Rifle case mouth flaring?

[dyl]

I measured

So I measured the case mouth opening after sizing and trimming with the 300 Blackout Small Base 2 die set. The diameter is 0.304 for projectiles that are usually 0.308

Is that a little on the small side? I thought I read somewhere that for maximum accuracy you want the neck to be 0.001 smaller than the projectile, and for lead projectiles any narrower than 0.001 would swage the projectile and you lose accuracy (not that this is for a bench rest rifle or anything). These are hard cast so maybe not an issue. Would 0.304 be detrimental even to FMJ accuracy?

 

[Unclenick]

-0.001 to -0.002 is typical, but occasionally you run into someone doing -0.003 who finds it gives the best accuracy in their particular gun. My guess is the powder choice has something to do with it, as some powders need higher start pressures than others.

The M-die should leave it the right size. Keep in mind, dies have a tolerance, and RCBS told me theirs is 0.002". So you can have that much difference.