I have a M&P Sport II that I've been replacing the parts on. I replaced the barrel with a BA hansen mid length barrel. At the same time I also replaced the BCG with a NBS nitride bcg from AR15 discounts. When I took the rifle out to the range to test fire wouldn't reliable cycle. I switched out the bcg with a toolcraft from another rifle and it cycled fine perfectly. I've also tested with the original bcg from the M&P and that cycles fine. I returned the NBS bcg for a replacement one and while the rifle will now reliably cycle it still won't lock back on the last round. Should I chock this up to NBS just being garbage or is this possibly a problem with my gas system?
Gunsmithing AR15 Bolt not Locking Back on last round
- Thread starter ketillolafsson
- Start date
I love these "why doesn't it work?" threads. It sounds like you took a rifle that could've maybe worked perfectly fine and fucked it up. Welcome to the Hide, first, if you're going to survive here you're going to need a thick skin, if you're 10-ply...well...it was fun.
What ammunition are you shooting? Be specific, what brand and bullet weight?
Who did the assembly work when the barrel was swapped out? Look and see if the gas block is properly aligned.
What magazines are you using? Have you tried other magazines?
What ammunition are you shooting? Be specific, what brand and bullet weight?
Who did the assembly work when the barrel was swapped out? Look and see if the gas block is properly aligned.
What magazines are you using? Have you tried other magazines?
I understood it as the rifle cycled with the OEM, Toolcraft, and replacement NBS BCG...meaning that it fed, fired, extracted, and ejected but that with all the BCG's it wouldn't lock back. Maybe I was wrong though, I dunno, OP needs to provide more details and stop buying shit off AR15 Discounts.
Last round lock back is a function of the mag follower stopping the bolt/carrier from returning home. It's easy to see with an empty mag. If you have done something to change that engagement (making it less) then you're going to defeat the design. Check the mags first then all of the relevant components and how they interact with the bolt catch.
Sorry edit - follower interacts with the bolt Cath that stops the bolt
Sorry edit - follower interacts with the bolt Cath that stops the bolt
TM- has the spirit of the Hide dialed in! Take notes.
I would like to congratulate you on getting a M&P "Sport" to run at all. I knew a kid that bought one and the hand guard was crooked and parts would juts fall off of it. So if that discount AR runs, leave it well enough alone and thank your lucky stars.
Colt 20 round mags have a reputation of not locking back, but I doubt you running Colt mags in that gem. Just in case.
PB
Colt 20 round mags have a reputation of not locking back, but I doubt you running Colt mags in that gem. Just in case.
PB
The bolt locks back fine when mag is empty with both the Toolcraft and OEM BCG's just not the NBS BCG's. Mags are all Magpul 10 and 30 round gen 3. The funny thing is that both NBS bcgs cycled fine and the bolt locked back in the non M&P AR.
The M&P was my first AR and after changing things like the grip and handguard it made me want to build one which while it's no Daniel Defense it does seem to run flawlessly. I then replaced the barrel on the M&P and the BCG because it was on sale and I thought it would probably be better than the OEM bcg for now. To me it seems like it's NBS being crap but they do run fine in my other AR which made me question if it's the BCG or something else. That being said I do plan to replace the NBS bcg.
As for the M&P being low end I get that, it was my first lol. Other than this issue it hasn't failed to run in any other situation or with any ammo I've thrown at it and fires surprisingly accurately for being what it is.
The M&P was my first AR and after changing things like the grip and handguard it made me want to build one which while it's no Daniel Defense it does seem to run flawlessly. I then replaced the barrel on the M&P and the BCG because it was on sale and I thought it would probably be better than the OEM bcg for now. To me it seems like it's NBS being crap but they do run fine in my other AR which made me question if it's the BCG or something else. That being said I do plan to replace the NBS bcg.
As for the M&P being low end I get that, it was my first lol. Other than this issue it hasn't failed to run in any other situation or with any ammo I've thrown at it and fires surprisingly accurately for being what it is.
And yes I know I took a working rifle and made changes that caused problems but the tinkering and learning is fun for me.
Ammo is
Winchester Valor 55 grain 5.56
PMC X-Tac 55 grain 5.56
Barrel and BCG both swapped at the same time. The BA barrel came predrilled for a pin and I believe it's aligned properly although I didn't use a jig to align it.
Have only tried Magpul mags but the same mags that work with the OEM and Toolcraft are the ones that don't work with the NBS. Same Mags that work in my other rifle with all of the bcgs.
Ammo is
Winchester Valor 55 grain 5.56
PMC X-Tac 55 grain 5.56
Barrel and BCG both swapped at the same time. The BA barrel came predrilled for a pin and I believe it's aligned properly although I didn't use a jig to align it.
Have only tried Magpul mags but the same mags that work with the OEM and Toolcraft are the ones that don't work with the NBS. Same Mags that work in my other rifle with all of the bcgs.
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Well I think you've deduced that it's the NBS BCG causing the problems, possibly due to tolerance stacking in the M&P receiver. Put the OEM BCG back in the gun, stop buying cheap crap off a discount site, and drive on.
AR15Discounts is a good vendor that sells about everything. They sell plenty of good stuff and ship quickly the few times I bought from them.
I knew I was taking a chance with the NBS bcg considering price and an unknown manufacturer. Just wanted to see if I was missing another problem in addition to the NBS. Thank you all for the information.
Well that's what we're here for and hazing, sometimes both....no reason this can't be fun.
Post some pics up in the AR15 pic thread in the semi-auto section.
Start by checking gas flow. One round in the magazine, load, fire and see if it locks the bolt. If not, start checking the gas block, gas tube alignment and condition, bolt carrier gas key, gas rings. Check BOLT CARRIER ONLY, see how it rides in the upper, no binding and has full travel with little to no pressure to move it.
And yes, SW AR's are bottom feeders.
And yes, SW AR's are bottom feeders.
Also try looking at the buffer. If everything else is good, then you might need to go to a lighter buffer and/or spring.
Take a look at the silent capture springs by JP Enterprises. I have them in a couple of ARs and they are great. Nevertheless, check everything else that everyone has mentioned first before you mess with the buffer & spring.
Take a look at the silent capture springs by JP Enterprises. I have them in a couple of ARs and they are great. Nevertheless, check everything else that everyone has mentioned first before you mess with the buffer & spring.
Save your money, stay with standard springs and buffers. Hopefully you haven't read thread to where if you don't get the latest Acme buffer system you will die.
Make sure that the gas port in the new barrel is the same size as the old (assuming similar barrel length), and that the weight of the new and old BCG are the same. You might simply have a heavier BCG and/or an undersized port . . . When the new parts do work, where is it ejecting?
With 1 round in the mag the bolt will lock back with the oem and toolcraft but not NBS. None of the bcg bind within the upper from what I can see, free travel until the bolt locks in. I was mainly just concerned it might be a gas system issue but honestly the port in the gas block was large enough it would be hard to misalign it with the port in the barrel.
Both barrels 16 inch. Previous barrel carbine gas system, new barrel mid length. Cases are ejecting around the 4-5 o'clock position.
Both barrels 16 inch. Previous barrel carbine gas system, new barrel mid length. Cases are ejecting around the 4-5 o'clock position.
You have your answer - the OE and Toolcraft BCGs work, the NBS does not. Since you stated the NBS BCG works in a different rifle, there is likely some tolerance stacking going on - might be with the bolt catch location on the lowers, might be something going on with the buffer/spring/RE combinations. Driving this to a root cause is probably not worth the time and ammo - I'd reinstall either the OE or Toolcraft BCG and then go shoot this thing until a failure actually occurs.
That's my plan
The Hanson barrels have super conservative gas ports to assist with suppressed shooting.
My 13.9 won't lock back full house 77 grain with an h2 buffer without a can.
It really needs a standard buffer if you're not running suppressed. You're probably undergassed with your buffer.
My 13.9 won't lock back full house 77 grain with an h2 buffer without a can.
It really needs a standard buffer if you're not running suppressed. You're probably undergassed with your buffer.
The buffer and spring in the rifle are the oem parts that came with the M&P. I believe these are both standard weight. I'd love to run it suppressed but unfortunately I live in California.
Locks back with a round still in the mag but not with empty mag? something is definitely not right there. That round should push the follower low enough that the follower doesn't engage the bolt catch. If it's locking back the bolt with a round still in the mag there is something else likely stopping the bolt then the bolt catch or something may be out of spec or installed improperly.
How many rounds are through the rifle and now the new barrel & BCG ?
Lubricants ?
Run Dry or "The Wetter the Better" ?
Lubricants ?
Run Dry or "The Wetter the Better" ?
Best reply here IMO. I have 4-5 uppers with the Hanson series barrels and have assembled roughly double that. Great barrels but under gassed for sure. Especially the middys. Anything more than an H, maybe H2, and you'll have issues locking back unsuppressed with most commercial ammo.
I assume you installed the GB per instructions from BA? Block secured in jig, barrel supported, and used the provided cross-pin that comes with the Hanson series barrels/gas blocks?
That M&P was probably subpar (4140 barrel, overgassed, etc.) out of the box so congrats on 'fixing' that upper! I'd grab a second Toolcraft BCG as it's the best out of the trio you've mentioned, and you've confirmed function in your setup. Sounds like you're green to go from there.
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I didn't explain that well, with one round in the mag, once fired it will lock back with oem and toolcraft bcgs but not the NBS.
Based on earlier replies I believe it is just an issue with NBS being crap and the hansen barrel being being conservative with the gas port. Gas black was installed following the instructions and secured with the pin provided. I'm gonna pick up another toolcraft bcg today and call it good.
Sounds like a winning combo!
