Hello all.
I am fairly new to the AR line of rifles.I own a Bushmaster varminter.
I recently went to a local gun show and picked up a used 16" carbine upper without the bolt/carrier or charging handle.I purchased a new bolt/carrier and charging handle from another dealer at the show.
I installed the upper with my Bushmaster lower and took it to the range today.The problem is that the gun wont cycle and eject the spent case.I shot about 50 rounds with a few different types of ammo.I have to pull the charging handle back to get it to eject.About half of the time it was a real bugger to get the bolt to pull back.
The bolt pulls back fine without a round in it.I have no idea what to do now.Im thinking maybe the gas tube is plugged but dont think that would make the bolt jam after firing.I lightly oiled the outside surface of the bolt before i put it in if that helps.
I am thinking of using my factory Bushmaster b/c and charging handle on this upper to see if it works.
I am really upset and would appreciate ANY advice that you guys have.Just please,no "Gun Show" part bashing because im already upset.
The guy i bought the upper from said that he shot it before bringing it to the show.So if he was telling the truth then im thinking a problem with the bolt.Anything to look for on the bolt? Thanks ahead of time to any help the i can get on this.

A rough chamber will require too much gas pressure to extract the empty.  Inspect the chamber to see if it's rough.  Look at some of the spent cases for signs of a rough chamber. Is the chamber chrome lined?

Does the extractor sometimes rip a piece of the rim off?  I like to use the extra power extractor springs.  Some people put a small rubber o-ring around the extractor spring to beef it up.

Another posibility is a weak extractor spring or an extractor that doesn't grab the case very well.  Is the empty round sometimes left in the chamber to interfere with the next round?

One symptom of insuficient gas is that it won't lock open on the last round. Chamber an empty and blow into the muzle.  The gas leak should be inperceptable.  

oldguy,
 The chamber looks chromed.The bolt doesnt budge after the shot is fired.I have to pull the charging handle to get it to eject and sometimes it is very tough to get the handle back.once i get the handle to pull back,it ejects the shell fine.
  I took the bolt out when i got back from the range and compared it to my other bolt.The bolt seems very stiff when trying to move it in and out of the carrier.My other bolt moves back and forth easily.I removed the bolt to ispect the gas rings and they seem ok.
   Another thing that i noticed is that when i have the upper off and go to put the handle and bolt in the uppper it is tight as it bottoms out.I can take my good bolt/carrier and it goes in the upper just fine.Im thinking maybe a bad bolt/carrier?
   I may swap bolt/carriers tommorrow.I just dont want to damage my good bolt.
   Do i sound like im on the right track?

Check the head space of the barrel with both bolts.  

Anytime you stick a case in the chamber after firing (read hard extraction), the ammo is showing signs of overpressure.  Since the rounds are spec, then chances are that the combo of the chamber/bolt are creating a undersized head spaced chamber.

It could be a headspace issue which would indicate a bad bolt (most likely) or barrel extension.  The fact that you say the carrier/bolt seems to bind up as the bolt locks into the extension (roughly the last 1/2" of carrier movement cams the bolt to lock it in place) would seem to point in this direction.  

Another cause for the hard manual extraction of fired cases could be a short recoil issue.  If the bolt/carrier does not move back far enough to eject the case once a round has been fired, it will rechamber the expanded fired casing.  Since the casing has been expanded, the rechambering of it will wedge it into the chamber pretty good, making it hard to manually extract afterwards.  The causes of your short recoil problems can be varied, such as a rough chamber, inadequate gas to the carrier, buffer or buffer tube issues, carrier/FCG interaction, etc.  If fact short recoil problems are probably the hardest to diagnose, since there is so many variables that can be a factor.  Pretty much have to be there in person to figure it out.

well i took the new upper and bolt to the range today along with my Varminter upper.The Varminter bolt/carrier works fine with the new upper that i bought so im assuming i bought a bad b/c.
   I hate to scrap a new bolt/carrier.Maybe i can keep working the bolt back and forth wial watching tv for the next month or so.I took the bolt back apart and it seems to be very tight where the gas rings touch the carrier.Any idea on how i can save this thing?
 BTW.Thanks for the great advice so far.

Use a Set of head gauges to determine how out of spec (tight) the bolt is regarding head spacing.  Then either turn (lathe) the back of the lugs, or face the bolt to get the correct head space.  

Also, depending on how much steel you remove, you may have to re-case harden the bolt.