better than I was expecting.  The one thing I did notice is that just looking at the fired brass you could tell that it had grown a lot at the shoulders. 
 
So when I was in the process of reloading for this Mosin and was measuring with the Hornady Lock and Load headspace gauge to bump the shoulders back a couple thousands, I was curious how much this brass grew from the factory loads that I shot, so I measured a loaded factory round.  I was shocked at the caliper reading, on average this brass grew 0.029" at the shoulders.  That seems extremely excessive since most of my rifles move about 0.003-5.  Now I've never fire formed brass into other calibers, so maybe this is all hunky-dory. But for it to be the same cailber and move that much seems odd...
 
And yes I did read that you should check the headspace of these rifles prior to firing them.  Instead (which may be stupid) I went by the Cabela's guy who sold it to me, insuring me that all the rifles at least pass the no-go test...

[swifty22]

Do they headspace on the rim?-Muddy

[anweis]

I had the same experience with a MN years ago. I no longer have the rifle. I cleaned the bore really well with the electric bore cleaner and with JB bore paste and Kroil. I used .311 bullets and they worked very well.
I used Lapua brass and i neck sized only, by setting the full sizing die higher up in the press.
Everything is probably fine in your rifle- chambers were made large to ensure function in battle; actually you should neck size only the brass if you want it to last. 
Those rimmed cases headspace on the rim. In fact, rimmed cases are in many ways superior to rimless, and easier to reload with accuracy. If neck sizing only (the best way to go), good brass should last 10-15 reloads with mild charges. If you full size, considering brass expansion in your rifle, brass will probably be ruined in 4-5 reloads.

[max1138]

being  ex mil Im sure the chambers are loose to insure functioning in the worst frozen muddy battlefield conditions without sticking, I dont think the czar was worried about brass life so much.


find some .0025 shim stock to go under your dies and let that be your new headspace reference and enjoy the longer brass life. or just neck size only, but sooner or later the brass will grow and need to be trimmed and re-headspaced with the full length die or at least have the shoulders bumped back a bit.

[shot1]

Your rifle is OK. As has been said typical military large chamber so it works in all conditions. The 7.62X54R head spaces on the rim not the shoulder. I use my FL size die to only part size my cases. You can smoke or color with a marking pin the neck of a case (LUBE IT) with the size die backed quite a ways off the shell holder run the case into the die and remove it. You can see where the neck is being sized. Screw the die down until you see that the neck has been sized and the shoulder just bumped. Your accuracy will be better and case life will be much longer.

[Buckskin]

I just bumped the shoulders back about .002".  I must have gotten lucky, because for some reason it was the easiest shoulder bump set up I've ever done.  I bought a Lee set so I could get the crimp die with it, and hit the .002" mark on the second adjustment.  Sometimes better lucky than good...