So I was looking for a new AR barrel a couple months ago and ordered a Faxon Gunner Barrel. It finally came today and when I looked at the packaging, I saw .350 Legend on it. I clearly did not read the item description enough to see that it wasn't for .223/5.56 caliber.

I looked around and saw there is some compatibility between the two calibers. Hypothetically, would I be able to fire .223/5.56 through the .350 Legend barrel without any issues? Also, how would the gas system and a muzzle brake fit into this situation? Don't be too hard on me, I'm pretty new to the AR platform.

This is the specific barrel: https://faxonfirearms.com/faxon-12-5-gunner-profile-ar15-barrel-350-legend-nitrided/ (Yes I know it's <16" and I was going to complete a Form 1 before installing)

Oh HELL no, not hypothetically, theoretically nor metaphorically! They are different size bores, different sized chambers.  The only thing they can share is the bolt.

As said HELL NO - you'll be lucky to not end up in the ER.  Or worse.

As far as parts compatibility, everything but barrel and likely mags.

So the case diameters and overall length of the .223/5.56 and .350 rounds (as well as  .300BLK) are pretty close, but bore and profiles are very different.

Here’s a picture that helps bring it into perspective:

Attached File


.300BLK.....350Legend....223

Not sure where you found info stating you might be OK to chamber and shoot .223 in a .350 barrel, but I wouldn’t for many reasons with the primary being non existent case support In key areas for properly containing and directing the high gas pressures during the ignition and the inability to properly guide the bullet down the barrel in a true and straight trajectory.

Here’s the lowdown from a reloading perspective....

Legend 350 Specs

Attached File

@Exorpmtech this isn't GD.  ~d16man

It will not fire in a 350L chamber.

When it first came out we made a cutaway 350 chamber to show how 300blk and 5.56 look like in the 350 chamber. We even loaded both to see if we could get them to fire.  The 350 is a proprietary case with no parent.  It is .014" larger at the base and will severely bulge or seperate a 5.56 case if you thought about necking up or using basic brass.

OP......you have much research to do.

The only ammo that should be fired through a barrel is what is marked on the barrel.