Crimp or not to crimp in a S/S with the 45/70 Govt...
First off, RC is right. With proper and consistent neck tension you don't have to.
But with a non-bottleneck slightly tapered case like the 45/70 that I may have belled the mouth on a dash to seat a flat based bullet I would kiss it with a medium crimp.
Why? Because I've read a couple of articles mentioning more consistent load performance with a crimp using middling burn rate powders in the caliber. The supposition was that the extra slight delay of bullet release allowed for more full ignition of the charge before the bullet began it's trip down the barrel and the volume started increasing leading to a more consistent burn and performance from the powder.
I tried it out, playing with my crimp type with my standard cast loads and Rx7 and jacketed with H4198. Didn't try uncrimped as I use the caliber in lever actions and tube mags. My check out showed a light crimp had a lower avg. velocity and greater (~15%) standard deviation than those identical except for a medium or heavy crimp (which were nearly identical). So I use a medium crimp for my 45/70 rounds, I'll take all the consistency I can get.
atvalaska: Yeah, I meant to start playing with your powder charge and nail that thing down.