The best way to attach an M7 bayonet to your Remington 870 is to get S&J Hardware Bayonet Mount. It is extremely good and reliable, it is CNC machined from solid steel. Will work with Remington 870 factory magazine extension or S&J Hardware magazine extension for Remington 870.
Remington 870 looks fantastic with bayonet attached.
You can get S&J Hardware Bayonet Mount for Remington 870 separately or together with M7 bayonet and magazine extension.
Highly recommended! There is no better way to attach a bayonet to your Remington 870!
Official website: S&J Hardware Bayonet Mount Kits (Very good holiday deals available)
More photos, test and installation instructiosn are coming soon!