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.htmlhttp://www.65grendel.com/forum/showt...ing-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/showthread.php?t=551414&page=2

[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.

[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]

IMG_0724.jpgIMG_0732.jpg

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]

[LRRPF52]

What is your ejection pattern? Clockwise