Capacity is a matter of the design of the receiver and magazine tube.
The Remington will ALMOST handle another shell, but not quite.
In extensions here's the best:
The Remington factory.
Has the best barrel clamp design, and it is held in position on the extension by the screw which interlocks with the groove in the extension tube.
This means the clamp will stay in place under recoil without having to tighten the screw so tight it might strip.
Expensive, and harder to get, but the best.
The Choate.
It's the best for price/quality/availability.
It comes as a complete kit with tube, spring, heavy-duty magazine follower, barrel clamp, and forward sling swivel.
It comes in a satin black sulfate or dark gray parkerized finish, in lengths to fit guns from 18" to 26" and for many brands of shotguns.
The Wilson/Scattergun Tech.
Very good quality, but doesn't come with a barrel clamp.
Some people don't like the sling swivel setup.
The Hans Vang.
Extremely good quality, but again, no clamp.
The Tank's Rifle Shop.
A very high quality, very close copy of the Remington factory extension, only the clamp screw doesn't interlock with the tube.
The Tac-Star.
Also comes in blue or parkerized, and in several lengths.
Only the longer models have a clamp.
Tac-Star does have a past reputation for some quality problems like tubes out of line and out-of-round mounting collars.
The AT plastic unit.
I don't recommend this one. It's a plastic tube cast around a steel mounting collar.
If the unit you buy does not have a barrel clamp, buy one.
It makes NO difference how strong the extension is. It can be a solid steel bar.
The weak point is the gun's magazine tube which has wide, deep threads that weaken the already thin tube.
A bump can knock the extension off the gun if there's no supporting clamp, and this almost always damages the gun's magazine tube.
This means a trip back to Remington to have a new tube brazed in and the gun refinished.
An unsupported extension is a handy lever that can break the extension off the gun.
USE A CLAMP.
My personal preference for extensions is the Choate.
It's a very good quality, and Choate has been making them for many years with no problems.
They come as a complete kit with nothing else to buy, and they give no trouble.
