[Flewis]

I know the general answer is there is no definite answer because there has never been an official specification for 300-221 and a ton of different manufacturers have been cutting the chambers.

But, if I have a 300-221 chambered AR and I have a box of Remington 220gr 300 Blackout ammo, how would I go about determining whether it is safe to fire?

I noticed that the Remington 220gr ammo has an OAL of 2.089. That is shorter than my 220gr Matchking load (mine is 2.170) that works well, so I can assume I am safe there. Also their published safe powder type and load are well within the ranges that I've worked successfully with.

I don't actually have box of the ammo yet, so I can't try this yet. But assuming it chambers and extracts ok is there anything else I should check before firing a few rounds?

I see people saying the chamber is .004 larger in diameter than a whisper, so I'm guessing I could have an issue chambering the Blackout ammo.

I guess the question is that if the manufacturer can't answer the compatibility question, how can I determine it myself? We know some manufacturers (Shilen?) have cut chambers that turned out to be compatible with the Blackout before the Blackout was ever mentioned.

Any input/advice?