Not particularly germane to US martial shotguns but...
It seems that not all shotshells are created equal. I just picked up some NobelsportItalia 12 ga. buckshot rounds. I already had several other brands in my stash. All are nominally 2-3/4”/70mm which is the chamber length necessary to accommodate the fired hull, not the length of an unfired round
The unfired PMCs are the shortest at 2.22” AOL
The Remingtons are 2.30” as are some Kent birdshot loads.
The Winchesters are only slightly longer at 2.31
The Nobels are a whopping 2.48”
All the brands other than Nobel use the familiar star crimp; the Nobels use a plastic “card” over the shot charge and a short roll crimp. It appears that the fired hulls will be very close to the same length as the other brands.
None of this means much until I load my 11-87P. It has an 18-1/2” barrel with a barrel length mag tube. The tube will hold 7 of the PMCs but just misses being able to take a 7th Remington or Winchester. With the Nobels the limit is also 6 but with very little unused tube length.
So if you have a shotgun that comes close to accepting one more round, it may be worth looking at some off brands to see if they are sufficiently enough shorter fit in that extra