Due to many confusions I am requesting a sticky on this matter.
The Burris Fastfire II can be bought in two configurations:
1. Picatinny/weaver Base.($209.00 Cabelas)
2. "No Mount"($199.00 Cheaper than Dirt)
The picatinny/weaver has an extra part that allows the sight to be attached to any weaver/picatinny base like a regular scope.
If you want your sight as low and as light as possible you can use a ''marlin 336 bases" made for the fastfire.
This bases was designed specifically to mount the "No Mount" sight on a Marlin 336 rifle.($47.00 Gunbroker)
It has been adapted to be used on flat top receiver of shotguns like the Remington 1100,1187 and 870, the Mossberg 835,935,535 and Browning BPS and Gold. It requires two 8-40 thread holes in the receiver 1/2'' apart.
These are very common configuration and found on many predrilled receivers.
Use of the Marlin base replaces both the Weaver/picatinny part included in the first package and the weaver or picatinny base that is on the gun or provided by you.
I am sure the sight can also be mounted on many other flat top shotguns.
The Protector from Gobbler74 can be used with the Marlin bases. He includes slightly longer screws to accommodate the thickness of the protector.