New Grendel build finally completed, now gas or buffer issue...

[bigbrianaustin]

I finally finished my first Grendel build with the help of a few friends and took it out yesterday to the range. Started at 25 yards to get on paper and it only took 5 shots to get 2 on top of eachother. Had no problems and seemed to run flawlessly ejecting with good trajectory.

Then I moved to 50 yards and after some quick adjustments for bullet rise, shot this group of 5.


I was very happy, although I knew it was only 50, so then stretched it out to 100, and that was where I started having issues (although I know the change in distance wasn't what caused it).

After my first shot at 100, it ejected, but did not load a new round. I racked the CH back, fired again, and the same thing happened again. From here on out, it always ejected, but never picked up the next round off the mag. I tried several different magazines to rule that out as well, but no luck.

My first thought was undergassed, but it didn't quite make sense with the first 15 rounds fired. I also had done some reading when building about the difference when running a rifle length gas system with a carbine buffer/spring, so i thought it might have been an issue there. Because I originally had an ACE Skeleton stock on the Grendel, I used the rifle buffer spring with a carbine buffer after reading about the combo on arfcom. That is what was in it to begin with, so since I had issues, I swapped in a standard carbine spring and buffer and tried those. Still ejected, but would not pick the next round up. I am under the impression that I can't use a rifle buffer in the carbine length tube without high risk of it cracking the rear.

So here's my setup:
20" BHW Medium Heavy barrel
Rifle Length Gas System- Did not open up the port at all
Centurion Gas Block
Rifle Buffer Spring and Carbine Buffer
RGuns BCG

Any help or tips would be greatly appreciated, as this is going to be my hunting rifle and deer season just opened here in Oregon. The accuracy is not an issue, because even manually racking it every time, I am shooting sub-MOA groups, but I want to get the reliability down. Thanks!!

[cory]

I'd suspect over gassed or bad mag. Leaning towards the mag.

Where was the brass landing?

Did you try a different magazine?

[bigbrianaustin]

I tried the 5 round mag I started with, then a 9 round, then a 25 round, and they all seemed to do the same thing. What could be happening if it is overgassed that would cause the brass to eject fine, but not allow it to pick up another round from the mag? The brass was ejecting around 5-8 feet back, and appeared to have a good arc to it.

[LRRPF52]

Rifle Buffer Spring and Carbine Buffer

You'll kill your spring that way, as the coils should never impact each other. You need a carbine spring with a carbine buffer.

Need to know what load you are shooting, factory or handload, details.

What weight is your buffer, 2.9oz, H, H1, H2, or H3?

Your BCG isn't cycling enough to the rear. Was your gas block installed with careful consideration taken into account as to how it aligns with the gas port, or was it simply abutted to the shoulder of the barrel at the gas block journal? How is it secured, and did you take any measures to ensure alignment with the carrier key in the bolt carrier, with the bolt removed to test articulation?

[bigbrianaustin]

I was shooting Precision Firearms Sierra 123gr MK's. I will have to figure out the buffer weights, as I previously weighed them, but didn't mark them. I will have to look back at the thread on arfcom where it was suggested to run a carbine buffer with a rifle spring. Maybe I misunderstood the post. I switched to a carbine buffer and spring after the first failures and even put my Larue Super Duty Chrome Silicone spring in with a carbine buffer and it didn't change anything.

I marked and aligned the gas block for center of the port and the tube feels identical in the carrier key when moving it manually to all my other AR's. I'm wondering if being its virgin shoot that something from the install process worked its way out and either clogged the gas system after the first 15rds, or if it had just enough oomph to get it back then, but once it got a little dirty, didn't quite have enough. Thanks everyone for the input so far. Will probably tear it back down and see if I can find anything obvious.

[NugginFutz]

My BHW 22" barrel (RLGS) had a bit of debris in the gas port, which caused a similar issue. The load then and now is a 123 Amax over 28.0 grains of 8208 XBR. I successfully cleared it with the smooth end of a drill bit. A couple of swipes with a bronze brush ensured I had no burr or debris in the barrel.

It sounds like you've aligned your GB and Gas Port, but be aware the BHW barrels need that ~0.025" gap between block and shoulder, unless your GB is manufactured to butt up to the shoulder.

Lastly, mine runs quite well with both a standard carbine length spring and buffer, and standard rifle length spring and buffer. No over/under weight buffers were needed. I did, however, install a BTE adjustable gas block to tame the case mouth denting I was getting, once I had it cycling. It now delivers brass into neat little piles ~8 feet out, at 4 o'clock.

Nice looking rifle, btw.

[bigbrianaustin]

Thanks NugginFutz, I am actually getting the case mouth dents as well, and was wondering what I could do to fix that. I think I will get into an adjustable gas block soon and see if that helps. I will take another look at the gas block and ports to make sure everything still lines up tonight.

[cory]

I completely missed that rifle spring and carbine buffer setup.

I'd correct that before you do anything. I do suspect this is an over gassed issue. IMHO if you were under gassed you'd be getting failure to eject.

[bigbrianaustin]

Ok Cory, I did switch the buffer and spring out completely and was having the same result. If I am overgassed, how would I solve that? Never had that issue before.

[cory]

Heavier buffer or adjustable gas block.

[lamrith]

Was the entire build new? When I built my 1st AR I had cycling issues, even with rounds that I now know were on the sun side of hot loads. I just started running it dripping wet for the 1st 200 or so rounds. Now it cycles like a champ with 2grains less powder using the same case, primer and bullets.

I am wondering if it was nice and lubed for those 1st 15 rounds, but then got dirty and dry and started acting up? Maybe do a full clean and lube it up and try again before you change out hardware? Like you said she ran good at 1st..

Maybe try some other ammo, grab a box of Amax to try?

[NugginFutz]

Come to think of it, I had a short stroke issue, first, and over gassed, after I cleared the gas port. Memory, they say, is the second thing to go...

[LRRPF52]

You are having short stroke issues. In addition to checking the gas block alignment and possible gas port obstruction, check that your carrier key is not loose. This can be very difficult to see, so look and feel very closely.

Take another bolt carrier, and see if the same thing happens. Load only one cartridge, with the magazine, and see if it locks back on firing.

[bigbrianaustin]

So I took the gas block and tube back off last night, and based on the carbon pattern, it looks like the block was lined up dead center on the port. The gas key is definitely tight and the rings are not causing any sort of leak, so I think it must have been the dirt and grime. I did run the BCG pretty wet to start off, but may need to do a little more to the buffer and spring. I weighed the buffers I have and they are both standard Carbine weight, so I'm thinking with the Chrome Silocone spring and a better lube job, maybe some Froglube, I will go back out and see how it cycles tonight. May be able to wrangle up a heavier buffer to help if it is overgassed as well. Thanks again for all the help everyone!

[cory]

Did you get the Rguns semi auto or full auto bolt carrier. It's normally best that a mil spec full auto m16 bolt carrier be used for the Grendel.

[bigbrianaustin]

I traded into the rifle in a package deal, and he already had the Rguns in there, so I'm not sure. Is there an easy way to tell FA vs. SA?

[bigbrianaustin]

For clarification, I got the gun in a very different form and tore it apart to build this, but it had only had a few rounds through it at that point. I pretty much only used the barrel and the BCG from it.

[cory]

There should be a difference in weight. Someone here probably know the which weighs what. Hopefully they'll jump in.

[montana]

http://www.youtube.com/watch?v=1_1B6ltGWCU

[davidj]

Thanks Montana.