wrong upper receiver

[SinisterRob]

I had my A2 style upper receiver swapped for an A3 w/ m4 feedramp cuts. The cuts on the barrel extension are regular rifle cuts. So my barrel receiver looks like the the picture below labeled "bad". The best the gun has performed is 20 rounds w/o malfuntion. I thought I had a gas problem. At its worst it jams with every shot.

The malfuntion is always the same, bolt is stuck on top of case that it is trying to load. The round is moved about an inch forward with the bullet starting into the chamber, but the rear of the round is stuck under the bolt.

Found this info in an old thread:

How about something wrong with the feed ramp?

Maybe, but I would expect a problem to be visible, or at least a bur you could feel with your finger. Does your barrel extension have M4 feedramps (i.e. cuts that go down to the aluminium of the upper receiver) or rifle feedramps that are only in the extension itself... see photo below (courtesy AR15barrels.com) ? Is there any evidence of damage or roughness on the offending side ?

Anybody ever throated their rifle barrel extension -- making it into a m4 feedramp?

[GentlemanJim]

It seems your rifle is short cycling...If the bcg is going over the next round that is about to feed...thats it

Check gas key for leakage along with gas rings and gas tube.

Also ckeck that the gas port is in alignment with the gas block port.

Jim

Edited January 17, 2010 by GentlemanJim

[mike_pinto]

Try it wet too! Jim is right that it is short stroking, but it might not be a major issue. Mine does this after I clean it and put what looks to be "enough" oil. Put some more in it and try it again.

M

[bigbrowndog]

to answer your post I've filed/stoned the receiver to make a smoother transition and not had any issues with it, but a gas problem is what seems to be your problem, check everything related to your gas system, gas block, tube, gas key etc. I've had a similar problem and it was the gas block leaking around the barrel. Once it was replaced it worked perfectly.

trapr

[carharttfarmer]

its still giving ya trouble rob that short ar of mine is the same way i had to dremel the barrel extension to get rid of the lip between it and the reicever other wise the bullet would catch on it but my bolt was behind the case when the jam would occur

[EkuJustice]

You can dremel the cuts in or send it to adco and they can do it if your not comfortable with your dremel skills

[GentlemanJim]

A small round precision file is the way to do it, a diamond file is best as most files will not cut the hard extention

A dremel is not as easy to control, but with a good quality small round stone it can be done.

Quick test for gas system problems....one round in the mag..shoot and see if the bolt carrier fully locks back.(locks on bolt face, not the end of the carrier)

do this a number of times to insure it is consistant.

Jim

[TMC]

If the nose of the bullet is not getting caught in the pocket created by the ramp in the receiver below the barrel extansion then its problem is not the barrel/reciever combination. If it is then I'd get a new receiver, its the cheapest part for the fix.

[Ron B]

Have you tried a different magazine? Or does this occur with all mags?

[SinisterRob]

I tried out many of the suggestions. So far,

4 different magazines-- same problem

extreme amount of lube -- less frequency, but still same problem

one round in mag -- Wolf HP 55gr, consistently not enough gas to lock back bolt

-- PMC Bronze fmj 55gr, locks bolt back everytime

-- AE XM193 fmj 55gr, locks bolt back everytime

Before changing the upper reciever the gun would shoot the Wolf HP 55gr but not the Wolf fmj 55gr. I also chrono'ed the HP wolf at about 2700 feet per second. The 5.56 XM193 chrono'ed at about 3200 feet per second.

Small amount of leakage at gas block

Also tried bolt out of M4, same problem

And M4 lower, same problem

The most interesting finding was that I had the same jam once while hand chambering a round. That was an insert magazine on closed bolt and try to rip the charging handle off to chamber the round.

I don't see evidence of bullets catching on ledge between the m4 style reciever and the rifle style barrel extension. I am still weighing my options on correcting that issue.

How about that for a jumbled mess of info!

Rob

[Ron B]

It seems your rifle is short cycling...If the bcg is going over the next round that is about to feed...thats it

Check gas key for leakage along with gas rings and gas tube.

Also ckeck that the gas port is in alignment with the gas block port.

Jim

Jim's advice is sound given your latest post. Could be the gas port/gas block alignment or debris in the gas tube.

[SinisterRob]

Since my last post I have had the rifle apart a couple times. With the barrel out a friend helped me throat the feedramps. Reassembled it and still had the same reoccurring jam, but I did finally notice it was happening only when using the right feed ramp.

The gun is working now, BUT that is because the old A2 style upper receiver is reinstalled.

My conclusion is that something is out of tolerance with the new flat top upper receiver. I'll be trying to get my money back soon.