KAC RAS Top Rail Taller than Receiver

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Cowboy7

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Hey all,

I've got a KAC RAS on a BCM upper, and I'm noticing that the top rail seems to be just a tiny bit taller than the receiver's top rail. If I mount the EXPS-2 on the receiver, I have enough visible front sight for lower third cowitness, but with it mounted on the handguard I can only see about a tenth of a millimeter of the post. Is it something common to all RAS units that I'm just gonna have to live with, or could perhaps a gunsmith get the bracket to sit lower? I can make do with the visible portion of the post, but it will definitely be slower to pick up and potentially harder to center in the rear aperture consistently.

I'm not losing any sleep over it, just curious if there's anything to be done to improve it.
 
Bought it off a user here with good rep, markings are all good on the top and bottom. Was used overseas, so probably wasn't a factory second (I also don't see anything visibly wacky about it). It's less than 1.5mm off judging by the difference in sight picture.
 
I'd like to see a picture maybe?

Going from a lower 3rd co-witness to a 10th of a mm visible (maybe I mis-interpreted the post?) is a LOT of distance and it sounds like something is wonky.
 
Cowboy7 said:
Hey all,

I've got a KAC RAS on a BCM upper, and I'm noticing that the top rail seems to be just a tiny bit taller than the receiver's top rail. If I mount the EXPS-2 on the receiver, I have enough visible front sight for lower third cowitness, but with it mounted on the handguard I can only see about a tenth of a millimeter of the post. Is it something common to all RAS units that I'm just gonna have to live with, or could perhaps a gunsmith get the bracket to sit lower? I can make do with the visible portion of the post, but it will definitely be slower to pick up and potentially harder to center in the rear aperture consistently.

I'm not losing any sleep over it, just curious if there's anything to be done to improve it.
Click to expand...

Why are you trying to put the Eotech on the rail?
 
Big_Red said:
Why are you trying to put the Eotech on the rail?
Click to expand...

Personal preference. I've always mounted optics just fore of the receiver on FF rails, putting it on the RAS feels about right to my eye, although it's a bit further than usual.

CarbineOne said:
Are you sure both the front and rear bands are under the front hand guard cap and between barrel nut and delta ring?
Click to expand...

Yeah, everything looks OK with the RAS, you should be able to see if anything's up (barely) in the side profile photos below.

F22_RaptoR said:
I'd like to see a picture maybe?

Going from a lower 3rd co-witness to a 10th of a mm visible (maybe I mis-interpreted the post?) is a LOT of distance and it sounds like something is wonky.
Click to expand...

It may not be a full lower third cowitness when it's on the receiver than, or maybe I'm judging the visible amount of front sight wrong when it's on the rail, but hopefully the following pictures clarify.

Mounted on receiver:

mSRXUm7.jpg


Mounted on rail:

dTZC6Wz.jpg


Position on the receiver:

ly1GqpY.jpg


Position on the RAS:

Sle07JP.jpg
 
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While I respect the opinions and experience of the CGNers chiming in here, we are getting sidetracked from the original purpose of this thread. The consensus among folks who have run optics on the RAS seems to be that shift in zero on even a smoking hot rifle is negligible out to 200 yards or more. This thing will never have more than 3 power glass on it, so I don't see it becoming an issue. If it does, however, I wouldn't hesitate to change the setup.

I'd still like to know if anyone has suggestions or experience with this rail height issue.
 
Wow you can tell the height difference in that.

One more thing to ask, if you take a strait-edge, and put it (spine or edge down) onto the top handguard picatinny rail, does it come back completely level with the flat-top upper rail? It almost looks from the picture that it's not perfectly level (aka the front is higher or something)

Does it appear locked into the front handguard cap properly? If it's really tight a small rubber mallet or a piece of wood could help seat something fully.


If it's locked properly into the handguard caps, then I can really only assume something is out of spec, which is weird since BCM is usually good, and KAC is usually really good. Maybe if you Email Farmboy (One Shot Tactical) he can give you some advice on known compatibility issues? I'm not sure filing anything down would get it to fit better, unless it's not locked properly into the delta-ring as GreenTips asked about before (like it's too tight on the barrel nut and needs fitting to lock in all the way)


Anyways, hopefully this gives you some ideas on what to check, but to me it looks like the quad-rail is an older one not necessarily designed for the exact same height as the flat-top upper, as what you see the old JTF-2 guys doing with the A2 uppers and quad rails!
 
I have a feeling it has to do with the age of the rail. It's definitely 100% locked in place, just seems to be a smidge higher than the receiver. Ah well, that's what zip ties and QD levers are for. If it ends up making too much of a nuisance of itself then I will probably shift the optic back to the receiver.
 
Something doesn't look right. I checked mine and its pretty level from the receiver to the rail. I took a rail cover and you can easily side it back and forth from the rail to the receiver. Mine is a Colt receiver no BCM but if its to spec you would think they would line up.
 
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