With a magazine that takes 6 standard (2 3/4") cartridges, the shotgun would still be classed as holding 7 rounds because you can have another one loaded in the chamber.
The "one up the spout" factor is significant in Europe and may have caused a slight translation issue, for example in the UK unless you are prepared to jump through another level of paperwork and usage justification, most semi-auto shotguns are physically restricted to accept a maximum of 3 shells, which is 2 in the mag plus one in the chamber.
Thanks for the insights, Eric. As you suggest, one round may have been lost in translation.:confused2: But as delivered, the magazine did hold 5 rounds as advertised, and the capacity problem only showed up with the aftermarket parts. Over here, the replacement magazine tubes are sold by their capacity, leaving little doubt as to how many 2 3/4" shells they're supposed to hold.
I've now found the
drawings the CIP (Euro standards org) uses to regulate shot shells, and I think I see the problem. A couple of them, actually. Unlike SAAMI, CIP doesn't specify an overall length for the loaded shell, only the overall length for the ready-to-load shell. Makes sense from a safety standpoint, where you don't want the shell so long it opens into the restricted forward portion of the chamber. But the CIP drawing leaves it up to the manufacturer to determine the loaded length, and that will impact magazine capacity. SAAMI provides maximum loaded length dimensions, which apparently Benelli ignored when they picked the spring. That might not be the whole story, either, since the US military has adopted the M4 as their service shotgun, and I don't think they'd be very happy having to use special shells to get 7 rounds in the mag. Regardless, I'm hopeful that a replacement spring will resolve the problem.
So far, I've found no restrictions on magazine capacity for shotguns, as long as you're not going to use it on game. I've also leaned that you can not only have one "up the spout", but also one on the shell carrier. The net result is +2, using this "ghost loading" technique.
Curiously, though, there are federal regulations on what alterations can be made on imported shotguns. It involves lists of parts that if changed, need to be replaced with a certain number of other parts of domestic manufacture. Not sure what the real purpose is, but the regs have fostered a cottage industry that makes the replacement parts.

:confused2: