This question has been plaguing me. Reviewing specs:

v3 tac 13 has a 13 inch barrel and magazine capacity of 5

V3 tactical has 18.5 inch barrel and magazine capacity of 6

V3 tactical “competition” has 22 inch barrel and magazine capacity of 8

As far as I can tell, all three of these models have magazines that extend to the end of the barrel. What am I missing here? From the tac 13 to the 18.5 inch V3 you gain roughly 5.5 inches of magazine tube yet only one extra shell (which are less than 2.5” unfired) .  To further confuse things, if you go from the 18.5” barrel to the 22”, only 3.5” more tube, you get a full two more shells capacity.  

Obviously the 18.5 must almost have room for another shell, but why wouldn’t two shells easily fit in the extra 5.5 inches of tube gained over the tac 13, even accounting for a bit more compressed spring?

2.75” is the standard.

Take one of your 2.75 unfired shells and measure it

That is all assuming when fully loaded, the spring is no longer compressable and the tube is full-full.  Some magazines allow you to fully load a mag, but then allow you to stick in about 1/2 a shell more.  Since you can't load partial shells, you either round up or down. Example, Tac13 allows you to 5 load with no wiggle room in the mag tube. Then the Tactical model allows 6.5 shells. Can't load half a shell, so you get a max of 6 in the tube.

Another part is that a longer required magazine spring will also take up more space. But I wouldn't see that as being a deal breaker in terms of making a 1 shell difference with magazine size differences.

Just some ideas. I don't know any of this for a fact specifically for the Tac13.

Yea that’s what I was thinking, worst case let’s say 5 shells just barely fit in the tac 13.  So moving up to the 18.5 shotgun, as far as I can see, gives you another 5.5 inches of mag tube. Realizing that shotshells can vary a small amount but I measured two typical shells together end to end they come to just under 4 and 5/8. Seems to me that leaves plenty of room.

While I won't lose any sleep over it, this is a very interesting idea though. I wish I had all 3 shotguns to compare as I am very curious to why.

Another idea would be that the extension tubes are difference. The endcaps may be thicker. After looking at my Scatterguntech +1, the follower is what takes up all of the space within the extension.  Which doesn't make sense since the extension is a little less than the length of a 2.75 shell.  And the threads overlap each other, reducing the actual overall internal extension.  But the extension is still suppose to give me +1.

Unfortunately I am still waiting on my Tac14 to see if I can load up 4 shells with no space left over.

What shells are you measuring? Two 2.75” shells measured end to end are 5 5/8”...

I measured two normal 2.75 shells with digital caliper.

Remember, In unfired state, shells are less than their full length.  My Fiocchi samples are under 2 3/8 each.

What shells are you measuring?

2.75" shells should be well under 2.5" in length.

Two 2.75" Winchester 9 pellet OO buckshot shells measure 4-5/8".  Just measured it.

ETA: my TAC-13 barely holds 5 shells

I’m wondering if remington has artificially capped the 18.5 model at six for state compliance reasons? To make it legal in the more states? Just a guess.  I’m not familiar with laws regarding semi auto shotguns.  

My 18.5 590 only holds 6 (6 and a half actually) but the mag tube stops almost 2” shy of the muzzle.  With an extension I could easily fit 7.

I can load 7 total in my Tac13.
5 in the tube, 1 in the chamber, 1 on the lifter.

Some brands of 2 3/4” shells are shorter than others.
I believe Rem shells are shorter than Fiocchi. Rio brand I think are on the short side as well.

8+1+1 with my Competition Tactical.

I am very curious about this. I tried many different ways to set up this 5+1+1 and I cannot accomplish it. I suspect it has something to do with the magazine cutoff, but the loading order must also be a factor. Could you please share your secret.

Thanks,
mark4

No secret. Load 5 in the tube first.
Now pull the bolt back far enough to allow you to put one in the chamber but not so far back that the last shell in the tube pops out onto the lifter.
While holding the bolt partway back, pull the chambered round back with your finger and let the back of the shell drop down below the bolt then push the shell back and down under the bolt completely onto the lifter.
Now put one in the chamber and close the bolt.

My Tac-13 cycles and shoots just fine like this.