Aaron Little is building me a 450-400 3 1/4 BPE on a petite toplever hammer gun with peninsular locks. I need to know the twist rate for 250-270 grain led projectiles out of a .410 bore. Normally I shoot 420 grain lead out of my BPCR rifles with 1 in 16 inch twist but this is for IMR 4198 and 250-270 grain projectiles. Thanking you in advance...

I am sure it was very slow, but I would use one in 16 .

I agree, dpcd. Wish I had measured my last one. Pac Nor only makes them in 1:16 so that's what I am going with.