Depends on your chamber.
300x221 is NOT a SAAMI or CIP approved cartridge so there is no standard dimension.
0.335" or 0.336" is a pretty common reamer diameter at the neck.
I have also been known to open up neck diameters to 0.340" for people that request it because they don't want to worry about neck clearance, EVER...
Choose your desired neck clearance accordingly.
If your loaded round necks are running much larger than about 0.332" in a chamber that's 0.336", I would keep a watch on your neck sizes.
You will know when you were too tight right away.
The recoil goes up and the primer usually leaves the case and ends up in your trigger mechanism.
Randall Rausch
www.ar15barrels.com
Threaded Mode
