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Posted: 8/12/2004 7:32:03 PM EDT
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Is a taller front sight required for use with an RRA M4 16" upper for use with an ARMS #40 buis. I remember reading something about this but cant find it now. Thanx 
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Good question. It applies to all flat top AR rifles. With the exception of LMT or Colt, all commercial AR flat tops need a taller front sight post to zero properly with a Mil-Spec BUIS, like ARMS, GGG, KAC, Colt Carry Handle, etc. Reason being is there are 2 types of front sight bases. One made for the A2 model upper receiver and another (.032 taller) made for flat top upper receivers. But most AR manufacturers use the A2 type Front Sight Base on all of their barrels, and as a result on all of their barreled uppers - (A2 and Flat top). So the A2 Front Sight Base is machined a little too short when used on a flat top, and the taller front sight post helps to fix that problem. Just to mix things up a little more....the manufacturer will use a shorter carry handle to compensate for the A2 Front Sight Base being mounted on the Flat top upper receiver. So it zeros ok, ... until you remove the carry handle and use a Mil-Spec BUIS. |
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Thanks for helping clue me in, Bravo. Let me make sure I got this clear. First off, this applies to flattops only, whether 16" or 20"? My current 2 flattop uppers have the following config. Here's what I have: 1) Colt A4 (flattop) upper w/ ARMS #40 BUIS, factory FSB. (Aimpoint) pic 2) Bushy A3 (flattop) upper with Bushy brand A3 carry handle, factory FSB. pic My 2 Qs: On my Colt upper, I shouldn't have to worry about the #40 BUIS and the FSB being out of alignment because the rifle is Colt (milspec)? On my Bushy upper, I shouldn't have to worry about the irons being out of alignment because the FSB and A3 carry handle are both Bushy? However, if I were to put an ARMS #40 BUIS on this Bushy upper I would have to get the taller FSB because it is Bushy and not "milspec"? |
That is exactly correct. As a side note: Most (I have seen exceptions) of the time a front sight base that is machined for a flat top barreled upper (Milspec) has a stamped "F" (F for flat top) on its left side. (again there are exceptions) EDITED TO ADD:
Yes - exactly correct
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While working on a variety of uppers, I have seen instances where the taller front sight base (flat top type) was used (although it was not marked with the "F"), but it is rare. For what it is worth, I would try to zero the rifle. If it is good to go, you're done. If you find that your front sight post is raised to the point where it is almost unthreaded to get a zero, then you might need a taller post. Taller Front Sight Post From Bushy |
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