The 300 Murphy (BLK Irish)

Gun Geekery:

I'm intrigued by the 300 BLK (300 ACC Blackout 7.62x35.) It approximates the performance of the 7.62x39 Russki and is much better suited to the AR platform. It can be loaded super or sub-sonic, and is a natural for a suppressor. In a short barreled/ suppressed firearm, it's well suited for home and urban defense. The donor-case is the ubiquitous 5.56x45 NATO/.223 Remington, cut down and necked up. Use of the 5.56 case allows the 300 to use the same bolt, magazines and somewhat the same chamber design.

And therein lies the problem.

300 BLK in a 5.56 AR Pistol Barrel -- guns.com

Murphy, the patron saint of all things mechanical, initially formulated his infamous law as: "If something can be installed incorrectly, it will be." A 300 BLK round can be installed in a 5.56 gun. And as any math major can attest, 300 goes into 5.56 only once.

The design of the 300 BLK allows it to fit into a 5.56 chamber. The Intrawebz holds this to be impossible, but there are too many reports and photographs of 300 BLK being (un)successfully fired in a 5.56 firearm. The shooters have not been noobs: most have been experienced and competent AR gunners. The .30 caliber bullet's ogive can neatly nestle into the 5.56 chamber's shoulders. The bolt closes and the trigger is pulled, and barrel, gun and shooter comense to having a very bad day. So far, there have been no reports of serious injuries. However, it's only time before "Someone gets kilt." 

300 BLK v 5.56 - onpointsupply.com

It's obvious that 300 BLK must be kept out of 5.56 guns. But, how do you bell that damn cat when the 300 uses the same mags ('cuse me: clips), bolt face, and at a glance can look much the same as the 5.56? The obvious answer is a strict policy of separating the 300 from the 5.56: in gun cases, range bags, mags, ammo, work tables, shooting benches, etc. Good luck with that in the chaos system that is the usual shooting trip. Some shooters mark their mags for 300 only, some use only one mag type (PMag or aluminum) for the BLK. I think I'd also paint the 300 BLK's mag's feed lips red, International Orange or Yum-Yum Yellow. And perhaps paint the mag well and magazine sides the same colors.

Who am I kidding? I'm much too much of an adherent to Murphy's Law to be completely  comfortable with these solutions. This idiot needs idiot proofing. Perhaps, I'll just have to continue with my 6.8 SPC II experimentation.

Until someone comes out with a foolproof solution, I fear the 300 BLK might well be renamed the 300 Murphy, or alternately, the 300 BLK Irish.

It's best to keep in mind  O'Toole's Observation on Murphy's Law: Murphy was an optimist.