In general you want your cast bullet to be .001 to .002 over groove dia. many will size the bullet to fit the throat or as large as the chamber neck will take with the given brass/steel case to be used. Brass or steel cases can vary so finding a happy medium can sometimes be tricky. I have Prvi and Win. 54r brass which is S & B brass that works well with the .314 dia. bullets how ever if I use Bulgarian surplus brass which is thicker in the neck area,the loaded rd. chambers hard unless I turn them down a couple thousands.
Over sized lead bullet with an equal load generate less pressure and therefor can be squeezed down much easier as they enters the leads,for jacketed bullets I like to keep those around .001 to .002" under groove to groove dia. In my SKS and Mosin both which have a .300 bore and are .312" groove to groove dia. I size my gas check 170 gr. bullets to .314 they chamber fine in both rifles but anything larger give occasional chambering problems, .310" dia. jacketed bullets shoot the most accurately in the SKS while .311" dia. J bullets shoot the most accurately in the Mosin. .311's in the SKS and .310's in the Mosin shoot well but not as accurately,go figure.
As far as using the surplus ammo I've Mexican Matched lots of surplus FMJ ammo with SP bullet to make hunting ammo. Often times I have found that reducing the base powder charge by two grains and working the load back up gives better accuracy as well as reduces pressure as noted by the primers below.
Original surplus ammo
After equal or lesser weight .311 jackated bullet replacement and charge reduction.
If you choose to use the original surplus powder I would not go below 80% of the original charge weight with cast bullet loads and regardless of the gas check the bullet has to fit the bore,just having a gas check bullet want save you. Gas checks allow you to shoot a cast lead bullet faster than you normally could with a given load at a point where both pressure and velocity can overcome the alloys hardness,they also help seal the bore agents any rifling induced defects on the base of the bullet that those high pressure gases will take full advantage of which in turn causes gas blow by,burns off the lube and leads your bore.
Also if you plan on using the original surplus powder with a reduce load you may or may not get some vertical stringing due to the powder charge not being at it's optimal charge density/case fill capacity. You can take up the extra space with a small tuff of Dacron/Poly fill,here is a more in depth article of it's use
http://castboolits.gunloads.com/showthread.php?t=109280
I personally only use the surplus powder when Mexican Matching surplus FMJ ammo with SP bullets or when I'm reworking the FMJ loads to shoot more accurately generally about a 3% to 5% reduction is the range I work in with those loads. For cast bullet loads in the 54r regardless of if I'm using the berdan primed surplus case or commercial brass I use powders like Bullseye for plinker loads or Alliant 2400 or Alliant Red Dot for loads in the 1600 to 2000 fps. range with 170 gr. gas check bullets. Here are a couple links to some cast loads Ed Harris has worked up using 2400 and Red Dot.
http://members.shaw.ca/cronhelm/TheLoad.html
http://castboolits.gunloads.com/showthread.php?t=13425
A couple of other notes if you plan on using cast bullets,you will need to flare the case mouths so as not to scrap lead off your cast bullet and make seating/loading easier,it's the same principle as loading pistol rds. I use the Lee Universal expander die for all my cast rifle bullet. You also need to expand the case neck somewhat larger than you normally would with J bullets so that you don't downsize you cast bullet when seating it. This can be accomplished by using an appropriate size expander ball or rod depending on your die set or using the correct size Lyman M die.
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), and would like to use it with a cast bullet - which will require removing some of the powder to bring the bullet down into safe speeds for the lead. Seems wasteful to dump the milsurp powder if it can be re-measured and used. Now, I'm not so stubborn that I wouldn't go buy some replacement powder, but it would be great to use the milsurp components with a cast bullet. Any suggestions?
