Custom Grendel AR with Function Problem

[therbert]

A friend of mine gave me his Grendel AR to play with, because he's having function issues with it. The rifle is built with a 20" heavy barrel on a TNW billet upper and a rifle-length gas system. It's built as kind of a DMR/counter-sniper rifle, and my friend, who has a lot more money than I, has a Nightforce 3.5-15x50 scope on it.

I took the rifle out and fired a small variety of factory ammo and some of my handloads in it, and it very definitely has a problem. It will only occasionally kick out a fired round. The usual behavior is that the bolt is not cycling with enough speed to eject a fired case and pick up a new round. When it does kick out a fired case, it doesn't eject very far.

I cleaned the barrel and chamber, and made sure the bolt and carrier were well lubed, and tried again, with the same result. We also put the upper on another lower with a standard A2 rifle stock, spring and buffer in it, to see if maybe the recoil spring was too stout for it, but no better.

I fired AA factory 123 grain Scenars, and Wolf Gold 120 grain ammo, and my loads that function fine in my Grendel.

I pulled handguard off drove the roll pin out of the gas block/sight assembly (I don't know why anybody would build a rifle like this with that kind of gas block, but that's just me), removed the gas tube and checked it for obstructions, and it seems to be clear. It's leaking gas around the tube and out the back of the block, as there's soot on the outer surface of the tube and on top of the barrel behind the gas block, but I would think that's pretty normal, as there's no sealant in there.

I haven't yet taken the gas block off to check the alignment of the gas port in the barrel with the one in the block, because the rife has a FDE Ceracote finish on it, and that would damage the finish on the barrel ahead of the sight assembly.

Right now, I can think of two non-invasive options: put some sealant on the gas tube to make sure all the gas is getting into the tube and to the bolt, and getting a lighter recoil spring.

Does anyone have any other suggestions?

Thanks in advance,

Tom

[VASCAR2]

I don't know if you could get an idea of the gas block alignment by blowing air into the barrel with an air compressor.

[Vasux86]

Is the bolt locking back when you have an empty mag? If so, your problem shouldn't be with the gas block.

You may have a weak ejector spring that is causing this. The ejector spring is the same as the safety detent spring. You could swap those out and see if it makes a difference. Be sure you don't knock the roll pin for the ejector all the way out. Just tap it out enough to pull the ejector out. The shape of the bolt makes it a pain to get it back in if you remove it completely.

[LRRPF52]

Could be a number of issues:

* Gas block not aligned with port
* Chamber is rough
* Carrier key loose

Pics of the malfunction would help. If it doesn't lock back on an empty mag (should be the first test you do live), it's usually a gas system problem.

[therbert]

I'll check the carrier key when I get home this evening. It never occurred to me to check to see if the bolt locks open on the last round, I never got that far into a mag. I should be able to get out this coming Saturday to check that, weather permitting.

Some additional information on the rifle -- it's damn near brand new, so it's unlikely that the ejector spring is the problem, although I understand that it's possible for a brand-new part to not be up to spec. I doubt it has 200 rounds through it. It started this issue immediately upon purchase, so the owner should have gotten back to the maker on it, but sometimes life gets in the way with things like that. Now it's out of warranty.

I took some video of myself shooting the rifle when I took it out the first time. I'll get that off the camera and upload it to my YouTube page, and maybe you guys can glean something from that. I'll post to let you all know when it's done.

Thanks for all your help so far.

[ah1whiskey]

i was gonna say those-- LOL--

the usual suspects on a a short stroking ar-15-- loose or improperly sealed gas key, bent gas tube, blocked gas port ?

i had one of those el-cheapo aluminum gas blocks that was causing this same issue in a 5.56 it was blowing a lot of smut out around the barrel.

if the gas block is out of line with the port that can be an issue.

the ar is an easy thing to get to run-- once you know its tricks.

[therbert]

Checked the gas key, it is secure. Not the best staking job I've ever seen on the screws, but they're not loose. I'm going to reassemble it tonight, in preparation for testing the bolt hold open this weekend.

[LR1955]

Checked the gas key, it is secure. Not the best staking job I've ever seen on the screws, but they're not loose. I'm going to reassemble it tonight, in preparation for testing the bolt hold open this weekend.

TB:

Although I too think it is an alignment issue with the gas port and block, run a pipe cleaner down the gas key tube to ensure the port that goes into the tube is clear of obstructions.

LR55

[therbert]

Sorry, it has been so long since I started this thread, and haven't been able to follow up -- I'm sure you have all be waiting with baited breath...

I was finally able to get out today to shoot the rifle after reassembly. I shot 11 rounds, one at a time, loaded into the magazine, to test if the last round hold-open was working. Ten of the rounds ejected all right, one did not eject, and in not one case did the bolt hold open. Tonight, I have pulled the rifle back apart, and removed the gas block/sight assembly. According to the carbon marking on the top of the barrel around the gas port in the barrel, the port in the barrel is perfectly centered in the port in the gas block. As far as I can tell by looking into the port in the gas block with a bright light, the hole in the gas tube matches up perfectly with the hole in the gas block when the roll pin is inserted. Although I do not have any pipe cleaners, a piece of stranded #18 copper wire with the plastic insulation passes through the gas key into the bolt carrier with no problem, and there was carbon on the bolt in front of the gas rings. Teh same piece of wire will pass entirely through the gas tube from the receiver end to the gas port, and I did not feel any obstructions or tight spots.

The only remaining cause that I can think of is that the gas port drilled in the barrel is too small. The port in the barrel is the size of a #43 drill, .089". A #42, at .093" does not fit. First, my question would be, is the gas port the correct size? If all else fails, I could pull the gas block from my rifle and measure the port, but mine is a carbine-length system, and this is a rifle-length unit, so they may not be the same.

Thanks everyone for looking at this.

[SG4247]

I have had to drill a few out to .110" on rifle length gas systems.

Sometimes the port gets copper or a burr pushed into the port, reducing the diameter.

On rifle length gas system barrels I have custom made, I have them all drilled to .098".

One of my own drilled to .110" just recently.

Ran like a champ!

[LRRPF52]

Yes, the port is too small, and explains why not enough gas is getting into the Stoner expansion system's chamber formed by the bolt and carrier.

It should be .0935" for RLGS/20".

[therbert]

Thanks very much for the information. I'll put on the milling machine and drill it out with a #42 drill, reassemble and test.

[LRRPF52]

Make sure to limit your depth so you don't go into the other side of the bore. Clean out burrs with a brass brush, go blast.

[therbert]

Done.

This is when I need a range out behind the house!

[SG4247]

Here is a tip,

I install an old brass cleaning rod into the bore, as a stop for the bit.

Keeps me from going too deep, you can immediately see the brass chips come up the bit.