I am putting together a M4 Flattop. Which of the Forearms (SIR, KAC, ECT.)line up the sight plane correctly for am ARMS #40  and eotech combination. Also what is the exact definiton of the Term co-witness, so I know what I'm doing.

Cowitness means that the iron sights are still visible through the window of your optic. The idela setup is where the reticle of the optic sits just above the irons when you head is low, but your cheek isn't in a traditional weld with the stock.  Uf your optic goes down, you lower your cheek to the stock as you normally would and the iron sights are visible through your optic.

Basically any rail system that doesn't raise its rail higher than the flattop receiver rail will allow cowtness.  The lower rail on the SIR bilevels are slightly higher, but just low enough to allow the EOTech to cowitness.  It is more desireable to have an optic mounted on a free float rail system to avoid deflection of the barrel, but units such as the RAS, and possibly the Surefire M73, are rigid enough for optics mounting, however barrel droop may become a problem.

Any of the SIR bilevel units (#50 and above) will allow you to cowitness although it will sit a fraction oif an inch higher than any other rail system.  Any of the KAC RAS, RAS II, FFRAS, URX units will allow you to do the same.  Any Daniel Defense unit wil also allow cowitness.  So will the Lewis Machine & Tool MRP.  These are the highest qulaity rail systems available.  However, for light use, any of the other brand free float rail systems will probably do the same.

Ok, cool. Now how should I approach thngs if I
can't spend the money and have to go with Plastic handguards and have to mount both the eotech and arms #40 on the reciever?
               [heavy]

You can easily mount the Eotech and the ARMS # 40 on the receiver. And if you want to as long as your not using M4 handguards you can "forward" mount the Eotech far enough up that the battery compartment hangs over the plastic forgrips.

In fact I ran this exact pattern of positioning with a KAC 300 mtr sight prior to having to move the Eotech fully forward onto the RAS itself.

Damn, I knew someone was goint to tell me that about the  M4  hand grips.  Would that mean that if I wanted (no $ left) M4 foregrips I should buy the eotech that takes the (N?), shorter batteries,
or do the M4 hand grips shoot me in the foot on that scenario also?

You can put a standard size han guard on the top and leave the oversize on the bottom. That way you end up with an aprox. 1/16" ridged on each side and it is easier to grip. The Eotech will go right over it, no sweat.
Jack

Well, if you really want the M4 grips and the AA battery Eotech then just position the Eotech fully back on the receiver.You ought to have room for both.  Otherwise you do have the option of the N batt model that's shorter.

I like the M4 handguards, the upper is going to come down to budget. How much upper is my dollar going to buy vs dollar spent on Furniture
and accesories.  I am going to have to fit a .45 in there also, but the .45 is getting littler and littler each week. I may end up with a .45 cartrige, a piece of water pipe and
rock.
Thanks for your replies.

You should have room for both an ARMS #40 and AA EOTech on the receiver even if you have M4 handguards.  I'm waiting on my ARMS, but I'm confident it will fit.

[img]http://www.sargenthome.com/allpics/hobbies/shooting/AR-15/CavArms/MKII/cav15_mkii_eotech_512_001-1.jpg[/img]

EoTech, ARMS #40, M4 Grips

[img]http://www.supercharger.net/private/m4a3-2.jpg[/img]

That works. What happens as you move the eotech farther away from your eye?

If you have a good solid rail out front, you get a better field of view when using both eyes open and thus faster targeting.
Good shootin, Jack

Those pictures illustrate perfectly what I was trying to get across. Thanks guys.

[img]http://www.thermaldynamics.com/pictures/guns/buis-7.jpg[/img]

where the 552 is mounted will not allow the battery compartment to be remove. but if you notice the rail space available between 552 and the #40 you will be able move the 552 back a copule of rail space and still make the #40 fit and have the ability to remove the battery compartment.

I had the eotech mounted on my flattop, but after having my eotch bounce off the rail a couple of times, I decided back up sights are a pretty darn good idea.  So I put my A3 handle and sights back on this weekend and dialed it in.  I was going with a gooseneck type mounted on handle, thinking it was the way to go.  But I really don’t want to have to observe my rear iron sight; I like a nice uncluttered eotech sight.  But I do want back up sights available and it looks like the ARMs 40 is the way to go.  But….  if I did go back to the flattop mount, I really would like the eotech mounted higher for a more heads up view.  I kept having problems with my glasses and the angle I had my eotech mounted on the flat top.  What are my options to raise the sight picture for eotech and still have back up sights?

Uh, or get Lasik or contacts……

ARMS have #38 rails of various lengths with the #40 built in. Since te Eotech is high you wouldn't be able to co witness, but you would be about a 1/2" higher for your better heads up wants.
Jack

I remember someone showing an ARMS throw lever mount for the Eotech that allowed the rear iron to look under the scope, and gave an even higher heads upnmdisplay. Anyone out there that can give a pic for him?
Jack

You mean the LR1EO?

[img]http://www.calssportingarmory.com/ARMS%20Images/ARMSnumberLR1EO.jpg[/img]

[img]http://www.calssportingarmory.com/_derived/eotech.htm_txt_Copy_of_ViewthroughARMS5440.gif[/img]

He would still need the #38 or SIR if I remember correctly, but it has been used that way by QCB folk as in heads up display with iron sight back up.
Jack

Anyone have a pic of their EOtech mounted with a LR1EO?

[img]http://www.thermaldynamics.com/pictures/guns/arms eotec.jpg[/img]

Wow, that is high.

Wow.  It's as high as I first figured... I was thinking, "nah, that can't be right... it'd have to be really high if the mount would place the Eotech above the BUIS".  How well does it work?

Center the rifle on your chest with the top of the stock right under your chin, look straight ahead and the optic window is right in your FOV. super fast for CQB combat.  Some teams even mount the EOTech on top of the detachable carrying handle to get the sight up higher.

You are better off in CQB with your head strait up for fast movment all round, than head tilted to one side. Thats why it is chest mounted and on the go shooting, vs general and bench rest. A lot are in service the heads up way.
Good shootin, Jack