The only time I had a problem like yours was with light hand loads.
Is the case catching on the forward end of the port and making a gouge in the plastic forward edge?
That was what mine did but again it was with light loads.
How about the bolt, when you pull it back (without a mag and no ammo) does it move smoothly and with the same amount of pull or does it feel rough.
The bolt on the storm is large and there are a lot of arias that it makes contact to the rest of the gun. I use break free and give the entire bolt a light coating. Doing this does not make mine pick up a lot of crud and I do shoot mine a lot.
The one thing I would try is to dissemble the gun and with just the upper receiver slide the bolt in and out and see if you find any rough spots where the bolt covers the end of the barrel when they make contact.
Have you owned the gun long? Are you sure it was assembled correctly. I can’t think of any way it can be put together wrong.
Wish I could be more help, mine is in the thousands of rounds and with normal loads I have never had a problem.
The following shows the gouge that light loads caused.
http://thefiringline.com/forums/showthread.php?t=391388&highlight=beretta+storm+carbine