The Arms 45 and 46 are free floaters that have a top rail that slides back over the flat top recievcer.  I want to forward mount an EO Tech sight, but I want potential back up co-witness, and I don't know if there is room on the back of the rail to fit the GG&G BUIS

As usual, Wes is 100% right on the money. No GG&G sight will fit behind any of the SIRs. The ARMS #40 is one of the boards most recommended back up sights. It is made to fit behind a SIR, has a lot of great features most others dont, and is one of the most affordable ones you can get. Look hard into a #40.

What makes an ARMS #40 better than a GG&G BUIS?? (besides that it fits behind SIR's)

Why won't any of GGG sights fit behind the sir?

The ARMS #40 has a lot of features and designs that almost no other rear sight has. These are generally why people here like them as much as they do. For starters, it is spring loaded. Which means it is gets knocked, it will pop right pack up into position. To assist this, the protective ears on the #40 are shaped in such a way as to fascilitate the sight to give way and fold down. If you choose to get the model with the horizon line cut into it, that horizon line will act as a BDC for extra long shots on the very very off chance that you might need such a thing.

The GG&G MAD is sort of like looking at a wall with a hole in it. I have one and have had it for a few years now. Good sight, but not an ARMS #40. The GG&G A2 has a locking detent which requires you to release it when you want to stow the sight. The ARMS sight is more easily deployed and returned. In addtion this locking lever will actually fascilitate the sight breaking all together if it is dropped. Much unlike the #40 which has the above mentioned ears that will allow the sight to give way then pop back up into position. These are just a couple reasons why most of us like the ARMS over the GG&G's or about anyone elses. The fact that it costs considerably less money than all of them is just the icing on the cake.

The rear of the SIR systems are machined in such a way as to make it virtually impossible to use any other rear sight except the #40. They are sort of made to be used with one another. The ONLY other rear that will work is the Knights 300M rear. And the only reason it works is because it is so low profile.

The SIR's were designed based from the way the #38 sleeves are set up, they work together. You get about two more  notches so the rear to allow NV monoculars to be back where they need to be, plus give extra space for mounting red dots and PEQ-2 lasers in front on a short M4. Well thought out for militaty needs, and civ. optics have more notches to drop into.
Good shootin, Jack

Does someone know why a GGG rear sight will not fit on a sir. The sir allows space in back for a rear sight to attach to the upper. So why does only arms and kac work.

You're getting the word from two of the most knowledgable SIR people on this board. The GGG DOES NOT FIT.  It doesnt fit because it wasnt meant to fit.  ARMS didnt design the rail of the SIR to interface with other sights, they designed it to fit with the ARMS 40.  The base of the GG&G sight has a squared edge that will not fit with the rail section that goes over the 40's sloped front edge.

THATS why it wont fit.

The Ford hub caps don't fit on the Chevy's, they were not designed to. The same for the SIR, it goes back to the rear to allow more notch and rail area for the reasons I already gave. The #40 is contoured in the front to mate the back of the SIR rear extended rail. This combo gives the troops more rear area to mount equipment they need and thus also gives civilians the those options too.
Good shootin, Jack

This is interesting... The #40 was released LONG, LONG before the SIRs. Was it just a lucky coincidence that ARMS machined the #40 with the slant in the front so that the SIR they would create years later would allow rails to go further back?

AS I mentioned the SIR came from the #38 and I have always known that I could get our NV, etc. back on the #38 than any othe rail set up. The #38 and SIR are the only set ups that I know of that also actually put the rear notch over the rear notch that's on the receiver, matching eye relief, nice.  No one else took the time to helps us on that. All ARMS had to do then was to cut the #38 in such a contour instead of strait, that then automatically gave the same rear notch ability to the SIR and kept the #40 so nice and shot, so as not to take up rail space with a long rear sight that others make. I just put the two same basic products beside each other and could see what they were able to do.
Good Shootin, Jack

I have heard that if you REALLY want the MAD, you can file away the notch on the SIR !!

I can't imagine cutting away a notch that gives added mounting ability, to put on a rear sight that isn't as good as the sight that was made to go there, plus ruin the value of the $400.00 main system. What a great idea.
Jack

Ideas are not always great ideas or the best ideas, but you gotta do what you gotta do. I honestly don't think that cutting the SIR notch is the best idea, it was an answer given to me when I asked a similar question to someone. I would go with the #40 for all the good reasons mentionned.