AR bolt hold open

[Sprocket]

I have 2 lowers - 1 pre-ban, not really sure on the history of the internals - the other an S&W M&P, I installed the internals.

I have 2 uppers - one was built by a local guy - the other is factory

When I attach each upper to the S&W lower the bolt doesn't stay back on either upper. I pull the charging handle back and engage the latch manually - the bolt stays open until I release the lever - then it slams shut in the usual fashion. Both uppers/BCG stays open on the pre-ban.

Can someone help me troubleshoot this issue or do I need to put some Hoppes on my skirt and wipe the whole thing down with it?

TIA

 

[WaldoOneFive]

Do you have magazines in the lowers, or mags out?

 

[HTRN]

Photos of each upper looking down from above would help.



Bolt should not hold unless the mag follower is pushing the catch up from an empty mag.

Does the catch move freely, or is it sticky on the pre-ban lower?

 

[Sprocket]

On the new lower - with or with out a mag in - everything seems to move freely

The pre-ban seems to function properly both with/without a mag in

At first I thought it was the new upper because it wouldn't hold open in the preban upper - I finished the new upper and tried both uppers - it's both upper on the new lower...

I'll try to get pics tomorrow - I'm assuming you want a top pic of the lower without the upper attached - do you need a pic of the upper at all?

 

[HTRN]

With the upper off, on the new lower, with an empty mag, does the catch flip up?

No upper pics necessary.

 

[kristo]

What kind of ammo are you using?
Are you using a notched hammer?
What LPK did you use?
Does you rifle cycle reliably?

My observations:
Certain mil-spec ARs won't cycle reliably commercial .223 loads as they are meant to use the more powerful 5.56 NATO load.
Most S&W M&P won't cycle any Russian ammo like Wolf or Bear. I've found that the problem is the friction between the bolt carrier and the hammer and specifically the notched hammers.

My solutions:
You can install a weaker hammer spring and a different shaped hammer. I prefer Timney triggers (which come with the hammer in a modular assembly) or for less money you can get a JP speed hammer.

 

[vellnueve]

The M&Ps should cycle fine with Wolf and Brown/Silver bear.

 

[Sprocket]

no rounds fired as I have just completed assembly of the lower - the upper was bought a few weeks ago, unfired - if it doesn't function properly, I won't shoot it.

"With the upper off, on the new lower, with an empty mag, does the catch flip up?"
Yes - the latch is fully engaged with a mag in - no play at all

 

[HTRN]

no rounds fired as I have just completed assembly of the lower - the upper was bought a few weeks ago, unfired - if it doesn't function properly, I won't shoot it.

"With the upper off, on the new lower, with an empty mag, does the catch flip up?"
Yes - the latch is fully engaged with a mag in - no play at all

When you remove the mag, does the catch lower under spring force??

 

[Sprocket]

When you remove the mag, does the catch lower under spring force??

yes - hit the mag release and the mag pops out quickly, the lever drops smoothly

Possibly BCG related?

 

[HTRN]

yes - hit the mag release and the mag pops out quickly, the lever drops smoothly

Possibly BCG related?

With the upper on, you can pull the charge handle back and visually inspect the engagement of the BCG and the catch through the ejection port. When you press on the catch, you should see it come up and stop the BGC from forward movement and when you release it, you should see the catch drop out of the way. When you do this, what do you see?

 

[Sprocket]

With the upper on, no mag:
pull the handle back, manually press lever, slowly release bolt forward - lever engauges but when I release the charging handle the lever slides off the bolt and the bolt slams home.

Same with a mag in...

 

[Sprocket]

Checked the working rifle and found the difference - the lever is not catching the bolt face, just the carrier face and slipping off - I'm gonna lube up the bolt and try again...

 

[Sprocket]

Problem ID'd - Buttstock screw preventing full motion of the rifle length buffer

Screw Too Long - never thought I'd have that kind of problem. Huh, go figure.