AR SBR problems

azn_hitman

azn_hitman

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hi guys,

i searched for about 10 minutes and couldn't come up with much. i have a 10.5 inch AR SBR that's giving me loads of problems. after 3 range trips i've decided that its time to get some help. the rifle is set up as the following:

-LMT 10.5 inch upper (was taken apart at the delta ring then put back together by me)
-LMT "full auto" bolt carrier group
-Stag arms stripped lower
-DPMS lower parts kit (with MIAD grip & ambi fire selector)
-Mil-Spec buffer tube
-tried both normal carbine and heavy buffers
-buffer spring that came with the buffer tube
-tried both pmags and metal STANAG mags

the 2 main problems i am having are:
1. the bolt fails to lock back and closes on the last round.
2. the bolt fails to move all the way to the back and hangs right before it reaches the bolt catch.
3. failure to feed, chamber empty while there are still rounds in the mag.

i have a friend who built almost the exact same rifle as me but his is 14.5 inch and is having issues as well. i will post pics of the malfunction later on.

its my first AR build. so go easy on me. any wisdom is greatly appreciated.
 
Last edited:
I was going to say that you are having gas issues until I read this

the bolt fails to move all the way to the back and hangs right before it reaches the bolt catch.
Click to expand...

What do you mean fails to move of the way back? Is it hitting something?
 
Sounds like classic short-stroking to me. Gas issues, most likely. Is the FSB fixed, or is it a gas block?

You could always just mask the problem and buy a KX3 flash hider. :)
 
What ammo?

Try shooting it with one round, empty mag - that will give you abetter idea on the short stroking aspect.

Have you tried it on another lower?
 
Have you tried it with the lighter Semi auto bolt carrier?. Is your shooting stance solid ?I mean is your shoulder moving back when the rifle is fired. A test to figure out if this a cause is to have the rifle rested against a solid object and fired.. Do the problems only occur when the rifle is fired?. What type and bullet wieght is the ammunition?. You are aware that there different length Buffer Springs this also translates into Spring Rates IE: lb's of force to move the spring 1" of compression .
 
Yes, what ammo? Nice to have a rifle work with all types of ammo but sometimes that's not going to happen.

Short stroking = gas problem (not enough)

As Savage suggests lighten up the moving parts ie. go to a semi bolt.

If you're using Remington UMC that stuff is pretty weak.
 
wow i didnt expect so much helpful info in like 5 minutes. thanks guys!

paulinski: its almost like the bolt carrier doesnt have enough force to go all the way back to engage the bolt catch. its stopped in place by friction on the bolt catch. the bolt head it self is in front of the bolt catch.
kevinB: i was using the RUAG stuff and there was some improvements but not much. its hard to find good 5.56 these days. i will try the 1 round in the mag method next time. maybe i should just go to BigRed's house and see if he can work some magic.
canucklehead: the FSB is a standard A2 type sight post put on by LMT at the factory
savage: i have tried the rifle with a bushmaster semi HBAR bolt. and the problem persists. the shooting stance i used was the standard CF "gunfighter" stance so i'd think its solid. bullet weight tried was 47g (i know:p) 55g and RUAG 62g. i am not familiar with the diff spring rates so maybe i should look into that.

thanks again guys
 
Short stroke

azn_hitman said:
hi guys,

i searched for about 10 minutes and couldn't come up with much. i have a 10.5 inch AR SBR that's giving me loads of problems. after 3 range trips i've decided that its time to get some help. the rifle is set up as the following:

-LMT 10.5 inch upper (was taken apart at the delta ring then put back together by me)
-LMT "full auto" bolt carrier group
-Stag arms stripped lower
-DPMS lower parts kit (with MIAD grip & ambi fire selector)
-Mil-Spec buffer tube
-tried both normal carbine and heavy buffers
-buffer spring that came with the buffer tube
-tried both pmags and metal STANAG mags

the 2 main problems i am having are:
1. the bolt fails to lock back and closes on the last round.
2. the bolt fails to move all the way to the back and hangs right before it reaches the bolt catch.
3. failure to feed, chamber empty while there are still rounds in the mag.

i have a friend who built almost the exact same rifle as me but his is 14.5 inch and is having issues as well. i will post pics of the malfunction later on.

its my first AR build. so go easy on me. any wisdom is greatly appreciated.
Click to expand...

If after checking for all routine causes of short stroke - measure barrel gas hole diameter - if .075 or thereabouts , open it up a little - Use someone who can do it while retaining the integrity of the chrome bore.
Some routine causes -
Carrier key not sealed to top of bolt due to uneven machining,not tightened and staked
Bolt gas rings to tight in bolt (burr or bad diameter)
Gas tube misaligned
Gas tube not properly fixed in gas block
Gas block not positioned properly over barrel gas tube hole
Bolt dragging (oversize or burr)
Buffer roll pin not flush and dragging on buffer tube
Buffer spring to strong or to weak or wrong length
Insufficient or excessive lube in the receiver
etc.
Remember AR's sh!t where they eat - shorties have a lot of unburnt powder fouling the receiver quickly and slowing the bolt - keep it clean.
 
Meet up with BigRed.

Swap him bolts - then swap lowers - and see if it that.

I prefer to try to isolate the trouble area, rather than tear the gun apart looking for trouble part by part.

LMT is usually good with QC - so I would intially look elsewhere.
 
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