(How) is neck friction (cartridge/bullet) an issue to
a) safety
b) accuracy ?
As to accuracy, there is considerable effect. I've done the most testing in 338's. The ideal grip for the bullet in several Win Mags seems to be about .002" with recenly annealed brass. Bullets in the magazine are able to withstand the recoil without being driven back and accuracy is not adversely affected. In my experience there is a bit more accuracy with a .001" grip but this light friction enables bullets to be driven into the case. For single loading a light grip seems to produce better accuracy, probably because of consistency in the friction in the neck. Accuracy deteriorated for me at .003" grip and above. Any more than that seemed to make little difference as there was not enough elasticity in the brass to matter much. These values would probably change in smaller calibers in the varmint class. Because of reduced recoil I generally settle on light grip in varmint class rifles. I've not done much testing on smaller calibers so can't be too specific there. However, in single loading the lighter grips seem to give the best accuracy across the board on all calibers.
That nice clean case with a nice smooth bullet produce very little friction. That is why "all" factory ammo is crimped.
With fired cases, you have a "harder" case with carbon on the inside and the neck tension is greatly increased, and crimping is seldom needed.
A normal hunting rifle will have 200 to 300 thousands expansion allowed and that's fine...
On big bores with recoil I always turn three or four thousands off the resizer button to facilitate holding the round tightly so as to prevent bullet set back and it also takes the place of a crimp when used with a powder that fills the case....this theoretically has some bad effect on accuracy but in a big bore you won't be able to find it....
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Ray Atkinson