.223 vs 5.56mm chamber

[Coastie2010]

No, this is NOT the "what's the difference?"

I recently bought a second-hand homebuilt AR, and discovered that the barrel is made for .223 and NOT 5.56mm. From what I understand, the major issue with the higher-pressure load is that the .223 has a shorter gap before the rifling starts, which causes the excess pressure buildup. I like the rifle, so I want to keep the barrel and upper, but would it be possible to have a gunsmith work on the barrel so that it could effectively handle 5.56 ammo, since it's pretty easy to find cheap?

 

[Underwhere]

For the cost, couldn't you just swap for a new barrel?
I think I'd trust a manufactured barrel over a modified one.

 

[bpm990d]

Yes a gunsmith can run a NATO chamber reamer or separate throater to open it up. They can also inspect the chamber with a casting or a borescope and a stony point tool and let you know if it's safe to shoot ball.

If he has already fired a bunch of rounds, you might have the equivalent of a 5.56mm chamber. The 5.56 is throated much longer than a .223. The potential problem comes from a brand new match .223 chamber shooting military ball that is jammed into the rifling.

B

 

[Coastie2010]

I've only put about 140 rounds through it, and 100 of them were definitely .223. It's a chrome-lined barrel, but I don't think it's match-grade. Is that an issue?

Also, who might be a good gunsmith in Eastern MA to ask to do this?

 

[bpm990d]

I've only put about 140 rounds through it, and 100 of them were definitely .223. It's a chrome-lined barrel, but I don't think it's match-grade. Is that an issue?

Also, who might be a good gunsmith in Eastern MA to ask to do this?

Yeah, I'm not sure that a smith is going to want to run a HSS reamer into a chrome lined barrel.

I'd just shoot a few ball rounds out of it and check them carefully. If they have flat or blown primers or brass flow on the head of the case or if the thing locks up and does not cycle you will know it's way over pressure. If things function and the cases and primers look good then you're probably fine.

Even with new chambers that are marked the same there is a range of dimensions. You could have one .223 that is very tight and will blow primers on a ball round and in another one the same load would be fine. You never really know what pressures your ammo is producing without rigging your gun with instrumentation so people still do rely on observable parts of the case to make a determination if things are safe.

I can't really recommend a smith in Eastern Mass. Maybe someone that reloads in the area would have a Stony Point that they could check how much jump a ball bullet has into the rifling.

B