Ar15 pistol

jos51700 said
This is the first I've heard of marking a lower on the 4473. I gotta look into that.

From my experience, I'd go 10" barrel. I don't want my hands that close to exiting projectiles and remember no vertical grip onna front. See the NOT PLEASANT pics from the Kel-tec shotguns to see what I mean. Pistol rounds don't kick like .223/5.56.... UPDATED: Another devastating Kel-Tec KSG injury - The Gun Writer

Unless you're committed to visibly marking ammunition, mags, and guns, and keeping them FAR apart, I would not run 300 Blackout, either. Get one of those 300 BLK rounds in your 556 ammo, and you will have a bad day. It WILL chamber and fire. I kinda want to build a 300 BLK but I can't imagine my wife or anyone coming along after me realizing what's up until it's blown up in their face.

Nowadays, I feel that the 300 BLK, although neat as hell, was poorly executed specifically for this reason.

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AGAIN: 300BLK In 5.56 Rifle -

You are absolutely correct about the 300 in a 223 chamber. That can end very badly. I already have both I have. 556 rifle and a blackout rifle I have specifically marked magazines for my blackout and am very careful when I take them to the range I will not shoot one without the other put up its case. It's really no different that owning a 243 and a 308 you have to be safe and mindful about the ammo your using. I also own 2 glocks a 26 and a 23 one is a 9mm the other is a 40 same thing again.
I dont even understand how you would accidentally get your hand in front of the barrel without knowing it? That's just simple gun safety. I hate that guy lost his hand but damn he what was he thinking?
You are correct about the 4473 but in this case I'm using a 80% so I wont have to fill one out.