I tried mine this past Saturday after getting it back from a smith and it does hold six BUT somewhere in the translation this gun has come back from to smiths with the same problem unresolved. Whether four or six it does not want to feed from the magazine. Even extreme gung-ho on the pump will fail to reliably feed one in. A couple of pards have suggested that I look for a burr in the tube but I would think that if two professionals coundn't find the problem, who can? Both returned it with the claim that it was working!
Nope, not a burr.
The carrier activates the two shell stops as the action is opened, thus releasing a cartridge. It is the width of the carrier that determines positive operation of the shell stops.
If you look at the action from the bottom when it's closed, you can see the two, reed-like shell stops holding a cartridge in place, inside the magazine. As the action is opened, the carrier falls and wedges between to cartridge stops, opening them and releasing a fresh cartridge.
If the carrier is worn -- i.e. too narrow to do the job -- the cartridge stops will work intermittently. The gun might have worked for the 'smith, who probably used different cartridges in testing it. The design seems to benefit from quality shells. Aluminum base shells are problematic since aluminum has the property of being "sticky." Thus, a carrier that's on the verge of not opening the cartridge stops might fail in the case where aluminum base shells are used, but work fine with brass-based shells, since brass has the property of being self-lubricating.
It still needs to be fixed. The fix is to weld some additional material to the front, side portions (i.e. both sides) of the carrier and contour it to run smoothly through the cartridge stops. In other words, the carrier is worn out and needs to be rebuilt.
Along this line, cramming six cartridges into a magazine that has not been otherwise modified can cause additional wear on the carrier at the cartridge stops because it now has to work harder to release the stops when they are under greater pressure from a full magazine. So, fitting six in the magazine has to go beyond just (barely) getting them in there. There should still be some margin of space in the column of shells to insure the pressure on the cartridge retainers is not excessive.