Unless there is some issue with twist rate, it will be much easier to make a subsonic load with a 45 LC than a 45-70. Even with 300gr bullets you have to work to keep them subsonic out of a rifle barrel.
It’s actually very easy to make subsonic loads for the .45-70, especially with cast bullets. Eleven grains of Unique (no filler ) under a 300-grain RCBS bullet nets 1079 fps out of my 30” rollingblock. Accuracy is under 4 moa with iron sights at 100 yards. Seventeen grains of BlueDot gives the 400-grain RCBS bullet 1075 fps out of my 16” Contender carbine.
That said, it
is easier with my M94 Trapper in .45LC. Eleven grains of AA-5 sends the 255 Lee RNFP out at 1084 fps. This bullet is a gem in Lee’s bullet line, very accurate with excellent terminal effect on game thanks to its large meplat and a surprising BC of 0.20. The very long bearing surface enhances accuracy, and it feeds well from my M94 too.
Supersonic the .45LC gave me 1570 fps out of the Trapper with a healthy dose of H110 and a Hornady 250 XTP. I think that the OP’s idea has a lot of merit.
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