Some good posts on this topic. I reload 9mm and put the theory to the test.
I took Hornady 115 and 124gr FMJ and crimped them to . They measured .376 some .3755"
I also pulled the bullets from factory Blazer that measured .375. They had no ring on the crimp.
Now this is far from scientific as I was going for how easy it was to remove the bullets. with a kinetic bullet puller.
The "over crimped bullets with the ring dents where much harder to get out of the brass. 3-4 good raps on the cement to release the bullet.
On the bullets that had no crimp ring only 1 hammer rap was required, they came out so easy I had to do a few more to believe it.
So I am not so sure the theory that over crimping makes the bullet looser is correct. At least in this test it pointed to a stronger hold on the bullet.
I've not conducted a accuracy test but I can see how a bullet with a ring around it might behave differently.
