7x57 backs out primer

[roysclockgun]

I recently acquired a hunting rifle that began life as a Model 1910 Mexican Mauser. Other than the receiver, the rifle appears as new. I pulled some 175gr RN bullets out to where they were about .005" off the lands and the rifle grouped four rounds inside 1 1/4" @ 100yds., which for my 64 year old eyes, is good. I had mounted the rifle with a 3-9x40 Burris Signature scope. So far, so good.
I had 1000 rounds of 7x57 Chilean military surplus rounds in my gun room, so even knowing that these have corrosive primers, decided to shoot some up. I have no problems cleaning up bolt rifles after shooting corrosive ammo. The Chilean ammo was loaded with 39gr of Ball propellent that looks like H380, but who knows. The bullet is a 135.5gr. FMJ-BT. The OAL was 3.068", which I pulled out to 3.140". On the first round, I observed that the primer had backed out very slightly. I than fired the as issued round that had not been pulled out and the primer still showed signs of backing out, but not as much as with the pulled out bullets. The headstamp is: "F.M.F.-1976-CHILE"
I then loaded three other test rounds using all commercial components and like with the factory 175gr bullets, there was no sign of primer back out.
My conclusion is that that the Chilean ammo is loaded too light and the pressure does not cause the case to swell against the chamber and then retract. The result is that the primer is slightly blown back, but also has not enough change in pressure to retract. The primer ends up just slightly overhanging the base of the case. One can barely feel the overhang.
Does my theory make sense to anyone, or am I all wet? Am I correct in believing that I am not experiencing too high pressure by firing this Chilean ammo?

 

[Brian]

Your "low pressure" theory does makes sense.

You might be interested in this thread, where we discussed protruding primers and what they might betoken.

 

[SOTN]

roysclockgun:

I'd say, you nailed it.

Smitty of the North

 

[Guest]

Only one thing I'd add, and that is the problem sounds like a mild headspace issue, as the cartridge is moving forward enough to allow the primer to back out. If you have a headspace gauge (like the stoney point design) you can check the shoulder length with a caliper, and compare it to specs. I don't think you're in real trouble here, but I would certainly use the bent paperclip trick to check the cases before reloading them.

 

[roysclockgun]

Thanks for the helpful responses.
I now believe that another issue with this custom rifle, is that it was "long chambered", in order to get the max out of 175 gr. RN bullets. The heavier RN bullets are the only ones that I have been able to load and pull out to the point that they show signs of touching the lands. I seat the bullet just enough that it will hold onto the case. I then coat the bullet with a black fiber tip pen and rack it into the chamber. The mark from the lands shows clearly on the inked bullet. I then continue to recoat the bullet with ink, seat it a tad deeper, until I get it .005" off the lands. Working with this long seating has gotten me the best groups in the past, but with this rifle, the best seating turns the rifle into a single shot, because the cartrdiges are then too long to fit into the magazine.
With 162gr SP bullets, I cannot reach the lands at all.
The pulled 175gr RN bullets are producing consistant groups of three rounds inside one inch, measured center to center. The best that I can get with the 162gr SP bullets is one and one half inch groups. Good enough for deer hunting I suppose. The 175 gr. RN, when set to an OAL that will feed from the magazine, produces groups of one and one quarter inch.
One remedy would be to have the bbl. set back to accept shorter OAL and still be just off the lands, but I am not that in love with this rifle. The entire exercise has just been a good learning experience. I have been hand loading rifle cartrdiges for forty years and I am still learning how to do it!

 

[clearwater]

I have a military mauser that loves light bullets (110-120 grain) and fast
powder (IMR3031 and W748). I can't even begin to get even a 175 grain
to the lands. Go figure.

 

[Guest]

I wouldn't worry so much about the freebore in your rifle, which is the correct term for a long throat area, allowing for longer bullets. Most Weatherby's are long freebore, and for that matter, my Browning BAR is such that I cannot seat out near the lands and still get the rounds into the magazine, but it still shoots close to one inch with factory, and I am starting loadwork on it to determine if I can get down within .75", which is my goal.

What is likely the case is that your gun does not like that particular bullet (the 162's that aren't shooting well) or doesn't like it in front of the powder that you're using with it. Try some other combinations, maybe something in a nice 140gr (which would be about as light as one would go with that gun, I imagine) and see what happens. There's always heavy construction (Partitions, A-Frame, TSX) to make up for a few grains in weight, if you think you really need it.