Reloading for Mosin And 303

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Bob7.62

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I have load data for Mosin and 303 Brit that say I can use .312 dia 150gr SP bullet but the thing is that I read somewhere else that you sould have your barrel sluged. I have already used .310 dia 122gr FMJ with no problem. I just don't know if I should slug my barrels before using the larger dia bullet. I would like to add that I am new to reloading and I don't know if I am being over cautious.
 
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you should slug your barrel and mic the slug....some mosins had tight bores and some didn't. Shooting a .312 out of a barrel that was not set for it can cause dangerously high pressures that could possibly cause a serious injury and/or damage to your firearm.

Always err on the side of caution and slug your barrel to be sure!

Happy reloading.
 
"...slug your barrel..." On the Lee-Enfield. Lee-Enfield barrels can measure from .311" to .315" and still be considered ok. Larger than .315" the barrel is shot out. Mosins don't usually have this issue.
Hammer a cast .30 calibre bullet or soft lead fishing sinker of a suitable size through the barrel with a brass rod and a plastic mallet then measure the bullet/sinker with a micrometer. That'll give you the barrel's groove diameter. Then use the closest diameter bullet.
Most factory ammo uses .312" bullets. Few regular bullet manufacturers make a larger bullet either. Steve at .303british.com is making 200 grain .313" and .314" jacketed bullets though.
"....310 dia 122gr FMJ..." Those are made for the 7.62 x 39 cartridge. It's not unsafe using them, but both the .303 and 7.62 x 54R used 150 or heavier bullets.
 
The Czech surplus 7.62x54R uses a 0.310" diameter bullet. It is a steel core with a copper plated steel jacket.

Someone I spoke to has loaded these into a 303 and found them to preform "surprisingly" well. (his words)
 
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