For process of elimination, start with a thorough cleaning.  This would include using a chamber brush and good solvent, make sure the chamber is clean, then swab out the chamber with some denatured alcohol to confirm that there is no oils or solvents left in the chamber.

Inspect the bolt carrier key (gas key) where it mates with the bolt carrier, specifically looking for any signs carbon deposits blowing out from underneath (usually at a 45 degree rearward and downward angle if there is a leak, which would indicate a possible broken gas key screw).

Thoroughly clean the bolt carrier assembly.  While cleaning it, check that the gas rings have a proper seal.  Then inspect to see if the gas key is tight, and that it is properly staked.

If the rifle has a gas block, and is held onto the barrel with screws or a clamp, inspect to see if it is tight and properly aligned with the gas port.

Inspect the build to see that the gas tube is in proper alignment with the gas key, as the gas tube goes inside the gas key about 3/4".  Turn the upper receiver upside down, and look into the charging handle track.  You should see about 1" of gas tube sticking out in the charging handle track.  Is the gas tube centered inside the charging handle track?  If the gas tube is diagonally off center, this may be a contributing factor where the barrel not is not in proper alignment, or the gas block/front sight base is off center and is causing the gas tube to be off center with the gas key.  If the gas tube is off center, then it will rub and cause resistance as it mates with the gas key, and this rubbing can be just enough resistance to cause the issues you are experiencing, another part of this rubbing can also wear down the back end of the gas tube (There is about 1/8" of the back of the gas tube that is slightly flared out, which creates a seal inside the gas key, if this seal rubs off then it can be the cause of lack of enough gas pressure and will result in short stroke issues on some short barrels).  A simple inspection test for this, is to take just the bolt carrier (No bolt, cam pin, firing pin or retaining pin), and slowly insert it into the upper receiver, wiggle and jiggle it as you insert it all the way forward, it should be a loose fit as it goes in/out of the upper receiver and not bind anywhere.

I would suggest using a standard carbine buffer, with standard carbine action spring (Buffer spring), and see if you get any different results.

Then when you go to shoot again, I would suggest soaking all 7 bolt lugs with a quality lubricant, my preference is Slip2000 "EWL" Extreme Weapons Lube.  Then try loading 1 round of ammunition at a time, and see if the bolt carrier locks back on the empty magazine.  


CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

From the pic, the gas tube appears to be off center.  Since you have a pinned front sight base, look at the barrel nut where the gas tube passes through it as sometimes you can visually see if there is a very little space on both sides of the gas tube where the gas tube will freely pass by the teeth without touching.  If it is touching on one side, and you have space on the other side, you might be able to barely move the barrel nut slightly to even the space out.  If there is no space on either side, it is a tight fit, and you can still adjust the barrel nut about a hair or two to either side to get the gas tube centered until it if free floating in a sense in the charging handle track centered with no resistance that pushes it to the side.

Also looking at your pics, I would again suggest a good cleaning, followed by a heavy lubrication.  Lubricate the 7 bolt lugs each with a full drop of a good lubricant so they are dripping, this will eliminate any friction issues.



CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Can you post a pic of the bolt catch locked back when it is manually activated ?

I am looking for the proper amount of bolt catch holding the BCG open.  I had a lower that only allowed the bolt catch to move so far... I ground the bolt catch tail to allow for more travel.