This is a FWIW more than anything concrete. I noticed this and found it interesting. I compared the barrel lengths and overall lengths of four Benelli tactical shotguns - I was a little surprised. Here's the measurements. Oh, the barrel lengths are from the receiver to the muzzle, not the actually bbl length. I was more interested in external dimensions.

______________overall_______barrel______Wt_(lbs)__
M4......................38-3/4".............18"..............8-1/2
M2......................38-3/4".............18"..............6-3/4
M2 (21" bbl)..........42"...............20-1/2"...........6-1/2
SNT PG..............40-1/2"............17-5/8"...........7-1/2
SNT Comfortec...40-3/4"............17-5/8"...........7-1/4

SNT = Supernova Tactical
PG = Pistol grip

Some thoughts:

One day I noticed that the SNT's were longer than the M2 & M4 and wondered why. The answer is the receiver. The SNT's are pumps and the M2 & M4 are semis and to my surprise the pumps have a longer receiver.

An interesting thing to me is that the M2 with the 21" bbl, 20-1/2" from the receiver, is only 1-1/4" longer overall than the SNT with the Comfortec stock. The 21" M2 also has a Comfortec stock. But, the barrel is actually 2-7/8" longer than the SNT which has a 17-5/8" barrel (external). This means that one should be able to get one more 2-3/4" round into the M2 (both guns with full length mag tubes), but only gain an overall length increase of 1-1/4".

Here are some pics - M2 (21") on top:



Here's one with the SNT on top of the M2; you can just see the M2 barrel peeking out:



And last a closer look at the barrel end:



Again I'm not sure that this means all that much; simply essentially a Benelli pump is going to be longer than the same size barrel in the M2 series. Also, the M2 (21") would is a bit lighter, 3/4 lbs, although it feels lighter than that.

One would think a 21" barreled gun would be 3" longer than a 18" barreled gun, but a 21" M2 is only 1-1/4" longer than an 18" SNT due to the longer receiver of the SNT.

Of course one is a pump and one is a semi and that may be of far more significance than anything else.