Velocity vs Crimp

I know you guys have a lot of experience here, so I’d like to ask a question. All other things equal, how much does the crimp style affect the velocity of a round? The reason I ask is that I recently loaded some 32 H&R Mag cases with about .02” varying length with 4 gr Unique and a .313” 78 gr cast round nose bullet, and got a large range of velocities. Interestingly enough, I got the same number of very low velocity rounds as short cases. I’m assuming that the longer cases got a good firm roll crimp, and the short cases got a very mild roll crimp, or maybe just a taper crimp (Lee dies). Could this account for about a 200 FPS variation in velocity?

crimp effects pressure... pressure is one thing that will determine velocity.

PS.. I'd try something other than unique if you have it. try titegroup for small loads. titegroup doesn't care if you load it in small loads.

A good roll crimp raises peak pressure, helping many powders to burn more efficiently and completely.

In a 45 acp, going from 0.470 to 0.463 crimp raises velocity 50 fps. I saw no effect on point of impact. I would only be concerned for function in automatics.

200 fps difference, Crimp, could be? Position sensitive powder, if there is a lot of free space in the case? But not normally a Unique powder problem?

If you have a weak crimp the primer alone could be pushing the bullet forward as the powder begins to burn. That is a fairly small casing it wouldn't take much pressure to move the bullet.

I believe a lot of it is powder dependent. With typical handgun powders it can be a lot especially with mid to slow burners. Unique is one that would like a crimp. Also try positioning the powder by tilting the gun up before firing.

If cases are not trimmed to a uniform length, any discussion about crimp style and velocity is like a dog chasing his tail.

Quote:

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Originally Posted by Char-Gar

If cases are not trimmed to a uniform length, any discussion about crimp style and velocity is like a dog chasing his tail.

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Makes sense......

Especially for roll crimps. They work really well if your cases are all trimmed to same length. If not, then it is like rolling dice.