Good day. I'm a newbie and currently load and shoot a ShilohSharps .45-70. I never could find a taper Crimp Die, but the other day I got a LEE Factory Crimp Die at Buffalo Arms. I used it yesterday and following instructions I re-pressed 10 loaded rounds into this die and applied extra pressure that put a tiny rolled crimp on the case mouth. today I thought I'd re-do another ten rounds and look at the collet in the die and stop pressure as soon as I saw the four elements inside the die just touch each other. This would not put the rolled crimp on the case, but it would assure that I'd be getting the same pressure on the cast bullet such that I could expect consistent velocities. Do you gentlemen have a peference for either a rolled crip or just a flat crimp. Either one, I shall try out at our range up here in Sandpoint, Idaho. Thank you in advance.   Shiloh Jim

Neck tension only. Fire a crimped round with your rifle and look at the fired case the crimp will still be there!So how did the bullet squeze out of the case?

For black powder loads in several single shot rifles, I shoot the SAECO #1881 500 grain Government round-nose bullet. This bullet gives good accuracy and casts well with 25:1 Pb/Sn alloy. 

I have a RCBS .45 Colt taper crimp die that also works well for the .45-70 round. Testing showed a slight, and I mean slight, taper crimp that just snugs the bullet from turning in the case has given the best accuracy. 

Before this, the bullet free in the case sitting against the wads gave the best accuracy. 

I use the roll crimp on the RCBS 300 grain gas check smokeless powder (Reloder #7) 1400 feet-per-second hunting load. This load is shot in a Marlin 1895 magazine rifle.

I think it could be argued maybe, maybe not. There are other variables at play. If you are shooting smokeless certainly. Some powders may burn more cleanly and uniformly with a little increased tension on the bullet whether it is from reduced neck size at loading or the variable tension afforded by various amounts of crimp. For black powder the tension can also affect burning. If fouling in the throat causes a variable position of the bullet at loading the compression on the powder can definitely affect burning. More neck tension or a tighter crimp can keep the bullet in a stable position relative to both the compression and the bullets position in the chamber. As someone mentioned a slight crimp gave better accuracy. It is just one of those other factors that has to be explored, possibly extensively, that can make a difference in accuracy.