Chuckles, you may need to open your gas ports a bit in order to capture more gas from the lighter shell. Is your spent shell jamming on extraction or is it a feeding problem with the second shell below?
If the problem is the shell not driving the bolt all the way back u might want to think about changing the recoil spring in the buttstock. If it has become worn or fouled it could cause problems. I take it that u did not have problems with this kind of ammo before recent problem began.
Sounds like a gas port problem. Joe is correct in using drill bit to clean out ports. Once you disassemble the gas valve assembly, you can open the ports with a bit that is ever-so-slightly larger than the port. I have done this several times with friends that want to shoot x-tra light shells. Works great with no problems. Care must be taken to go up in bit size in very small increments...You may have to repeat the process once or twice to allow enough gas to pass to operate the bolt. DW
This is a common problem. The spring in the forend cap assembly may be too weak. It controls the excess gas venting. If it allows the gas vents to open too soon or too far the gun will vent the gas it needs to cycle. That is the self adjusting part. Cole Gunsmithing sells a kit with two springs. There is a red and a blue. I think the red works for light loads. It keeps the door closed so the gas works the action. If I remember correctly, the kit is around $20.
I don't mean to be contrary. The bolt isn't making it back with light loads. This could be any of the above suggestions. The gas ports could be clogged. The recoil spring could be dirty. The spring that controls the gas valve could be weak. If you take apart the recoil spring assembly to clean it you should replace it. I've done it 2x. Use goggles its frisky. The red valve spring worked for me.
Just reread thread. I was refering to chuckles problem. Tinman88's problem may be the recoil spring. Don't go tearing apart the thing just yet. Borrow someone else's trigger assembly and try it for a while. I think all 391's are interchangable. Some 303/390 one's may also fit. If the exchange works, its the trigger assembly/floorplate/lifter. If it still fails it is probably the recoil spring. I have also seen reloads so bad that they stick in the tube. A lot of sporting clay shooters drop the shell onto the plate after putting it in the mag. Just push the little button at the back and the shell preloads onto the lifter. It doesn't seem to hurt anything and may help cycle the gun.