In addition to the visual checks already suggested for your extractor/shell stop suggested already, honing/polishing the chamber would be the next step. More info on what your malfunctions look like when it happens would help narrow it down. Sometimes in the heat of competition it can be difficult to recall the process as you're simply trying to get the gun running in a hurry. When you have the time to analyze one, here are some observations to make that will help narrow it down:
1) How did I first know there was a problem? Did I pull the trigger and get a click? Did the next round jam into the back of one still in the chamber? Did more than one feed from the mag tube?
2) Once I noticed a problem, what did I do? Did I attempt to quickly charge the gun? Did anything eject or feed from the tube when I did? Do I know where that hull went?
This happens a LOT in 3-Gun with every make/model out there and it is tough to reconstruct what the issue is without knowing the above.
If you got a 'click', the bolt went back far enough to reset the trigger and the shell may have extracted just fine, but didn't eject. It may have then been crammed back into the chamber (where it is now REALLY stuck) or may have had its rim ripped on the initial way out and already WAS stuck. This is usually a lousy ammo or rough chamber issue. If you have other rounds involved in the jam you may also have issues with feeding from the tube that can be magazine spring or shell stop related.
Since you are only having this problem with one kind of ammo, it would be fair to blame it on the ammo until proven otherwise. Some ammo is simply too poor of quality to run right in even a pump shotgun. That said, you may simply be on the edge of running it reliably and need a few tweaks to get it 100%. Start with chamber polishing (the cheapest fix) and then look at reducing the resistance in your gas and recoil systems. We have parts and related suggestions for each of those, but more info on model, round count and knowing what really happened with #1 & 2 above should happen first.
William
http://www.OR3GUN.com