The Remington extension has been around since the early 1950's and has been as well proven as it's possible to get.
It's been very extensively used by police and military units, and if they can't destroy it, it has to be good.
The Remington barrel clamp has a screw that passes through he groove in the extension tube and interlocks with it. This prevents the clamp from moving on the barrel.
The Choate has been around since at least the 1960's, and it's been very well proven in civilian and police use also.
It comes in sizes from 2 shots for an 18" barrel to one that gives a 26" barrel 10 rounds. It's available in satin blue or parkerized and comes with a clamp, heavy duty magazine follower and sling swivels.
I can't seem to find anything on a Pmag shotgun extension.
On the Remington and Choate, reliability and quality are high. Life expectancy is unsure since no one seems to have ever worn one out.
