There is no need to crimp 7mm Rem. Mag. bullets. There is also no need for crimping 223 Rem. bullets even if they're to be run through an AR-15. I don't even crimp bullets of cartridges to be run through an M1A or M1 Garand.
There are several reasons to crimp:
1. Crimp for heavy recoiling cartridges to be fired in a gun with a magazine or cylinder. That would be stuff like a 458 Lott, 460 S&W Magnum, etc.
2. Crimping for some low pressure loads such as 45 Colt may help accuracy.
3. If in loading certain types of bullets (Barnes comes to mind), the bullets are a little loose, crimping with a Lee Factory Crimp Die will tighten them up.
