I've been doing some research in preparation for getting a new pre-ban carbine upper (and yes, it will be going on a pre-ban lower). I had planned on going with a KAC FF RAS but now I'm starting to look at the ARMS S.I.R. systems.

I did a search on the boards for any posts regarding problems with RAS accessories having problems fitting on the S.I.R. and didn't find anything.

IIRC, the new Tango Down foregrip won't work. Right now, I'm happy with the KAC foregrips I have but, if TD makes one that will go on S.I.R.
I'll probably pick one up.

The only things I'm planning on putting on the S.I.R. would be the following:
- Surefire M-900A (I know that will go on fine.)
- KAC RAS Front QD sling swivel (I've read on the boards that these will go on fine as well.)
- EO Tech 552 (if I keep this optic, I'll be getting a #50 Bi-Level (if I remember the model # correctly))
- Maybe an Aimpoint M2 w/an ARMS #22M68 (I know this will work and, if I decide to go with this optic, I won't be getting the ARMS #50)
- TA-31 w/#19S (I know this will work fine as well though I starting to wonder about the height of the optic in regards to cheekweld).

I'd really appreciate hearing any experiences anyone has had with the S.I.R. and mounting RAS accessories.

TIA

I think your term "RAS" accessories is incorrect.

They are simply accessories that attach to the picatinny standard railing, which comprises many products besides KAC.

Of your accessory list, I have the Surefire M900, ARMS #22M68, and KAC grips, which all happen to fit just fine on my #45 and #51.

The only problem I've had with an accessory mounting to railing was when it was a low quality, "other" brand.

This particular item didn't fit on my SIR's, my KAC M4 RAS, or a GG&G UFIR.

That was the ONLY mistake I made when it came to buying accessories.


Chris

If you decide to go Aimpoint, I believe there is a attachment for the #50 that brings the forward rail up to the height of the rear rail, turning it basically into a #46.  That may be the way to go, so if you eventually switch optics again, you have the option of the lower forward rail.

It is easy to figure out. Companies that build rails to the mil std. 1913 male dovetail dimentions that are actually sold to the US military, are required to go thu gov't inspection, and must accept any female attachment device that the US military buys from genuine mil spec and inspected makers of optics, lasers, NV, etc. There are some after market rails and varios attachment items that may claim it is mil spec, but doesn't fit, because it realy isn't mil spec. If it fits the RAS or SIR it doesn't automatically mean it's to spec, but if it doesn't fit at all, then it def. is not to mil spec. The KAC RAS and ARMS SIR rails are without doubt to the mil spec for attachments. So you can go from there.
Good Shootin, Jack  

I think all your items will work beautifully if you don't mind a bit of added height using TA31 over SIR.

I really like my SIR but personally KAC's vertical grip looks extremely "AWFUL" on SIR. Furthermore, I think since SIR is based on "selective" rails you have to put rail on the handguard to use grip, which brings the grip down somewhat.

Even though I bought KAC grip for SIR, I'm either going to use it with my hand or attach Surefire M900A (way expansive for me though).

I have been doing some thinking about the height of the TA-31 w/the #19S mounted on the SIR. I need to check around and see what others have experienced with the same/similar set-up.



While I agree with you on this somewhat, I don't care what it looks like. I care how it functions. I've had an ARMS #23 and I found that uncomfortable to use. I've got a couple of Surefire M-900As but I don't put one on a rifle unless I need to use one.



Actually, I'm more concerned about the lower part of the SIR placing any vertical grip lower.

I'm still kicking the SIR vs. KAC thing around. I've just started doing my research. Thank you for the input.

I thought the beauty of a free float system like the RASII or SIR was that they would allow you to add stuff to the rails without effecting the zero.

Please explain.  Do the accessories cause the SIR to "sag" pulling the optics with it?

Hmmm.  Now I'm wondering why I just bought that RASII....

Thanks,

Corey

I'm under the same impression.  Comments?

This would be concerning optics mounted to the RASII or SIR not your irons.......  If you have an aimpoint mounted and zeroed, then add a light, or whatever, it could affect POI.  Like Paul said, not noticeable at contact but punching paper at long range.

Hold on a minute here. Paul's saying that adding a light after zeroing your optic [b]will[/b] change POI. And Horik, you're saying doing so [b]could[/b] affect POI.

I agree with what others have said. I thought the whole point of a free-floating rail system was the fact it didn't change the POI. What are we missing here?

If the front sight is mounted to the barrel, the barrel isn't going to flex unless a rail sytem is also attached to the barrel, and only if you are putting some pressure on it. There are two types of pressures, one is hardware, but is not going to affect the zero, as long as you do not remove the equipment. The second type is hand pressure, espec. pulling down and rearwad via a fowrad pistol grip. If you use a free float such as a SIR or RAS2, they are more rigid than a barrel cause they are designed to be that way, apposed to a gun barrel that is only designed to shoot bullets, and not be a support pipe/closet clothes rail for hanging things. Any external forces (hands, bipods, on & off equipment)touching or pulling in an un-uniform inconsitant manor on a barrel will scew up accuracy, hence Free Float is best all round.
Good shootin, Jack  

Take an Aimpoint M2 and mount it on either the RASII hump or the SIR rail (but in the same position as the RASII hump, i.e., still basically on or over the upper receiver).

Is this mounting position rigid enough that mounting a reasonably sized light (M900 or M95X series) will not adversely effect POI??

By "optics mounted on the free float tube," are you perhaps more specifically referring to Aimpoints and Eotechs mounted out front on the rails??

I'm hoping that keeping the dot sight over the receiver will ensure a constant POI.  I have an M2 on the hump of an RASII.  If I had an SIR, I would probably be running it in a similar position.

Thanks for the info.  This is a real eye opener.

Not a problem where ever you put the sight, laser, bi-pod, etc. on a FF, you will not see any dif unless you are pulling down as hard as you can to flex the rail via something like a pistol grip. The barrels flex a whole lot easier than the rails we mentioned. The rails are designed to do what your wanting to do, mounting at the front mid or rear.
Good shootin, Jack

This makes complete sense now. I always new that intention of a free-floating tub/RAS was to free-float the barrel. I just never thought that adding weight (accessories) to the RAS would affect POI if you added and and removed accessories. Good info to know. Thank you all.