I installed a Knight RAS on my rifle and something is a bit sideways...  Just from looking, the rail looks a bit canted but so does the FSB.  Is there a test to determine whether or not the FSB is straight?  How is it done at the factory?  

BTT for info

What brand is the rifle?

If it's a Bushmaster, there has been quite a few canted FSBs from them, mine included...

Nope, it's a Colt but the brand is totally not an issue for me...so please, no OMG Colts aren't perfect or anything like that.  

Maybe it's just me...the rifle may shoot great and have zero issues.  I just need a way to know if the FSB is actually canted or if it is just my mind playing a trick on me when I look at it.  

I put a RAS on it and it's strange, the RAS isn't level with the Receiver and neither of them looks to be level with the FSB.  But like I said, it's hard to tell from just looking.  I need an actual way to test it.  

More than likely it's either the RAS is on canted or it's an optical illusion. I know that staring at the FSB, trying to aline it with the upper and a rail system will make it look like it's canted.

You will not get any Colt bashing from me...I have 2.

If your RAS is not level with your receiver rail, your barrel nut is not timed correctly.  With regards to your FSB, I just put two rear sights with the apertures centered on the receiver rail and look through the holes to see if the FSB is in alignment.

shoot it and see then, there was some incidents of canted fsb's on colts but im sure its a crooked mounted rail

Quoted: If your RAS is not level with your receiver rail, your barrel nut is not timed correctly.  With regards to your FSB, I just put two rear sights with the apertures centered on the receiver rail and look through the holes to see if the FSB is in alignment.

The barrel nut is a bit off from TDC, not badly but it is off.  I tried the test with the carrier only by letting it slide into the receiver in order to check the gas tube alignment...there is a bit of bind in the last 1/4 inch which will keep the carrier back.  

The RAS is twisted in the same direction that the barrel nut is turned too far.  

It's minor...but you can see it.

I have a Colt Match Target HBAR 20" 6700....... And I'm pretty sure the FSB IS canted. The FSB is canted to the right just a tad, nothing drastic, but enough to cause an annoyance with someone who's O.C.D. about weapon perfections. :-)


-Covert

Quoted: I have a Colt Match Target HBAR 20" 6700....... And I'm pretty sure the FSB IS canted. The FSB is canted to the right just a tad, nothing drastic, but enough to cause an annoyance with someone who's O.C.D. about weapon perfections. :-) -Covert

My 6920 is the same way and it REALLY bothers me, need to get if fixed I guess

A simple test I do is to turn your AR 15 upside down and place in on a level surface.  Some thing like inspection blocks, mill table, a piece of glass or the kitchen table.  The flatter the surface the better the result.  At this point the ears on your front sight and the ears on the rear sight will be the only thing touching.  If you have 4-point contact your front sight is top dead center.  If you have 3-point contact, it will rock with little pressure, the more it rocks the further it is off.  Remember this is a fast field epedient method.  If your ears are not of equal height you won't get a true result.  The rear sight (A2) is almost always canted, take that into consideration.

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