Here's what I read/heard/understand for what it's worth. The chamber in the Tactical is actually 3.5 inch. However, in early production / sales, there were reports that when the magazine was fully charged with some brands of 3.5, the weight under recoil caused shells to crush/distort in the tube. Subsequently and consequently they marked the VMT as 3.0 regardless of the 3.5 chamber.
When I got my VMT, it was plugged to 4 x 2.75 with a steel cup. In playing with it and without deliberately trying to remove it, it literally fell out. At that point the entire extension tube was available and would hold more than the allowable number of 2.75 which is all I'm interested in shooting.
I made a plug with tubing and epoxy that allows 5 x 3.0 to be loaded and just barely 6 x 2.75. As I shoot in the US more than in Canada, I have an extension that I keep there that gives me 10 + 1. I restore the gun to Canadian standards before coming back from the US. I also have a + 1 cap that gives me 9 + 1 if that's what the game rules call for.