The Remington factory extension is probably the best.
The factory magazine follower is believed to be "weak" by some people so you might replace it with a Choate or Wilson.
The factory is the only extension made with a groove in the extension tube that the clamp screw interlocks into.
This prevents the extension from moving under recoil like all the others can and do.
Other extension are:
Choate:
www.riflestock.com/Wilson.
Vang.
https://www.vangcomp.com/accessories.htmlTank's Rifle Shop.
www.tanksrifleshop.com/remshotgun.htmTac-Star.
ATI Plastic.
The best all around after-market unit is Choate from the stand point of quality, price, and availability.
The Choate comes as a complete kit, with the extension, spring, stronger follower, sling swivel, and the all important barrel clamp.
It's available in more sizes than all the others put together, and comes in black or parkerized.
The parkerized matches the Remington Parkerized finish.
Quality is HIGH, and the Choate has been around a long time and very thoroughly "road tested".
The Wilson is good, and EXPENSIVE. It does not come with a clamp.
The Vang is good and expensive. Also, no clamp.
The Tank's Rifle Shop version is a close copy of the Remington factory. Good, and expensive.
The Tank version is listed just under the Choate model on their web site.
The Tac-Star usually doesn't come with a clamp. They have some past history with quality problems like out-of-round magazine mount collars or tubes that are out of line.
Tac-Star is available in parkerized.
Quality seems to be lower.
The ATI plastic unit. This is a plastic extension molded around a steel coupling. Works, but the quality just isn't there. Also some reported problems with defective molding around the coupling.
Whatever you get, GET A CLAMP.
It doesn't matter if the extension is a solid steel bar, it's still screwed onto a thin magazine tube, and a good bump will knock it off the gun, damaging the gun's magazine tube.
This means a trip back to Remington for an expensive replacement.
If your extension doesn't come with a clamp, the best one is Uncle Mike's close copy of the Remington factory clamp.
www.unclemikes.com/adtemplate.asp?invky=9004490&catky=1465723&subcatky1=4548843&subcatky2=2000180
My recommendation: Buy the factory unit. These have the best reputation of any extension.