I thought the Spanish conversions had 21" barrels, but my memory plays tricks on me now and again.
QuickLOAD thinks you'll have about 31,800 psi with a large case like a Winchester and about 35,700 psi with a tight case like an IMI. This is with mild primers and the bullet seated one caliber into the case neck. With a 20" barrel, it estimates 2325 and 2400 fps for those pressures, respectively. With all the variations in bores and chambers it could be a good bit different. Actual measured velocities would make a better estimate possible.
The same program suggests about 37 grains of IMR 3031 would fill the case just as well or better and would give you maybe just 500 psi more pressures, but, being faster, would burn more cleanly and completely and would get you another 50 fps or so of velocity.
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Last edited by Unclenick; August 16, 2011 at 02:54 PM.