Well after putting a little over 700 rounds through my ARMI Spencer I have finally found a truly accurate load. All load testing was done at 50 yards because of the fairly crude sites on the carbine. I have used 4 different bullet molds in my quest. I used the Accurate 51-320S (with some changes), the RCBS 50-350FN, the Lyman 515139, and the Lee 518-360. The RCBS is the only single cavity mold in the group, all the others are two cavity molds. Since my carbine has a .516 groove diameter, I started there and ended up using .518 sized bullets to get targets with out flyers. Anything smaller than .518 resulted in flyers. I tried the traditional 1-30 alloy without success and ended up using an alloy of hardball mixed 3-1 with Lyman #2. I believe the fast twist of the barrel has something to do with the need for a bigger and harder bullet. I tried both smokeless and black powders, eventually focused on a black powder load. The powders I tried were IMR 4227, GOEX Olde Enysford 1/12F and 3F, plus Schuetzen 2F. I used Fed 210M primers and Starline brass. RCBS 56-50 reloading dies and #31 shell holder were used. The load is 38.0 grains GOEX Olde Enysford 3F, .030 vegetable wad, Accurate 51-320S bullet, Fed 210M primer and Starline case. Load OAL is 1.545. I compress the powder using the RCBS neck expanding die spaced out using the 38/357 die spacer. Here's a picture of the bullets used and a loaded case.
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Here is a picture of the load fired at 50 yards off the bench.
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I had accurate modify the 51-320S bullet as follows: I increased the upper driving band to .110 in length and had them make the as cast diameter of .519 instead of .515.
Bill