Hi guys, any help will be greatly appreciated. I have a 16" A2 Bushmaster that I recently decided to replace the upper on with a flattop. i ordered a DPMS upper from the great guys at Del-Ton(super friendly). I used the bolt and carrier from my Bushmaster upper in the new one.

My problem is with the gun cycling the rounds, it will shoot a round and eject it fine but it won't chamber another one. If I do it manually with the charging handle it will eject and chamber fine, it's just when I'm shooting it that it malfunctions. I am newbie at working on AR's, is this a easily remedied problem or something I need to let a gunsmith look at? I thought maybe the bolt wasn't coming back far enough but I wasn't having any problem with my other upper.
I also mounted a Surefire M97 light on the bottom of the gas block, is it ok to leave it there or does it need to be mounted to the forearm? Thanks for any information you can give.


Jamie

Make sure it's not "short stroking"

ie--insufficient gas delivery to bolt carrier.

Do the following procedure and report results.


---TEST FIRING PROCEDURE---
-Unload the rifle and verify that it is clear.
-Check the bore for obstructions.
-Load the rifle with one round from the magazine. Use quality ammunition and a proven magazine for this.
-Aim downrange and fire, the bolt should catch on the bolt stop. Hold the rifle as loose as possible.
-Reload and repeat the process if the rifle does not lock open Depress the bottom of the bolt stop before firing this time. This allows for weak magazine springs, sticky followers, or an overly strong bolt catch spring.
-If the bolt does not lock open then there is a gas system problem. Investigate and repair the problem.
-If it only locks open with the bolt stop depressed, repeat the process with a different magazine. If the rifle still fails to function then look at the bolt stop as a probable culprit.

If it ran fine before - and now doesnt run.... and ALL you replaced is the receiver:  here are a few things to check.

Explain the fail to chamber better.  Does it not even try to pick up a new round?  Does it try, but not strip the round from the mag?  Does the bullet strip but hang up on the feedramps?  Doe is almost chamber but just not lock?

Did you change ANYTHING else at the time of the receiver change?

Ammo change?  What type of ammo are you testing with?  Same type and lot you shot before?  Have you tried changing to some good milspec M193/M855 just to see?

Perform a check of the gas system:  load 1 round in a mag.  Fire.  Does the bolt lock back?  Every time?  If not - you have either poor lubrication, binding/rubbing, or a gas leak.  As directed - clean and lube with CLP per the TM.  I will cover the others in a minute:

1.  FSB - I assume you re-used your original FSB, and it was a pinned bushmaster.  If this is the case, nothing to check here.  If you changed something, let us know.  You might have a gas leak or misalignment.  You mentioned a gas block.... please give more details.

2.  Binding/rubbing.... this can be caused by a few areas.  Make sure you can pull the charging hand rearward, and then ride it forward - do you feel any rough spots?  When you let it ride forward, does it lock, or stop short?  One test is to remove your upper from the lower, pull bolt/carrier fully rearward, tilt upper down 45 degrees - bolt should slide forward and lock.  Places to check for binding are the bearing surfaces in the upper receiver, the buffer tube, and the gas tube.  When you torqued the barrel on, if the gas tube is cocked to one side, it will rub on the carrier key as the weapon locks... and cause a fail to chamber.

3.  Gas leak - this is caused by a leak at the FSB, or in the carrier key.  If you didnt change these, I doubt this is the issue.


Need info.

Ahhhh agreed.  This is a big can o worms until we get more info.  I took it as a receiver replacement.  I see where you are going.  Could be a LOT of things if it is a totally new upper/barrel....

especially ammo, need to know what he is testing with.

Hi guys, problem solved!  Thanks so much for the information, I checked the carrier on the bolt and it was a little loose.  I tightened it down and the gun works like a dream again.  A simple issue I know, but I had to idea what to check so I really appreciate it.  
My other question was about the Surefire M97 mounted on the bottom of the gas block, is it ok to put it there or do I need to mount it on the Knight's forearm? Thanks again for the help..............Jamie

Quoted: My other question was about the Surefire M97 mounted on the bottom of the gas block, is it ok to put it there or do I need to mount it on the Knight's forearm? Thanks again for the help..............Jamie

Gas blocks get VERY hot.  I personally would not mount any electronics on a block... they will transfer heat to whatever is mounted there.

Thanks...I appreciate it...Jamie