You seem to be the Big Man On Campus as far as Remington stuff goes.
I have a R870 18" barrel, I also have a factory 2 shot extention. When it is installed the extention is pushed down by the barrel band aprox 1/ 8 " . did Remington offer a different mag clamp? Or do I need to do some WECSOG work to it? Also does anyone offer an aftermarket magclamp that will work with this set up?

Thanks for your time
762minigun

Tried to IM you but it is disabled

Did you assemble the clamp on the barrel so the screw passed through the groove in the extension?

The groove positions the clamp and prevents it from moving forward under recoil.
If you don't position the clamp in the groove, it'll push the clamp down..... About 1/8".

Also, 1/8" deflection is within manufacturing variances and should have no effect on anything.
Many after market extensions deflect for one reason or another, including the super-expensive versions like the Vang and Wilson.  
"Nature of the beast".

The best after market mag clamp is Uncle Mike's exact copy of the Remington factory version.
The difference is, the Uncle Mike's will work on most any extension, where the Remington often won't.
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Thank you once again.