New build 6.5 Grendel J&T upper: bolt doesn't lock open after last round

[gdaddy]

Went to range to test out new 6.5 build; used 2 different magazines and three different types of ammo: Hornaday 123gr Vmax, Wolf 120gr and 123gr. Gun shoots well but bolt doesn't lock open after last round. I can manually lock bolt open no problem.
The J&T upper has 20in barel with free float tube. I do have a milspec 6 position stock with 3 oz buffer which I suspect may be my problem. I have ordered a H2 buffer which is about 5 oz based on reading other threads on this forum.
I also had one stove pipe round today which was the last round.
Any ideas appreciated.

[fkleinbu]

I would look closely at the bolt stop in the lower..

When you say you can lock the bolt open manually, are you doing that just by drawing the bolt back with the charge handle with an empty man in place??

Also same brand of magazines or different..

Since you said "to test a new 6.5 build" can I assume this was the first time the rifle was fired??

flk k

[gdaddy]

Yes, I am just drawing bolt back with charging handle and pressing down on bolt stop. Both magazines are AR Stoner; I was loading 5 rounds each time.
This was my first time to fire the weapon. I used a DPMS stripped lower receiver and a lower pats kit from Palmetto State Armory. I do have another bolt stop I could try.

The rifle was very accurate: shot 50 yards first to get on paper then moved to 100 yards. This was primarily a shake down test and was largely successful. My objective is to use the 6.5 as a hog gun and deer rifle. I have several grandsons that would love to shoot it :-)

[rasp65]

Did you have the carrier and bolt rings well lubed? The other possibility is the gas port is not properly aligned with the gas block. The heavier buffer will actually slow down the carrier due to its mass. It will help if the bolt carrier velocity is too high causing the carrier to override the bolt stop. Check these things out and let us know. It is important to the grandkids.

[RangerRick]

You should be able to pull the charging handle back with an empty mag in place and have it lock without touching the bolt release. If it doesn't lock, something is wrong with the bolt release mechanism.

RR

[fkleinbu]

RangerRick, that is exactly why I wanted clarification from gdaddy if the bold stop was being presses up when the charge handle was pulled back..

But in addition to the bolt release mechanism (most likely culprit) it could be a problem with the magazines..

By the way, in both Walt Kuleck/Clint McKee's (The Complete AT-15 Assembly Guide Vol. 2) and Glen D. Zediker's (The Completive AR15 Builder's Guide) books they discuss a number of checks to perform on a new build.. And you got it, checking the bolt remains open on an empty mag is one of them..

P.S. I do like Glen's book a little better of the two, but if you were only getting one of the books, I’d go with Walt’s.. But be warned.. The in the first part of Glen’s book, he goes over all of the tools.. If you are a tool horde (as I am) you could spend a small fortune if you get all of the tools.. Fortunately, to build lowers, you only really need very few..

Good luck gdaddy with the upper, and in getting the bolt hold open fixed.. flk k

[gdaddy]

Checked the bolt lock with empty mag and it works every time; both magazines.

One thing I did notice while trying to find something different between this 6.5 and another AR carbine is that the bolt lock spring seems weaker on the 6.5 build. On the carbine I would guess it takes about twice the pressure to depress the bolt lock versus the 6.5?? I don't have a gauge I was just using my little finger as a weak point to test with.

Any chance this spring tension would have an effect?

Thanks for all the troubleshooting help,
Gdaddy

[fkleinbu]

It very well could be the spring.. How does that 6.5 upper work on the other lower, or can you not use it on the other lower?? flk k

[gdaddy]

Since this was a new lower I haven't tried it with any other uppers; should have :-) That would isolate issue for sure.

I have changed the bolt lock spring with a new DPMS spring ( it does seem much stronger now) and will go to indoor range tomorrrow and test it out. I'll also try the new lower with a Stag 6.8spc upper that I have.

I'll update forum after trip to indoor range.

Thanks again for the help.
Gdaddy

[gdaddy]

Went to range this morning, test fired several times same issue. Used a 6.8spc mag that I had before trying a different lower and bolt latched open. Tried several times with a mix of rounds and the 6.8 mag worked every time. Went back to 6.5 mags still failed.
So both my AR Stoner mags exhibit same issue.
Any mag recommendations? other than I just keep using my 6.8 mags :-)
Gdaddy