AR bolt won't stay open on last round

MJS

MJS

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Newfoundland
Like the title says,my bolt won't stay open after I fire the last round,before I say anymore here's my build:
7.5" NEA barrel
Pistol length gas tube and NEA gas block
NEA slick side upper
NEA lower
NEA full auto BCG
BCM CHARGING HANDLE
Magpul BAD lever

The buffer spring and buffer are a leftover that came with an old stock assembly.not sure if it's a carbine buffer or not, I'm thinking this may be my problem,would a buffer spring that's too heavy cause the bolt to stay forward after the last shot?

I tried p mags, LAR15's and USGI's, and all the same problem,no hold open on the last round.

Here's the weird thing though.every so often (I put about 300 rounds through it today)it would hold open.

Any of you AR guru's have any sage words of advice?
 
I'm not an expert But I have stayed at a holiday Inn express. The buffer weight and spring would be my first suspect then I would check the bad lever and magazine.
 
Trinimon said:
First thing that caught my eye... BAD lever. Remove that and give it a go again. You might be surprised.
Click to expand...

Yup take it off and see what it does. You might need to shave a bit off the back of the bad lever. Or just throw it in the garbage.
 
Since you say you don't know if it has a carbine buffer and spring I'm going to suggest you do a little research and figure this out. A rifle buffer will not work in a carbine tube as it is too long. If the bolt will lock back when you pull the charge handle it's probably ok. If it is a carbine buffer and spring and the only time you have problems is when you fire it then it's probably the BAD lever.
Did you install the LPK yourself or did you buy a complete lower?
 
I removed the bad lever today.going to give that a shot and see if it fixes the issue.

I did the build myself.no problems with my prior 3 builds and did nothing different on this one other then the BAD lever and the buffer springs.

Are all the battery assist devices(magpul,troy,phase 5) all known to have this issue?i love how fluid it feels being able to use my right hand to reset the bolt without having to use my left hand
 
MJS said:
I removed the bad lever today.going to give that a shot and see if it fixes the issue.

I did the build myself.no problems with my prior 3 builds and did nothing different on this one other then the BAD lever and the buffer springs.

Are all the battery assist devices(magpul,troy,phase 5) all known to have this issue?i love how fluid it feels being able to use my right hand to reset the bolt without having to use my left hand
Click to expand...


I use phase 5 one piece levers on 2 of my AR's and have had no issues.
 
First suggestion is the BAD lever.
If it still doesn't lock back check your buffer weight.
A carbine buffer should be 2.9oz.
If you buy another buffer get an H3 buffer, an H3 contains 3 tungsten weights inside.
You can remove the roll pin and you can mix and match the weights with the 3 steel weights in a carbine buffer to make all the different buffer weights.
 
ok,so i went back to the range today without the BAD lever,no change

I tried out multiple types of mags,Gen 2 pmags,LAR15's,CAA 5 round steel.....the pmags would not lock the bolt at all,one LAR15 would on the odd occasion lock the bolt back,and once or twice the bolt would lock back just behind the mag,but not quite on teh bolt catch....I'm still leaning towards a buffer issue.
 
Check the length of the buffer spring. Pull out the other 2 springs out from the other AR. And put them side by side
 
Start at the beginning.
Remove the buffer and spring and stick a tape measure or ruler into the receiver extension (buffer tube).
It should measure exactly 7" at the top where the upper would meet the lower.
If you have less than 7" then your buffer tube is screwed in too deep.
This can cause lock back issues.
 
ok so the buffer spring in my 7.5" is 10 3/4",my 16" barrel DPMS is 11 1/2".

as for the receiver extension,its 6 7/8" deep,if i back it off anymore the buffer retainer will not stay in.
 
MJS said:
ok so the buffer spring in my 7.5" is 10 3/4",my 16" barrel DPMS is 11 1/2".

as for the receiver extension,its 6 7/8" deep,if i back it off anymore the buffer retainer will not stay in.
Click to expand...
You can always back it off another turn and try it without the retainer.
The only purpose of the retainer is so that you don't play jack in the box every time you disassemble the rifle.

Edit: I seriously doubt that this is the cause of the problem but it doesn't hurt to eliminate it as a cause.
 
looking closely,its appears the buffer assembly may be slightly out of spec,when i disassembled it and then reinstalled it,i had difficulty closing the upper,and it appears as if teh buffer extension is making contact with the upper.
 
I had the same issue. After several issues with the NEA BCG (not made by NEA though I don't think) I decided to by a DD bolt. Never had that issue since.

It's odd though. First 200 rounds I put through my NEA were factory. Always locked open on last. Soon as I tried my own reloads I noticed at first that the bolt rarely locked open. Seemed to improve as the charges got stronger. I assumed it was because the rounds on the lower end of the load didn't have enough jam to lock it open. Even near the max load I was working on it improved but never 100%.

When I changed to a DD, I went back to my load development. Sure enough, even the weakest charge that gave the last bolt fits were no problem for the DD. It has never failed to lock open.
 
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