No bolt hold open on P-Mags

[spleify]

On one of my AR's the bolt will not hold open after the last round is fired. It only happens on some of the mags and not all of them. I know I remember seeing this before and someone posted pics of what they did to modify the mags so the bolt would stay back.

Anyone know what I need to do here or can someone point me in the right direction with the pics?

Thanks

[Blowby]

Is the bolt catch operating smoothly? Have you changed anything on the AR? Can you provide barrel length, carbine or mid, ammo used, buffer weight and buffer spring used. This will help to diagnose. I would not recommend chopping up the magpul.

[spleify]

Is the bolt catch operating smoothly? Have you changed anything on the AR? Can you provide barrel length, carbine or mid, ammo used, buffer weight and buffer spring used. This will help to diagnose. I would not recommend chopping up the magpul.

Ok, your right I left out quite a few details there, sorry.

Yes the bolt catch is operating smoothly and if I manually pull the charging handle back on an empty mag, or after the last round fired and it didnt catch, it will grab fine and hold open.

14.5" carbine length gas system, my hand loads, 23.8 grains of H335 behind a 55grain mil spec FMJBT projectile(been shooting these for years and have fired thousands and thousands) Carbine spring and Spikes ST-T2 buffer.

Nothing else has changed besides the mags. I recently sold all of my steel mags and switched to the P-Mags(thinking they were better)

This is the very first time I have had this issue and it is not on all of the mags but just some of them. I number all my mags so I know which ones they were.

Thanks again

[spleify]

Also, this is a DPMS lower and DPMS lower parts kit

[Blowby]

It seems the BCG is not traveling all the way back past the bolt catch while activated by gas. Simple steps to bring out the problem.

Leaking gas key, gas tube alignment, leaking at the gas block or FSB.
Do you have an H or H2 buffer? Should be lighter than the current one used.
Do you another buffer spring? Shorter on the acceptable scale (10.25" or so)

I know the Wolf spring has fewer winds so the compressed length is shorter while the over all length is about 12". I think without altering the gas activation (ammo loads) this might be the answer.

[spleify]

It seems the BCG is not traveling all the way back past the bolt catch while activated by gas. Simple steps to bring out the problem.

Leaking gas key, gas tube alignment, leaking at the gas block or FSB.
Do you have an H or H2 buffer? Should be lighter than the current one used.
Do you another buffer spring? Shorter on the acceptable scale (10.25" or so)

I know the Wolf spring has fewer winds so the compressed length is shorter while the over all length is about 12". I think without altering the gas activation (ammo loads) this might be the answer.

Yes, I do notice around the FSB(low profile weaver base style) that there is gas leaking out around it. How can I remedy this?

I have an H2 buffer in it. I have an extra one I can throw in there along with(I believe) another buffer spring So I can try them both out.

Thanks so much

[Blowby]

"Try a lighter or shorter spring first". Then you can swap the buffer to an H if it's still not locking on last round.

I would then replace the gas tube to correct the root cause (if it's leaking). After everything has been check and functioning properly in the gas system you can try the H2 again if the AR was initially configured and designed to use an H2.

BTW approximate weights:

Spikes ST-T2 = 4.3 oz
H2 = 4.6
H = 3.8
Std = 3.0

[jscwerve]

Thanks for posting this up, I am having the exact same issue. I was figuring it was my buffer spring and was going to go and get a spring with a little more give to it. Mine doesn't have any gas leaks and still has the problem. I have been running my 55gn bullets with 24.5grns of 8335 with the same effect. Also same thing with all different factory ammo.

Please post up when you figure it out, I will do the same.

[Beer]

[spleify]

"Try a lighter or shorter spring first". Then you can swap the buffer to an H if it's still not locking on last round.

I would then replace the gas tube to correct the root cause (if it's leaking). After everything has been check and functioning properly in the gas system you can try the H2 again if the AR was initially configured and designed to use an H2.

BTW approximate weights:

Spikes ST-T2 = 4.3 oz
H2 = 4.6
H = 3.8
Std = 3.0

Ok, so correction, yes I have the Spikes ST-T2 NOT the H2. I also have a standard buffer and another buffer spring, along with a new carbine length gas tube.

I will change one thing at a time and see what my results are.

Thanks for the tips.

[DeusExMachina]

Take it to Rob at Bowers, he'll diagnose it. He's fixed random problems like this I've had.

[ray1970]

Definitely sounds like it is "short stroking" like others suggested. I personally don't run anything heavier than the "H" buffer in my carbines for reliable cycling. Also, if you haven't already done so, make sure to check the gas rings. I shot one of my first builds until the rings wore out and the first sign I noticed was it started "short stroking" intermittently. A new set of rings and it was back to running like a top.

[spleify]

Definitely sounds like it is "short stroking" like others suggested. I personally don't run anything heavier than the "H" buffer in my carbines for reliable cycling. Also, if you haven't already done so, make sure to check the gas rings. I shot one of my first builds until the rings wore out and the first sign I noticed was it started "short stroking" intermittently. A new set of rings and it was back to running like a top.

Good advice Ray, I have some extra rings too that I can throw at it!

Thanks

[Blowby]

Any results?

FYI: I went to the range yesterday and decided to change out my buffer spring with a longer spring to prove out my comment above and guess what. I fired 3 rounds per magazine and the last round of all 5 mags would not lock the bolt back. No other changes, same ammo, same upper, same bolt, 550 rounds since cleaned. Replaced to 10.25" carbine spring and ran the same test and the 3rd round on all mags locked back like it always has during the 30 rounds, each mag twice for a total of 580 since cleaned. I like to prove out reliability in real life situations where you can't always have time or supplies with you to have a perfectly clean AR. This is where you find the issues that need to be corrected so you don't have to focus on failures in the heat of battle. Zombies have no compassion. [LOL]

[spleify]

No unfortunately I have not had a chance to get to the range yet. I dont know if I will make it this weekend either, it might have to wait until next weekend.

Thanks for doing the testing and posting your results.

[rbeau30]

spleify, is this a newly built rifle?

approx how many rounds you got throught it?

[spleify]

Nope not new, it has about 900-1000 rounds maybe.

I did change out the buffer to a standard buffer and I am still have the same issues. I bought a new mag release button but I havnt changed it yet. I will also test to see what the gas rings are like also.

[rbeau30]

Ahh okay... I was having the same results on my new M4... I heard from others on other boards that for the first 500 rounds on a new build, it helps shooting it real oily.

As soon as I ran a bead of oild down the underside of my upper all my problems instantly ceased.



Store it with the bolt open for a couple days before goin out to the range?

There should be a "troubleshooting" section on here to organize these things a lil better.

[tarantula]

I once had a bolt catch that had casting marks on it and it caused me lots of problems until I changed it out. The problem was worse with p-mags too...

I have also had issues with hand-loads a time or two and once because I changed primer lots. Do you have the same issue with your carry load? I personally would not set up a rifle to work with anything other than full -power , quality defensive ammunition. I'll accept less with practice ammo and it gives a person a change to practice malfunction clearing.
Just my opinion and I understand why you and others might feel differently. Good luck with the problem.