Perplexed with 5.45x39 AR

[ABNAK]

What was wrong with these guns in the first place before you went and monkey****ed them?

Nothing.

Bought an Osprey (another of which I had before on a 5.56 carbine)and put it on the S&W MP15R and she ran like a top. Decided about a year ago I wanted an AR pistol and went with the 5.45 S&W upper as the base. It too worked with the Osprey albeit with a few tweaks due to cutting the barrel to 11". Of course after Sandy Hook and the scarcity of ammunition I decided to conserve my 5.56 fodder and keep practiced with the 2500 rounds of 5.45 I had on hand, figuring it was enough to hold me over until ammo became more readily available again. Carbine-sized weapon was my preference for skillset and so went back to that platform. No DI carrier available when I set out on this journey and THAT is where the problems began.

[M4C]

OP: did you ever get your 5.45 issues worked out? I have some thoughts on what I think may be going on here, and I think we can un**** this.

I *think* different manufacturers have used different bolt face dimensions, along with what I would assume would have to be corresponding chamber reaming depths to achieve appropriate headspace. All 5.45x39 bolts will have a larger diameter bolt face, but I believe some have the bolt face the same depth as a 5.56x45 bolt, and some have the bolt face a bit deeper. The deeper bolt face would be coupled with a barrel with a shallower chamber.

If you have a standard 5.56-depth bolt, and a barrel that is reamed to headspace with a more deeply recessed bolt, headspace will be tight, assuming my hypothesis is correct. That could explain your stuck rounds, and possibly your FTFs due to incomplete chambering.

I received a 5.45 barrel from Ballistic Advantage, and since no bolts were available I asked them for dimensions to have a machinist open up a 5.56 bolt for me. This is copy/paste of what they provided:

A standard 5.56 bolt should be:
Width .380 +.005 / -.000
Depth .122 +.003 / -.000

To convert a 5.56 to a 5.45:
Width .395 +.003 / -.000
Depth .130 +.002 / -.000

So, Ballistic Advantage uses the deeper bolt face and, presumably, shallower chamber.

If I’m reading it correctly, you have an S&W bolt with a Ballistic Advantage barrel. I suggest checking bolt dimensions to see if your depth is closer to .122 or to .130, and having it machined out if needed. Or source a BA bolt, good luck with that. I have a 5.56 bolt ready to go to the machine shop, I’ll post here with how that works out.

If I’m right and a deeper bolt fixes your chambering issues, next on the agenda is the over gassed barrel. You could try putting the Osprey kit back in, but I agree with the others that you’ll be better off not dicking with it, even if it does like the big gas port, which I doubt. Instead, I would suggest contacting Clint (he has already posted in this thread) about one of his insertable gas ports. Much cheaper than a new barrel, and should fix it right up.

I’ll add one more thought: I think we are all playing with fire by not using headspace gages with 5.45 builds, especially if we mix & match bolts and barrels. 5.45 is not standardized in the AR like 5.56 is, and I see serious potential for headspace problems. I field gage my 5.56 builds, and I will try to borrow or buy a 5.45 field or no-go gage before I fire this amalgamation of parts.

[ABNAK]

Yes, I got it squared away. Got rid of the rifle spring and bought a Sprinco extra power carbine spring (the "red" one) which is supposed to be 27-28% stronger than a regular carbine spring. Also using the H buffer I mentioned earlier with the protrusion. Still DI mode, no more piston. Ejects to 4 o'clock about 7 or 8 feet. Works fine now.

[vicious_cb]

I still don't know why people want to shoehorn pistons to replace a perfectly function operating system.

[Machinegunnertim]

Because if no one ever tinkered with or changed anything then we would all still be firing muskets.

"If it ain't broke don't fix it" right? Well that's the motto of the under achiever.