They are different. I have a FF2 on my 870 with a Burris 336 base. My dad has FF2 on his 870 with weaver base. The 336 setup is lower profile and cleaner looking. But i sighted dad's gun in and liked his setup just fine too. If your gun is already drilled and tapped you may just wanna throw a weaver on it. My gun wasn't drilled so I gave the gunsmith the 336 base for a template. If I remember correctly the hole patterns are not the same for the two bases. The 336 takes two holes about a half inch apart. And I think the weaver holes are just a tad more spaced. Another thing to think about is if you ever decide to change sights, the weaver can be used for different sights and scopes, whereas the 336 is FastFire specific. If I'm wrong about any of this I'm sure one of the boys on here will correct me. But yeah, I love my 870 FastFire 336 setup.
stilker
Live for God
