If a round chambers and ejects, it's not an ammo problem unless your overall length of the ammo is longer than 2.255" which could mean bullet tips catching on the front of mag. If loading 55gr bullets, make sure OAL is in the 2.225 to 2.225" range. If that is not the problem, try following.
Try locking bolt back, insert mag with one round, drop bolt catch and if round loads, fire it and see if bolt locks back. If so, most is good. Then try 2 in the mag and if first round loads, fire it and see if 2nd round loads. If so, we are part way there.
Now try inserting mag with 2 rounds less than full in it under a closed bolt. See if the mag inserts easily and snicks into place without having to tap bottom of mag to get it to seat. If so, pull bolt back with charging handle and let it fly forward and if 1st round loads, fire it and see if 2nd round loads. If so we are most of the way there.
If mag is very hard to insert and seat under a closed bolt with it mostly full, you may need to stone down the top of the mag catch to get a little more clearance under a closed bolt. An empty mag should seat easily under a closed bolt and still have a small amount of up/down play (about 10th of an inch). Try all of this with more than one mag.
All of this fails, take AR AND mags to a smith who knows AR's.
Try locking bolt back, insert mag with one round, drop bolt catch and if round loads, fire it and see if bolt locks back. If so, most is good. Then try 2 in the mag and if first round loads, fire it and see if 2nd round loads. If so, we are part way there.
Now try inserting mag with 2 rounds less than full in it under a closed bolt. See if the mag inserts easily and snicks into place without having to tap bottom of mag to get it to seat. If so, pull bolt back with charging handle and let it fly forward and if 1st round loads, fire it and see if 2nd round loads. If so we are most of the way there.
If mag is very hard to insert and seat under a closed bolt with it mostly full, you may need to stone down the top of the mag catch to get a little more clearance under a closed bolt. An empty mag should seat easily under a closed bolt and still have a small amount of up/down play (about 10th of an inch). Try all of this with more than one mag.
All of this fails, take AR AND mags to a smith who knows AR's.

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