Anyone ever hear of an "over-ride jam" with an AR ?
Most of the time, the carrier is not coming all the way back due to binding or a blocked/leaking gas system.
Since the bolt does not make it past the end of the round in the mag, the bolt grabs the top of the round and drives the round into the chamber mouth with the bolt sitting on top of the round, with the back of the round still in the mag.
Or, the mag is not held high enough in the mag well for the bolt to grab the rim of the round on forward stroke. 90* of the time, this is fixed by tightening up the mag catch to retain the mag higher in the well.
Or, the mag is a cheap type and the round is not seated correctly or the round is not forced up to the top of the lips.
To figure out what problem you have, try this.
Insert a single round into a good USGI mag,then cock and fire the rifle. The bolt should be held back by the bolt catch.
If the bolt is not held back, then the carrier is not coming all the way back.
Load the good USGI mag with 5 rounds and fire. If it does not have a problem, then you know it's the mags that were defective. If it does bind, then use a pencil and push the mag release as far in as it will go and turn the catch one more turn clock wise to tighten the catch up. If this does not solve the problem, then the lower receiver has been made with the mag catch milled too low in the receiver and will need to be replaced.
Granted that this may be a bit more that you needed to know, but I thought that I should just answer everything up front and save you the guessing games that seem to occur here as different people point to different spots on the rifle to check.
Hope this helps.