On a (COLT) M4 upper. A couple of questions. I have the 512 (AA) model, which is a little longer than most, and I want to put an ARMS 40 flip up behind it to co-witness. Will I need/should I get the Swan Sleeve, or will both fit? Is it more beneficial to have the EOTech mounted as far forward as possible? These are the only things I am putting on the gun (no offense, but some people put WAY to much stuff on their guns, they remind me of ghetto cruisers in the hood), so no need for all the panels/Knights system. Thanks for any advice.

I assume you are mounting to a flattop upper receiver so let's see if I can help because I have similar setups.  First off, ARMS #38 Swan Sleeves have the flip up rear sight built in the back in which case you would not need the ARMS #40 stand alone rear sight.  However, the EOTech sits up higher than an Aimpoint which means you do not need a spacer such as the Swan Sleeve (you should mount the EOTech directly on top of the upper receiver).  I recommend getting the ARMS #40 flip-up stand alone rear sight and the EOTech.  Mount these directly to the flattop on the upper receiver.  This should allow a perfect co-witness.  If you mount the Swan Sleeve on top of the flattop and then mount the EOTech to the Swan Sleeve, then the EOTech will be sitting too high and not allow you to use your iron sights, therefore not allowing a co-witness.  It is recommended you mount all optics such as EOTechs and Aimpoints as far forward as possible.  You should have enough room to do this.  I hope this helps.

Yeah, it's going on a flat top. I just hope the added length of the AA batteries allows room for both. Thanks for the input.

STANMAN,

With my Eotech I have found no need of moving it forward as some others have.  I do not foresee any problems with the way you talk about setting things up on just the flattop.  For clarification, mine runs at the front of the flattop with no problems.

Mount the EO directly to the flat top upper.  Put the #40 where it belongs and you'll be fine.  I dont throw my EOTECH way out on the fron either and since it isnot the whole bottom that hold the mounting portion our fine.  Put it as far out as you like although I use mine so the front(bat. compartment) just barely sticks out over the upper reciever.  I do have the shorter eotech though.  The site picture is so large that no matter how close or far away you mount the site you will see plenty of target believe me!  Good luck!

ps.  sorry for the %$#&@ spelling but I am too lazy to go back and fix it! he he he