I don't have a .300 Blackout but thought this might be of interest to those who have a .300 and a .223.
POTD: Informative .300 BLK Cutaway - The Firearm Blog
POTD: Informative .300 BLK Cutaway - The Firearm Blog
I guess if both of your weapons had identical setups I could see it being a concern.
We've been discussing at length on ARFCOM and I've run some of my own chambering tests. The picture above is the best/worst case scenario with a light bullet seated relatively short. Heavier bullets seated to mag length will typically not fully chamber, but after repeated attempts, or if the user uses the forward assist, they can create set back and cause the issue.
I'm not saying its not an issue, but with some basic gun safety techniques and making sure to use the right ammo (which you should do on EVERY gun), the risks are really not that great.
Yet ANOTHER .300 Blackout round fired through a 5.56 Barrel(pics of course) - AR15.COM