magazine tube size?

[JTF#]

what size extension do i need to make a gun that has 4 in the tube one in the chamber to 7 in the tube? an 8 shot or 7 shot extension?

 

[Dexter Morgan]

It sounds like you just want to add 3 extra shells.

In that case;
The extension you're looking for should just be labelled as a +3 extension.

Extensions aren't generally labelled for total shotgun capacity. Only for how much capacity they add to the gun.

 

[EL34/6L6]

Extensions from ATI and Tacstar are labeled as 7 shot , 8 shot etc.
So a 7 shot would be a +3 extension for a 4 round factory tube.