Will a 300 blackout chamber in to a .223?

Well, I don’t suggest you ever do this. But this is proof positive of will it, or will it not chamber.


Nice demo! And definitely worth doing. I suppose it doesn’t surprise me that some folks think they can’t chamber. Lack of experience means that they might not have encountered bullet set back before. Or the reverse where bullets work their way out of the case due to recoil or repeatedly chambering the same cartridge (a sort of kinetic bullet puller).

We’ve had cartridge combinations like that for a VERY long time. I don’t mean to make the issue of no concern by that statement. The traditional solution has been to never have the cartridges in proximity to where it could be a problem. The classic mismatch I can think of right now is 12 and 20 gauge. And the solution there was to make sure that the game vest was always 100% empty of 20 gauge when swapping to 12 gauge. The solution for .300BLK and .223 is the same. Why? Because it works.

Labeling mags is all fine and dandy, but .300BLK cartridges should never be anywhere near a .223 rifle, labeled mags or not. IMO that’s still an invitation to disaster. It’s the cartridge that’s the problem and not the mags. As long as .300BLK will fit in a .223/5.56 mag with the wrong label, confusion can occur. Can’t put .300BLK in a mag labeled .223? Of course it can happen.

In practice, when shooting .223/5.56 that gun and that ammo ONLY is out and available and .300BLK is nowhere near. Want to shoot .300BLK? Then everything .223 is put away, not just to the side. The area is checked over to make sure it’s all gone then the .300BLK gets taken out (or vice versa). Maybe it’s a little more complicated than the 12/20 gauge solution but similar, and that has worked for many decades. Failure to clear the area of the other cartridge could result in the wrong cartridge being put in a magazine that externally looks like it contains .223. When the rifle self loads no one will hear or feel the difference between a .300BLK or the correct .223 when part way through the mag’. Yeah, a .308 bullet isn’t going to swage down to .224 when backed up by a charge of powder.

I take cartridge confusion so seriously that I got rid of my only 20 gauge and went with 12s only. But I’m about to enter that realm again with a .300 BLK. I’ll just need to take things super seriously, esp’ when at the range with both cartridges.


This topic is definitely worth revisiting, especially with new gun owners out there who have little experience with the potential for problems beyond the basic safety rules.


I agree. I know people with both Calibers as well. They are very cautious on how they separate and store their ammo.


It is only .08". Just use a little lube.


Just get rid of all those pesky 223’s. :upside_down_face: