I've got the same 870 slugger as Digi, and the MonteCarlo stock on it works for me. Looking at some pics on Rem's site, the top of the stock shares a line with the bottom of the barrel, just like in their 700 series rifles. Can't be too far off.

Hereare the Rem stocks...






[align=left]Model 870™ Synthetic Monte Carlo Stock & Fore-Ends
• Fits Remington Model 870 shotguns
• All weather fiberglass reinforced synthetic stock
material
• Monte Carlo stock ideal for turkey/deer hunting
• Rear swivel stud attached for easy addition of
sling with Universal Mag Cap installation
• R3 Recoil pad to manage recoil[/align]

[align=center][/align]

[align=left]Model 1100™ & 11-87™ Synthetic Monte Carlo Stock & Fore-Ends
• Fits Remington Model 1100 & 11-87 shotguns
• All weather fiberglass reinforced synthetic stock
material
• Monte Carlo stock ideal for turkey/deer hunting
• Rear swivel stud attached for easy addition of
sling with Universal Mag Cap installation
• R3 Recoil pad to manage recoil[/align]

[align=center][/align]

Model 7400™ Synthetic Monte Carlo Stock & Fore-Ends
• Fits Remington Model 7400 rifles
• All weather fiberglass reinforced synthetic stock
material