The out side of the mag. tube will get carboned up from burnt powder from the gas port and need to be cleaned . It will look black not silver .
I use WD-40 and a scotch brite pad to remover the carbon , then I wipe it dry . If you leave oil on the tube it will colect carbon 3 times faster.
Is the gun extracting ?
Is the gun extracting and not ejecting ?
Is the new shell feeding from the mag tube ?
Will it cycle if you do it by hand ?
Where is the jam happening ?