At the range last week an experienced shooter had a mishap on the 100 yard range. He had an AR in .223 and another one in .300 Blackout and was planning to do some sighting in and function tests. Somehow he managed to run a .300 Blackout round into the .223 AR and fired the round. The shooter was not seriously injured. The AR lower was damaged and the upper was wrecked. The bolt was fused closed. The magazine was damaged and some parts were not found. The shooter said he had 60 years of experience and still managed to make a stupid mistake. He was very lucky. It pays to be aware and very careful. The .300 Blackout round is a bit shorter than a .223 round. I was the Range Officer on duty and no pictures were taken. All the best...
Gil

This is why I don't take my 300 BO to the range when I'm shooting my .223/5.56. He was a lucky fella!!!

This is a big part of the problem when using rounds made from the case of a smaller calibre…and having the smaller calibre on hand to use as well. I think I’d be tempted to mark the custom rounds with some sort of mark that is very evident…maybe a bright red Sharpie or another colour on the primer that will stand out when loading the magazine. I’d also consider putting a bright fluoro band around the magazines for the larger round, in this case the 300 Blackout.

I know it seems like a lot of extra work, or is overkill, but with time, complacency slips in, leading to accidents like this.

Ive heard of it happening many times now...reckon a rifle dropped in combat during tactical retreat with a 300 round in bottom of mag full of 5.56 just has to be best booby trap ever....
I BELIEVE you can prevent it depending on projectile choice.... something longer and fatter wont allow chamber to shut....