Anyone know if anybody out there makes a rail system for mounting sights (like an Aimpoint) on an 870?

There are a number of what we call "saddle rails" available from people like Millett and B-Square.  I don't think any of them are true MIL-STD-1913, but they might take an Aimpoint, depending on the ring mount you use.

We're not happy with anything we've seen on the market, which is why we are developing our own.

Another alternative is a Badger Ordnance steel Picatinny rail that can be brazed to the receiver, if you can find one.

Beyond that, if you want to tap the receiver, we and others make aluminum Picatinny rails that will work (usually for the Mossberg with its factory-tapped holes).

PRI



pri-mounts.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=PRI&Category_Code=SHOTGUN

I have the one that mounts with the receiver pins and it works great.  
ONly thing though is you have to make sure u do not tighten it too much or it will bind when pumping it.

search for
if you can find one
Knights 870 rails
Mesa Tactical
Side Armor
i think i am missing a few more