I just picked up a new GP-100 a few weeks ago and I've got a pile of Lyman 358429s cast and ready to size. I slugged the throats .. 0.3585. I have sizer dies in .358 and .359. Bullets are probably about .360 unsized so I can go either diameter.

Which would you pick, 0.358 or 0.359? Is there a fairly clear answer or should I size a couple hundred at each size and do some load testing?

Might try a few unsized first then go with the 359.

^^^^^^^^^ This ^^^^^^^ memtb

lead slug thru the bore, measure it with your calipers, the measurement of bore size add .002" equals the size of bullet for the rifle.

you can go .003" or .004" or .001", but it usually .002" bigger. i did a .311" bore size and added .002" and it was .313". but the rifle ('91 Argie Mauser) did not like it, it put the bullet sideways. i did trail and error and it ended up .315" (.004" over).

I would go .359 if only fitting for a single gun.

.358 would also work with an alloy and load to bump it.

I'd try .359" first.

Thanks. I ran a 300 through the .359 sizer. I figure on 100 for my buddy / ex brother in law to try in his blackhawk. I think the cylinder is long enough to load 'em normally. I seated one to the crimp groove and stuck it in my GP-100 then slid a ruler across the face of the cylinder. I can feel a bump .. it is protruding a tiny amount .. less than the barrel/cylinder gap 'cause it would still go around despite the bullet sticking out of the cylinder. Guess I'll try them both in .38 cases loaded to the crimp groove and .357 cases crimped over the shoulder. Probably should adjust the lube/size rig to pump lube into the crimp groove for that rather than leaving it dry.

I'll second the bet on .359. If the bullets protrude past the cylinder face at all I would shorten the cartridges up a bit. The least little bit of fouling build up or a bullet that works its way out against the crimp a tiny bit, and presto - a locked up gun. Nothing says you can't seat the bullets deeper and crimp in front of the top band. If you're skating with maximum loads I would cut the powder back a skinch to compensate for slightly increased pressure caused by deeper bullet seating. Conversely you could also trim the brass to a length that keeps overall length short enough to keep the noses back inside their holes, if crimping in the groove is important to you.

Or, you can cut to the chase and determine throat diameter and size your bullets accordingly. I'd be willing to bet that said '91 Argentine Mauser has an egregious throat diameter which led to the need for a .315 bullet. Slugging bores and adding X amount to groove diameter is pretty passe among savvy cast bullet guys anymore.

This. The only reason I would go smaller than .359" is if they won't chamber in the cylinder, then go .358"

I’d go with .359 and get the throats reamed. Ruger has a well established history of undersized cylinder throats. I’ve got 6 Rugers and needed to get 2 reamed.

I tried to find a compromise in projo size the first time I ran across it. It leaded either way. After I got the cylinder reamed and was allowed to run the proper sized projo for the bore, everything was right. When I slugged the bore and cylinder on a later purchase and found the cylinder throats undersized, I didn’t even try. It just went off for reaming. No regrets.

Just my 2 cents.