Bolt not locking back

[Jakal]

Ok Horde I need some help here.

Firearm:: New 6.5 Grendel built by myself
UPPER::
Anderson Stripped upper; Cerro Forge (Keyhole I.D. same as Colt) http://www.masdefense.com/cerro-forge.html
Rock River Arms Upper parts kit
Lilja AR740 profile 1:8 Right-hand twist
SLR Set Screw .750 adjustable gas block
Rifle length Black Nitride Gas tube
Samson Evolution 15” Keymod rail
Larue QD LT745 20MOA mount
Zeiss Conquest HD5 5-25x50

LOWER::
Anderson Stripped lower
Rock River Arms Lower parts kit
Geissele SSAE trigger
Magpul PRS Rifle buffer tube, buffer and spring bought as a set from Brownell’s
Houge Overmolded grip
Maxim Firearms Bolt and Bolt Carrier
New Alexander Arms mags 5 and 10 round w/blue follower
Hornady 123grn SST factory loads

Condensed History:: After completing build, took it out for a test drive. Had feed problems, pierced primers and bolt not locking back or locking back on bolt carrier.

Found this explaining the 6PPC feed issue and that the ejector needs a radius. Followed directions, seems to have fixed this issue.
http://www.65grendel.com/forum/showthrea...issue-help!!!
http://bulletin.accurateshooter.com/tag/6-5-grendel/

Found this on the pierced primers and followed directions; seems to have fixed this issue.
http://thefiringline.com/forums/showthre...414&page=2

The part I am having trouble with is the bolt locking back.
I started with a new SLR Clamp on .750 gas block. I wound up with this all the way open and it would still only lock back about a third of the time. Called SLR and explained to Todd what was going on. He sent me out a set screw style with the thinking that the geometry of the clamp on may not allow the needed seal to channel all gas to the BCG. The new set screw gas block was installed and I sat the gas block right on the journal of the barrel. I immediately saw improvement. The bolt started locking back on position 8. I shot 1 round at a time to verify locking, after about 20-26 rounds the bolt started to lock back on the bolt carrier again but not to the amount that the original gas block was locking on the bolt carrier. I adjusted up to position 10 and a total of 60 rounds but was still having the same results.

I can open this up to positon 15 when I go back out but was afraid of over-gassing and running the BCG to fast.

I asked around on the board and it seems that most guys running a SLR with a Lilja barrel opens the gas block to between 7 and 10 positions. The gas port on the barrel is to the drawing spec on Lilja’s web site.

So far I have 320 rounds thru the pipe and am starting to spend too much on factory rounds trying to troubleshoot this. I need to get it running correctly before I start the load development part of the program though.

Also, the Hornady SST ammo seems to be really dirty not sure if this is part of the gas problem or not, but it is the worst I have ever seen an AR operate at.

Any help/advice would be appreciated!

[rwh]

The first thing to check is that the gas block lines up correctly with the gas port. A borescope makes it easy as you can look up into the gas block through the gas port. Second best is to use some calipers to measure the distance from the shoulder to the gas port and compare that with the distance from the back of the gas block to the hole. Sometimes a shim is needed to make the two holes line up.

After that make sure your bolt operates smoothly when you put it together and make sure it slides in and out of battery without binding. If the problem is the ammunition then the rifle will lock back on the first shots and then stop locking back as the bolt gets dirty.

If that doesn't fix it you might need to try a lighter spring. All rifle length springs should be the same, but some are stiffer than others.

[dmsims21]

Sometimes the bolt not locking back is caused by being over gassed.
The bolt moves back and rebounds forward so fast that the follower doesn't have time to move up and catch the bolt.
The thread on the pierced primers was interesting. No one had the cause exactly right. The guy probably had a mil spec firing pin and a mil spec bolt that was machined deeper for the thicker case rim.

[Sputnik]

The first thing to check is that the gas block lines up correctly with the gas port. A borescope makes it easy as you can look up into the gas block through the gas port. Second best is to use some calipers to measure the distance from the shoulder to the gas port and compare that with the distance from the back of the gas block to the hole. Sometimes a shim is needed to make the two holes line up.

+1 especially with the Lilja barrels. I seem to remember others having the same issue as the Lilja offers room for the forward retaining cap needed for some forearms.

[rwh]

Two more things to check, is the bolt catch moving freely? And if so try some different magazines.

[LRRPF52]

AA Grendel bolts are longer than 5.56 bolts, so the tail of the bolt takes up the deeper bolt face recess. Short story: You use regular AR15 firing pins. Measure your bolt length. It should be 2.810".

Next, RLGS and the 6.5 Grendel don't need the adjustable gas block if you have a 20" barrel, and Lilja cut the ports to the correct diameters. Yours should measure .094". Run it with full, open gas. Double check the gas block alignment to the ports, not the shoulders.

Your bolt carrier needs to be well lubricated as well.

[montana]

[ATTACH=CONFIG]5790[/ATTACH][ATTACH=CONFIG]5791[/ATTACH]

I have almost the same rifle. I have the same clamp on SLR adj gas block, trigger, and barrel.
My SLR clamp on gas block is set on #9 and runs flawlessly with both Factory Hornady 123gr amax and SST ammo. I would check everything the others have suggested. Check the gas tube and make sure it is centered with BC key, and the gas key on the BCG is sealed tight. Make sure the gas block port is centered on the barrel port. This is very important. I have never had a problem with either my Syrac or SLR adjustable gas blocks sealing on my AA or Lilja barrels. I have had stellar performance with both the screw and clamp on versions.
dmsims21 is correct about over gassing .

[bigbear_98]

I'm probably going to get stomped on for this....

I originally built my rifle, a 20" krieger, with a carbine buffer tube, buffer, and spring. The gun ran well. I bought a magpul PRS and rifle buffer and spring. All of a sudden had all kinds of issue. Pressure signs, not locking back, etc. I talked to a bunch of folks, posted here, etc. Most everyone told me to get an adjustable gas block because my suppressor was causing an issue. The gas block made things worse.
A local guru told me to get a spikes buffer spacer and go back to the carbine spring and buffer. Viola. Swapped back to a none adjustable gas block and all is perfect in the world again.
Long story short, my buffer was too heavy, causing the bolt to stay closed too long (pressure), and keeping it from locking back.

[bigbear_98]

Montana. Love the color! cerakote?

[montana]

Montana. Love the color! cerakote?

No one is going to stomp on you , and yes it is Cerakote. Sometimes things can get confusing with everyone giving advice that seems to contradict each other. In the end the person with the problem hopefully has enough advice to learn from others successes and failures to solve the problem. I don't always agree with others but I have also learned a lot from those same people. Building a reliable rifle can be a fun but some times frustrating journey so every ones opinion is welcome.

[NugginFutz]

AA Grendel bolts are longer than 5.56 bolts, so the tail of the bolt takes up the deeper bolt face recess. Short story: You use regular AR15 firing pins. Measure your bolt length. It should be 2.810".

Next, RLGS and the 6.5 Grendel don't need the adjustable gas block if you have a 20" barrel, and Lilja cut the ports to the correct diameters. Yours should measure .094". Run it with full, open gas. Double check the gas block alignment to the ports, not the shoulders.

Your bolt carrier needs to be well lubricated as well.

^^^ wut he sed.

I'd also like to clarify a point that, until last weekend, I wouldn't have considered in need of explanation. When LRRPF52 says "well lubricated", I'm quite certain he means with a quality oil or gun lube (many preferences among the horde), but not grease. As I was getting ready to leave the range, someone (a noob with his first AR) was having some FTF (fail to fire) issues, and asked me to look at it. I agreed, and looked it over.

I noticed that the charging handle pulled back with an unusual resistance, so I broke down the upper. The bolt carrier and bolt were both coated with a layer of a very tacky grease, which the owner could not identify. He'd just bought this 5.56 carbine from somebody who'd built it, and was told nothing about it aside from "it's an AR". (I'd could have guessed it was a homebuilt due the the Front Sight being mounted backward on the rail, ala "Buck Rogers".

I cleaned the bolt and carrier as best I could using CLR (the only thing resembling a solvent we had at the range) and got it to cycle much better. His FTF issues improved, (went from a reported 1 in 4 FTF's, to all the shots I witnessed going bang). I showed him how to function check his weapon, taking extra care to explain the need to ensure it goes fully into battery, and left him in the capable hands of a shooting buddy.

Ok, so it's not necessarily related to the thread, but it is a real-life experience I thought I'd share. Grease <> Oil. As I recall one of the Horde putting it, "Grease is for rotating parts, Oil is for sliding / reciprocating parts." or words to that effect.

[cst]

I have the same problem with Lilja 319....had to turn my adjustable SLR gas block way down..5 clicks from close...when the bolt moves too fast it wont engage the mag to lock back..

[bigbear_98]

I have the same problem with Lilja 319....had to turn my adjustable SLR gas block way down..5 clicks from close...when the bolt moves too fast it wont engage the mag to lock back..

I've had this happen as well but on a midlength 223.

[Errorhead]

I'm probably going to get stomped on for this....

I originally built my rifle, a 20" krieger, with a carbine buffer tube, buffer, and spring. The gun ran well. I bought a magpul PRS and rifle buffer and spring. All of a sudden had all kinds of issue. Pressure signs, not locking back, etc. I talked to a bunch of folks, posted here, etc. Most everyone told me to get an adjustable gas block because my suppressor was causing an issue. The gas block made things worse.
A local guru told me to get a spikes buffer spacer and go back to the carbine spring and buffer. Viola. Swapped back to a none adjustable gas block and all is perfect in the world again.
Long story short, my buffer was too heavy, causing the bolt to stay closed too long (pressure), and keeping it from locking back.

This is interesting, I had a sabre defense 14.5, with mid length gas system, had problems with two different BCGs, try a lot of things and ammo. I finally put in a heavy buffer with the stock spring. Two years ago and about 550 rounds ago ( I shoot my 24" over watch more!) and not one hiccup!

[dmsims21]

I

I have the same problem with Lilja 319....had to turn my adjustable SLR gas block way down..5 clicks from close...when the bolt moves too fast it wont engage the mag to lock back..

I also have a Lilja 319. Works fine with 8208, but with CFE-223 and Factory SST, I am over gassed.
I guess I need to get an adjustable gas block.
CFE is accurate in my gun and meters so much better than 8208. I would hate to give it up.

[Jakal]

This is my 1st Grendel build but not my first AR build, so some of the ideas put forth have been verified before posting!
The 1st gas block was put on the barrel using a hand guard cap as a spacer that I had cut into 2 pieces so that the top part would fit over the gas tube and fit between the gas block and the barrel journal (shoulder). By my measurement using calipers it came to .030 distance. This should be normal with that type of configuration.
After shooting 60 rounds, I pulled the gas block and saw a carbon 8 on the barrel meaning that the gas block and gas port were not lined up correctly. The .030 was too far out, so I moved it to the barrel shoulder. I shot about 30 rounds and removed the gas block and verified no more carbon 8 on the barrel.

The second gas block being a set screw, I was able to take the screws out and measure from the back of the block to the closest edge of the block gas port and it measured .294. I then measured from the barrel shoulder to the closest edge of the barrel gas port and it measured out to .294. This told me to mount the gas block right up on the shoulder with no gap.

I only have 3-- 6.5 mags at this time. The bolt and carrier is from Maxim Firearms as directed by Lilja for proper headspacing, bolt measured at 2.808”. The carrier moves freely in and out of the buffer tube. I use Hoppes gun oil on the bolt and carrier. No grease.

From what I can tell, the gas tube and the gas key line up with no binding. I tried a 5.56 bolt and BCG (not live fire) to see if any binding and it felt the same as the 6.5 bolt and BCG. Gas key is staked from the side and looks like a good seal around the BCG.

Sounds like we are about split on being over-gassed or under-gassed. I can run the gas again from 1 to 15 or from 15 to 1 and see how it operates.

To BigBear98’s post, I will put a CAR lower on and see if any changes or not. I also have another carrier that I can swap out.

I have heard that sometimes the Bravo Company Mod4 charging handles cause an issue but I swapped that out with a regular charging handle and still the same issue.

This leads me to believe either over or under gassed or a buffer/spring issue or a combination of these 4 items.

Thanks for the input and I tried to respond to everyone’s suggestions. I’ll try the few things mentioned here and will report back.

[LRRPF52]

What is your ejection pattern? Clockwise

[Jakal]

My case mouths have a slight dent from hitting the brass deflector so I would have to say about 3:30 to 4:30

[Slappy]

Do you have a muzzle brake and what length is it (the barrel)? Or did I miss that somewhere? BANG BANG!!

[arnie]

Is your barrel coated with any thing .I had one that had a Teflon mat black coating and it was blocking some of the gas port .had to run a .093 drill through it to clean it out .It locked back ok after that .