I build subsonic loads for lots of different cartridges. Trail Boss is my go to powder. Before Trail boss used all the typical old style ways to build SubS loads. Since Trail Boss, there is no reason to incur the risk of using pistol or shotgun powders to build SubS loads. I have never had a cartridge that I could not get under an inch at 100 yards with jacketed bullets and under 2" with cast. First step, crimp the loads if shooting cast, do not use gas checks, size your lead to .002 over normal in smaller calibres, as much as.005 over for larger, so for .224, size to .226 then try .227. Next major consideration, use flat nose bullets with wide metplat, much better killing effect. Round balls also work great and have the advantage they lose velocity very quickly so much less carry if using on tree squirrels etc. When tuning a load increase or decrease TB powder charge by .1 grains, you will be suprised how little it takes to really shrink groups or open them up.
In the 22 Hornet, I have loaded 900 FPS with 50 grain Honrady Spire point, under an inch at 100, in the 223, 880 FPS using 70 grain Sierra, under 1" at 100, in the 375 Ruger, 950 FPS us .378 260 grain cast, 1.5 inches at 100. All using Trail Boss powder and this is just a few. Have made cast loads even for the 460 Bee using 410 grain cast at 990 FPS. They shot just over an inch at 100.
308 Round Ball Grouse and Squirrel loads, 900 FPS. These round balls are .312 diameter. The loads I tried with .310 round ball, the groups were three times this size. You want the lead to seal the bore and get a real good bite on the rifling for best accuracy.
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Round ball groups at 25 yards. Both are 5 shot groups, one on the left is .1 grains more than the 5 shot group on the right.
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