Hello Everyone;

I own a Colt AR15 Matched Targets H Bar.  I bought the gun used.  It was modified with an Accuarcy Speaks trigger set, Hoge weighted stocks and it looks like the barrel was changed from the Colt to a NATO barrel (the barrel currently on the gun says NATO on it and I am not sure if that is standard issue to this type of gun).  

I have been having troubles with gun jamming and need some guidance.  I clean the gun according to the procedure listed here on this website found under the maintanence section (and I've cleaned it according to a M16 military issue manuale from the army).  I then take the gun to the range and begin firing.  About 100-120 rounds into the target practicing session the gun will start to jam and it does so in the following ways.

I load the first round of a full 20 round magazine (this would be about the 5-6 magazine of 20 rounds I have put into the gun to this point) by pulling back on the charging handle and then releasing and pull the trigger.  Then one of two things will happen when the next round as it is chambered automatically:

The next bullet will not chamber at all, it just stays in the magazine.  I have to pull back on the charging handle and load the bullet into the chamber manually. I fire that round and then again the next bullet will not chamber at all or it will chamber only paritally.  When the bullet loads paritially in general the bolt carrier will run into bullet base and crimp the brass.  I clear the jam and load the next round using the charging handle. That round fires  off and then again the next bullet will not either chamber or is loaded only partially.  Once the problem starts to happen it continues to happen for the rest of that target practicing session.  I go home, clean the gun, take it to the range and it starts all over again.

The ammo that I have used when this happens has been Winchester 55 grain (the kind that comes in the red and white boxes).  It was bought in a 1000 round box and all the individual boxes of 20 rounds have had lot numbers from the same lot (lot number 1650).  I have used 4 different types of Remington and have had no trouble with any of them (as a matter of fact I used some of these Remington in the same magazine along with the Winchester 55 grains and had trouble when the Winchesters went through but not the Remington).  I have tried South African Battle Packs, Winchester Suprems and have had no troubles with any one of these types. The one thing that I have not been able to do with the non Winchester ammo is fire off 100-120 rounds in a row (I only bought one or two boxes here or there as people suggested that I try it instead of the Winchester.  I don't want to buy large amounts of other ammo if it is the gun).  
 
I have cleaned the gun with gun scrubber, Hoppes#9, Shooters Choice.  I have cleaned the gas tube as best I could with pipe cleaners, and even snaked of bicycle brake through it while attached to a drill.  I have no trouble with a 30 round magazine that I use also.  I have cleaned the 20 round original Colt mag that came with the gun and a 30 round mag I bought.    

I am at a loss.  I don't want to sell the gun but it got so bad once that every bullet had to be loaded into the chamber manually with the charging handle.  I don't know if its the ammo, the gas tube, the NATO barrel, something that has to do with the gun getting hot, the bolt carrier.  

If anyone can help thanks.