I'm buying a M&P 40 soon. I know it has a built in rail underneath. I want to mount a reflex red dot sight on it. Is it difficult to mount a weaver-style rail on top?

It wouldn't be hard to mount a rail, but I would question the reliability considering the slide velocity of the 40. I would be more inclined to find a scope mount that attaches to the lower rail and come out around the slide so it stays fixed to the frame.

I don't know how to mount a rail but if you go to the Gabe Suarez site Warrior Talk Forums there will be information on the relfexs sights used on pistols.
I don't think they use a rail but mount right to the slide. I know he has
done it with glocks and I believe M&P's. The site has much info to offer.
BTW I have a 40 and really like it with my XS big dot sights. Another
way to go for quick acquisition if that is what you are after.

Is this what you're looking for?

It looks like it replaces the rear sight.

Leupold DeltaPoint Reflex Sight

Exactly what I was looking for, but I found one about $300 cheaper. The Browning Buck Mark reflex sight is only $60, but it mounts to a picatinny rail. One Source Tactical http://www.onesourcetactical.com/tec...sdivision.aspx modifies slides for various red dot sights, but I'll do more research before I spend my money.

Good luck with Botach. I would make sure they have it before you order from them, if you do.

Sig makes a bridge mount that mounts to the under lug, and surrounds the barrel and slide. It only fits the sigs, because they have a rounded rail, but I bet with a little dremel work it could be done. Heres a link to it at Topgun Supply $129 and it comes with a red dot.
Sig Sauer Red-Dot Sight with Bridge Mount - Top Gun Supply

I have a Browning reflex sight and it works fine on my 22, but don't know if it would stand up to the battering that the slide movement would give it.

I'll research the Burris Fast Five II 4 MOA dot. It costs about $200. Maybe I'll put a Buck Mark reflex sight on a M&P 15-22.

I think you would be better off with some type of bridge attached to the frame of the pistol not to the slide. Mounting a scope directly to the slide will increase it's mass and will likely effect reliability as well as ware. Not to mention that I doubt many scopes are designed to withstand the forces that would be placed on them by the slide slamming back and forth.

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Life Member B.A.S.S., NAFC

Here's a thread on our prototype which is exactly what you described, we should have these ready, with several different interchangeable mounting bases, in a couple of weeks, maybe a tad more.

M&P optic mount - Brian Enos's Forums... Maku mozo!