AR bolt hold open malfunction

[Blackmesa96]

Have an AR platform rifle. When I fire the last round, the bolt doesn't stay open.

Happens with every mag. The hold open catch will engage manually and work correctly. Cleaned, lubed and inspected, nothing pops out at me.

Anyone here wish to offer me guidance on what to check next?
Thanks to all in advance.

 

[Belt Fed]

I've seen cheap ammo create that problem, has just enough power to chamber a round but not push it far enough back to lock.

If it has done it from new with several brands of ammo then that ain't the problem.

 

[Funwithfuel]

What kinda mags you running, and with what follower? There were some really cheap Korean mags sold that specifically do not have the bolt hold open on the follower.

 

[10mmLife]

I recommend taking out the bcg and mag to test the spring on your bolt catch to see if it automatically pushes the bolt catch fully engages the receiver.

Low powered ammo and cheap mags can also be a culprit.

 

[Blackmesa96]

What kinda mags you running, and with what follower? There were some really cheap Korean mags sold that specifically do not have the bolt hold open on the follower.

P Mags

 

[10mmLife]

P Mags

Does the bolt catch work automatically on an empty mag or are you pushing it in manually?

 

[Funwithfuel]

P Mags

Well, that pretty much rules that out.

 

[Bassbob]

Is the weapon clean ? Does it have an adjustable gas block ? Did this just start happening ?

 

[HansGruber]

So: is this a factory rifle, or a home build?

Did it used to lock, but now doesn’t, or has it never worked?

 

[Blackmesa96]

So: is this a factory rifle, or a home build?

Did it used to lock, but now doesn’t, or has it never worked?

Bought used. Just started running it.

Weapon is clean, honestly don't know if gas block is adjustable or not.

Relatively new to this platform and am learning more as I go.

 

[Bassbob]

Bought used. Just started running it.

Weapon is clean, honestly don't know if gas block is adjustable or not.

Relatively new to this platform and am learning more as I go.

The most likely causes are the gun is undergassed, the bolt catch is F'ed up or bad magazines. With P Mags that is extremely unlikely. Could be the BCG or the buffer weight too. First steps:

Clean and lube weapon.
Stick an empty mag in and manually cycle the action/BCG via the charging handle. Does the bolt stay open ?

Shoot the weapon and see where your brass is going ( i.e. 1:00- 3:00- 5:00)

 

[Talyn]

The most likely causes are the gun is undergassed, the bolt catch is F'ed up... Could be the...the buffer weight too.

+1

Also, the recoil spring may be too high poundage.

 

[Funwithfuel]

So, do you have a home built weapon or name brand mfr? How long is the barrel? Can you attach a picture of the gas block? If you open the action and place an empty mag in, does it raise the bolt catch? Does the catch pivot smoothly or does it drag? Is there a chance that it's bent or even tweaked a little? Post up some pics.

 

[Bassbob]

The good news is that whatever the issue is it's a relatively easy fix and replacement parts aren't that expensive.

What kind of gun is it ? Lower receiver ? Upper receiver? Both are likely marked. 5.56, .308 ?

You don't seem to be too forthcoming with the info brother. It's easier to help you if we have all the info and can start ruling stuff out. I'd start by cleaning and lubing, especially the BCG and insert an empty P mag in and charge the weapon and see if the bolt holds open.

 

[HansGruber]

Bought used. Just started running it.

Weapon is clean, honestly don't know if gas block is adjustable or not.

Relatively new to this platform and am learning more as I go.

Factory, or home build?

I’m wondering if you don’t have an out of spec bolt catch.

Or it could be under gassed.

Or, it’s the ammo you at using.

Or—and this is a real thing—your rifle doesn’t like PMags.

Or it’s just a full-on case of tolerance stacking.

 

[Bassbob]

Or—and this is a real thing—your rifle doesn’t like PMags.

It happens but not very often. On the other hand I have had a bunch of Amend2 mags and all of them either won't hold the bolt open or won't drop free from the magwell on their own.

So to add to my suggestions, which are about all I can think of with limited information, I would also by a new P mag and a USGI with the orange follower and see what that does.

 

[HansGruber]

It happens but not very often. On the other hand I have had a bunch of Amend2 mags and all of them either won't hold the bolt open or won't drop free from the magwell on their own.

So to add to my suggestions, which are about all I can think of with limited information, I would also by a new P mag and a USGI with the orange follower and see what that does.

The only mag I’ve ever met that has never failed in any rifle are HK High Reliability mags.

They rock, but at $40+ a mag…

I got turned on to them by a trainer who also had experience with HK’s ITD…I remember buying a couple for $70 during the first Obama scare.

 

[Talyn]

I had issues when I tried some of the new "fangled" recoil springs. So I standardized on standard-weight springs (Brownells) but made of chrome-silicon for greater life-span. Problem solved.

Never had an issue with buffer weights & I use the various Spikes Tactical weights ("H's") to smooth out the recoil pulse.

 

[Bassbob]

The only mag I’ve ever met that has never failed in any rifle are HK High Reliability mags.

They rock, but at $40+ a mag…

I got turned on to them by a trainer who also had experience with HK’s ITD…I remember buying a couple for $70 during the first Obama scare.

I'd love to have a pile of those, but in reality every single AR I own, including my latest, loves P mags and I have never ever had an issue of any kind with any of those weapons. So I can't really justify changing course. Not to mention the fact that I now have a ridiculous amount of Pmags still in the plastic besides the ten or so I have in rotation.

 

[TEXASforLIFE]

Maybe a pic of the firearm from rooter to tooter and receivers separated would help.