Here's my story and I'm sticking to it:
My shooting is almost exclusively 100yd bench at my club's misurp bolt action matches. I was drawn to casting for $$ reasons.
Two rifles I'm working on: Finn M-39 (slugs at .300-.310) and Swiss K-31 (slugs .297-.308) I have not chamber/throat cast either rifle. Both barrels are in very good shape as milsurp barrels go, and were well cleaned when I shot my first CB loads. Results were not good.
Mold is a Lee six cavity oversized 155, the C E Harris Design, that drops just under the advertised .315 using WW alloy ingots, AC'd. All Boolits were weight sorted to w/in .5 gr.
I size with either of 2 Lee sizer dies, a .311 die that actually produces a .310 for the Swiss and a .314 die that actually does .312 which I use for the M-39.
I was trying to develope "the load" (+/- 16 gr of 2400) for either/both of those rifles.
I have tried the 45/45/10 tumble lube with 15, 15.5 and 16 gr of 2400, but w/o a gas check.
I don't know why that would be an "oops" necessarily, I shoot lots of things without checks at lower velocities just fine. The tumble lube may or may not prove to be sufficient for that application. I would most definitely invest in a lubrisizer with .309" and .311 dies, either a Lyman or an RCBS is cheapest.
Using the .312 TL's, the Finn leaded up very quickly (@10 rounds) to the point that a lead donut was built up at the muzzle face and lead pushed out with a patch. Not surprisingly, those were all over the paper. The rifle, with surplus, is sub 2.5 MOA capable. The Swiss, however, using the .310's, and same powder loadings, had only very slight leading, BUT quite frankly don't haqve a clue where they were actually going, (it wasn't on the paper....) But that lack of rather important info was my fault for not taking some spotting help along with me .
So, the plan is GC's on the same bullet next go round, still using the 45/45/10 TL. (Yes, I should have started with GC's.) I want to also take some lubed with a different recipe, particularly for the Finn to try at the same time. Its a piece from here out to the range. I'm leaning towards Gene Fryxells 50/50 just because it seems easy enough to do but I'm open for any info that would help ID'ing a good lube recipe for the above.
Here goes my two cents: Make some Felix lube, or buy Carnauba Red from White Label Lubes. The reason here is you're looking to shoot in competition, and those lubes are designed for such, being doped-out to give shot-to-shot consistency and superior accuracy through proper balance of carrier (lube base) and lubricants, which helps reduce flyers as the barrel gets warm. Other lubes will work, but that's what I would try first personally.
Also, when putting gas checks on (Hndy 30 cal.s), about 25% come off the boolit somewhere bewteen the sizer and seating the bullet in the case. Any tips on correcting that problem or is it likely that's caused by inconsistent casting technique?
Without good pics of the boolit bases without checks, it's hard to say, but you either have rounded bases from a sprueplate/mould that is too cold and not filling out, or the gc shank is too small.