crimping?

Crimping is a good way of uniforming the bullet pull weight with hunting bullets that are typically seated short to function through a magazine, but a target rifle is often fired as a single shot. Seating the bullet into the rifling provides greater uniformity as it requires the same amount of pressure to get the bullet moving round to round. When the bullet is crimped and seated off the lands, variations in neck wall thickness, and the degree of crimp become the uniformity gremlins.
 
I found very early on that crimping just added one more level of possible inconsistancy to my reloading regime so I stopped the process.
 
No, don't need to crimp any bullets, but for semi-autos it may be a good idea.

"It may be neccessary to crimp the bullets for tubular magazine guns."
~ quote from the rifle lee loader kit.
 
There are good reasons to crimp like semi autos, tube fed levers, heavy recoiling hunting rifles with box magazines etc. As to helping accrracy, there are some cases when it really does help. The 22 Hornet, with light bullets and Lil'Gun powder benefits from a light crimp. In some guns this light crimp drops SD from 90 fps to 15-18, and shrinks the groups as well. I have applied this same strategy to other small capacity cases with bullets of 40 grains or less, like the 218 Bee, 17 Ackley Hornet etc. The 17 HMRs are all crimped for the same basic reason, better SDs and accurracy.

In the larger cases and heavier bullets I have never seen crimping improve the accurracy but I also haven't played much with it as it would make getting exactly the same neck tension more difficult. I have never seen it hurt accuracy in my 338, 375, 45-70 etc as I have tried these, crimped and not when working up loads. That said, these are sporting rifles and would not show the effect like a precision target gun.
 
In my own testing I have found the crimp beneficial on loads with light for caliber hunting type bullets, ones with a cannelure, generally hornady 165 sst's or btsp. Allows a touch more resistance and makes for a more even ignition and burn. I tried a mild crimp on some 190 SMK's, my velocity levels did even out, like with the hunting loads, but accuracy changes were negligible. 300 win mag BTW.
 
rule of thumb: you only roll crimp cannelured bullets. if you roll crimp your loads brass will not last as long. they will start to crack at the crimp. if your brass stretches before it cracks you have a case stretching problem which will result in a case failure. you will have separation either at the webb or a little further up the case. maybe you'll be lucky and the primer pocket will stretch before seperation happens. no fun pulling a separated case out of the chamber. this only happened to me once and luckily i have a case extractor for that calibre.
 
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