that would mean you would have the length of 6 shells extending passed your barrel with that 10 round extension.
I believe he means he wants his shotgun to have 10+1 capacity. If so that means he would have the length of two shells extending past the barrel. Two unfired 2 3/4" shells measure about 4.5".
I would go with a +4 tube extension. I offer a +4 (8+1) tube extension that's even with the muzzle with a 21" barrel.
A +6 (10+1) tube extension would extend past a 21" barrel about 5". It will add a lot of weight up front.