Hate to see you hanging out here for 6 days with no response.
I don't have a rem pump but I am pretty sure it has a 3/8" dovetail on top of the receiver. Should be able to measure it with a ruler.
If your red dot is a "rimfire" model with built-in clamps, it will clamp directly on to it.
If your red dot is a "rifle" model with the built-in clamps, they are probably for "Weaver" rails, which are about 1/2".
They make a set of adapters that have the 3/8" rimfire clamp on bottom and the Weaver rail on top, to clamp a Weaver mount to a rimfire rifle, but it put the scope up so high, its ugly and impractical.
Better choice in that case would be to get the correct red dot.
Some red dots have reversible claws on the clamp, where you flip them around one way for rimfire, and another for Weaver.
If its the plain tube without built-in clamps, buy some scope mounts that fit the tube of the red dot and the 3/8" dovetail of the gun.
Mounts that are labeled "rimfire" fit the 3/8" dovetail.
Hope this helps.
