I'm AR dumb. bolt won't hold open.

[Revvin' Evan]

So I have an SBR'd YHM 9mm AR which is my first and only AR. Well my brother built me an 11.5" colt commando-ish upper in 5.56 for christmas. So we put the upper on the lower, shot it all day Saturday and it ran great except for the bolt will not hold open at all. Here are some pics. Any help is greatly appreciated.

 

[fivepointstar]

even with the magazine in it won't hold open?

if hte mag is out, work hte mag release/catch til it holds open

 

[rouchna]

Did you try a different mag?

 

[Lapua Dan]

Does the bolt hold open still move freely and is it still under spring tension?
My 9mm upper bent mine and had to be replaced.

 

[Blu Falcon]

Will it stay open when the bolt catch is engaged?

 

[Flipz]

I have the same YHM 9mm SBR. I haven't run a 5.56 upper on it yet but I don't see why it wouldn't lock back. I would think its a mag issue or maybe a buffer issue. Did you switch out the 9mm buffer for one suited for 5.56? The one that came in the SBR is for 9mm only and is longer than a normal buffer. This might he why yours isn't locking back.

 

[Revvin' Evan]

I used metal mags and pmags and it will not hold open with either. It won't even hold open when you push the bolt catch button down to attempt to lock it open. Thanks for the quick responses guys.

 

[Flipz]

It's the buffer. The one that came in it is for 9mm only and is slightly longer than a regular buffer. Because it is slightly longer it will not let a 5.56 BCG retract far enough to engage the bolt catch. You need to switch it out for an H buffer.

 

[Revvin' Evan]

I did not replace the buffer and the bolt catch does have spring tension and appears to work properly but like I said, I'm no expert. Just have this AR and a S&W M&P 15-22 for comparison.

 

[Revvin' Evan]

Thanks flipz. So I just Google "H buffer" and get one of those?

 

[Lapua Dan]

Flipz is correct. You have to change the buffer for a carbine length buffer.

 

[Flipz]

Trust me. It's the buffer. Replace it with any standard buffer and you'll be good to go.

 

[Flipz]

Thanks flipz. So I just Google "H buffer" and get one of those?

You could just use a standard carbine buffer. But many people use H buffers, or even H2 buffers, with SBR's to reduce recoil and extend lock time. Id do a search on what weight buffer people like for 11.5 barrels and go with that. Most likely an H buffer. You may want to give Mark a call and see what he suggests.

 

[Revvin' Evan]

So I just cycled the bolt and that's it. The bcg needs to come back another 1/8" to get behind the catch but it can't. Thanks.

 

[Revvin' Evan]

Y'all rock!!

 

[Flipz]

Y'all rock!!

We know, we know. You can repay me with letting me know if the girl in your pic has a sister. Atleast I really hope that's a girl.

 

[Revvin' Evan]

We know, we know. You can repay me with letting me know if the girl in your pic has a sister. Atleast I really hope that's a girl.

It's definitely a girl and she had two sisters.

 

[mdoffroad]

Are they single and do these look as good posing with firearms?