AR 15 Bolt Will Not Lock back?

[kcofohio]

I had a AR 15 pistol that I converted upper and lower to a rifle. I went with a A2 rifle buffer tube/spring/buffer. The upper is a 20" 6.5 Grendel.
I took it to the range to sight in and try some reloads. The reloads are 120 gr. Sierra's with Tac, starting point of 28.0 gr. and also some 28.4 gr. Start load is 25.8, max is 28.7 gr.
When shooting, the bolt wouldn't lock back on last round. Even pulling the charging handle back, it would lock back. If I pushed hard on the bolt release, it locked back until I let off. Then the bolt would go into battery. There is no sign of breakage on the bolt release lever. This lower, the only thing I changed was the buffer tube/buffer/ spring. I even reused the same bolt carrier, only changed out the bolt for the 6.5 Grendel.
So I did some changing around, swapping bolt carriers out. That didn't work. Swapped back. Then put the upper onto a carbine lower, and it locked back manually. I put the carbine upper on the rifle lower, and it wouldn't stay locked back.
My question, could the buffer spring be too strong? Or is the buffer too long? The tube/spring/buffer came as one package.
TIA!

 

[rsrocket1]

The bolt should lock back with an empty magazine in it. Put the empty magazine in it and see if the bottom part of the bolt release lever toggles inward and the oval top section moves outward. If it doesn't, the tab at the back of the magazine follower is not pushing on the inside of the bolt release lever. Try it with multiple magazines. Here's a method of fixing Gen 3 Pmags where the user scrapes away a little of the rear of he magazine interior to get reliable lock back.

Fix: Empty Pmag gen 3 doesn't catch/lock bolt

AR platform is new to me, and I hope I don't upset anyone here because I am a noob when it comes to ARs. I recently assembled this Noveske rifle and bought 3...

youtu.be

 

[kcofohio]

Yes, I agree. But even if I had more magazines, it should still lock back when I manipulate the lock back lever.

If I press in hard, the bolt will stay back. Once I quit pressing, the bolt releases.

 

[rsrocket1]

Does this happen even without a magzine in the lower? Meaning does the bolt release as soon as you let go from pulling the upper part of the lever. If so, there’s a problem with the assembly of the catch.

Are you sure that’s the right spring under the bolt release and not one that is too long? I would personally disassemble all the lower parts associated with the bolt catch, check the parts and reassemble.

If that’s the wrong spring, you may have the wrong spring somewhere else too.

 

[kcofohio]

Does this happen even without a magzine in the lower? Meaning does the bolt release as soon as you let go from pulling the upper part of the lever. If so, there’s a problem with the assembly of the catch.

Are you sure that’s the right spring under the bolt release and not one that is too long? I would personally disassemble all the lower parts associated with the bolt catch, check the parts and reassemble.

If that’s the wrong spring, you may have the wrong spring somewhere else too.

I will check. I'm going to look through my spare parts to see if I have an extra catch and spring and swap them. It worked before with the 300 BO pistol. But the weakness may not have showed with the pistol buffer and spring.

 

[rsrocket1]

I would get one of your other lowers, put the 6.5 upper on it (it shouldn't matter which upper), put an empty mag in the lower, pull the charge handle back and see that the bolt locks back. Pay attention to the bolt catch lever as you pull the bolt back and watch the lower part move inward. Then put your new lower on the upper and put an empty magazine in it and do the same thing. My guess is you won't see the lever move with the new lower while you will see a small double dip in a working lower as you pull the bolt back and that will lock the bolt open.

Another close view of your picture shows the roll pin is not centered in the lower receiver frame. It may not be in far enough to allow smooth motion of the lever.

 

[HKGuns]

That bolt release spring in your picture looks funny to my eyes. Are you sure the right spring was used in the bolt catch? It appears, I may be wrong, that the spring has slipped over the detent in that bolt release.

 

[kcofohio]

I took the uppers off both lowers and compared the catch position with and without magazines. Both lowers with the magazine in, the catches were both at their peak. I saw no differences and no sticking in one position or the other.

After switching between the uppers, when I placed the 6.5 onto the rifle lower, I noticed considerable slop between the upper and lower. I had missed that before. I will check the pins.

But in the meantime I have a couple spare lowers that I will check for tight fit and build up for the rifle.

Thanks for all the input and I will give and update.

 

[kcofohio]

I tried to get a decent picture of the slop.

 

[bfoosh006]

Can you take a photo of the bolt face, when "locked" back ? Also its depressed bolt catch without the upper / ... looking for complete travel / pivot of the bolt catch into the upper.

And when you pull the charging handle back.... can you poke a screw driver onto the bolt face and see if you can press the entire BCG further into the buffer tube ?
"If" your BCG travel is limited / restricted... the bolt catch might not be allowed to catch fully.

You should have 3/16" ish additional travel when pushing on the bolt face, in between the bolt catch face and bolt face.

PS DON"T Let the screw driver fly into your face .... Since your bolt catch isn't working !!!
Wear Safety Glasses !

 

[4v50 Gary]

Try different magazines.

 

[Ritchie]

Make sure the stock screw and spacer are correct. If the screw is too long it may limit the movement of the buffer/bolt.

 

[kcofohio]

Can you take a photo of the bolt face, when "locked" back ? Also its depressed bolt catch without the upper / ... looking for complete travel / pivot of the bolt catch into the upper.

And when you pull the charging handle back.... can you poke a screw driver onto the bolt face and see if you can press the entire BCG further into the buffer tube ?
"If" your BCG travel is limited / restricted... the bolt catch might not be allowed to catch fully.

You should have 3/16" ish additional travel when pushing on the bolt face, in between the bolt catch face and bolt face.

PS DON"T Let the screw driver fly into your face .... Since your bolt catch isn't working !!!
Wear Safety Glasses !

I checked out the bolt travel. With the charging handle pulled back, the bolt stops just before the back side of the locking lugs reach the bolt catch.

After switching out the buffer and spring with the one from the carbine, now it will lock back with an empty magazine.

 

[rsrocket1]

So the problem was putting a rifle buffer and spring into a carbine stock?

 

[illinoisburt]

So the problem was putting a rifle buffer and spring into a carbine stock?

Well that would certainly do it. The rifle buffer and spring combo are at least 3/4" longer when compressed versus a carbine buffer/spring combo.

 

[kcofohio]

So the problem was putting a rifle buffer and spring into a carbine stock?

No, it is a A2 rifle style buffer tube.

 

[rsrocket1]

It still sounds like the rifle buffer and spring combo are either too long for your A2 buffer tube or somehow they are binding up as you pull the bolt back and they won’t allow it to go back far enough to lock it.

As a test, see if your bolt locks back with the rifle spring and carbine buffer, then the carbine spring and rifle buffer.

 

[kcofohio]

It still sounds like the rifle buffer and spring combo are either too long for your A2 buffer tube or somehow they are binding up as you pull the bolt back and they won’t allow it to go back far enough to lock it.

As a test, see if your bolt locks back with the rifle spring and carbine buffer, then the carbine spring and rifle buffer.

I will give that a try tonight.

Here is the differences between the buffers/springs.

 

[kcofohio]

Just a side note, in reference to post #8. The slop between the upper and lower is from the upper when placing other lowers on. So I plan on buying another stripped upper.

 

[citizenconn]

So the problem was putting a rifle buffer and spring into a carbine stock?

I made that mistake one time while assembling a batch of ARs and it took me probably 30 minutes or more to figure it out.

 

[rsrocket1]

That explains the length difference but your A2 tube should allow the spring to compress without “bottoming out” or hitting a hard stop. BTW, the picture is somewhat misleading. The rifle buffer only inserts into the spring up to the first ridge, not all the way to the end cap as it does with the carbine buffer. The top portion sits above the spring.

 

[BushMaster-15]

I will check. I'm going to look through my spare parts to see if I have an extra catch and spring and swap them. It worked before with the 300 BO pistol. But the weakness may not have showed with the pistol buffer and spring.

Buffer & return spring WONT affect Latch assembly or lock back ,however will affect individual rounds feeding and extraction as well as failure to lock back by puny gas activation aka short cycling issues . Shot @ 200 yd. ( IMR 4064 just failed to cycle and IMR4895 failed to lock back in MY 6.5CM )
I purposely shot the orange square with N540 ,in case anyone was wondering .
I've seen this time and time again ; Persons swapping buffers springs carbine rifle pistol and it most certainly will effect cycling and extraction .
Normally gas bolt timing shows up with ejection port jambs or ripping rims scenarios .
A good many knowledgeable builders will purchase super brand of spring buffer set ,only to find the combo fails in their particular application . Extra heavy buffer spring will also negate Latch catch .

​

 

[rsrocket1]

As a reference, I took the lowers off my guns and pushed the buffer back until it stopped and measured the depth. The rifle buffer and spring in my fixed stock A1 measured a depth of 3-3/4". I did the same with the carbine buffer and spring in my adjustable stock carbine lowers and they all measured the same 3.75".

If your installed buffer and spring can't be pushed back that far, you've found the problem of not being able to pull back the charge handle enough to get the bolt to lock with the rifle spring and buffer but were able to do so with the carbine spring and buffer.

 

[kcofohio]

Here is a picture of the rifle buffer and spring.

It's back the furthest it will go, even when trying to push it back as suggested by @bfoosh006

 

[rsrocket1]

Take out your buffer and spring and measure the inside depth of your buffer tube. My rifle length buffer tube is 9.5” from the back of the tube to the spring loaded retention pin. My carbine tube measures 7.5” so when you put your carbine spring and buffer into the A2 tube, you have 2 more inches of depth and can easily pull the bolt back to where it locks.

When you put your rifle spring and buffer into that same tube, something is preventing that spring and buffer to compress enough to get the bolt all the way back.

Either the spring dimension is wrong (unlikely) or there is something wrong with the buffer tube. It could be an obstruction or a bend. You should get a flashlight and examine the inside of your buffer tube. Also measure the inside depth just to make sure.