In order to not hijack someone else's thread, I've started my own.  After reading about the front sights and zeroing with certain BUIS, I've come to find out that I may potentially have trouble doing so.  I've got a RRA Tactical Entry upper (flattop with 16" barrel), and from what I've read, BUIS may not zero properly without switching out my front sight post to a taller one.  I WAS going to get an ARMS #40, but if it will not zero properly I should look elsewhere.  Can anyone tell me if I've got this info correct and which BUIS is compatible with my RRA flattop upper for proper zeroing and aiming?  Or perhaps I'm just S.O.L. and only have the option to swap out my FSP.  Thanks for the help.

My suggestion?  Take it to the range and shoot it off of sandbags first to see where it actually shoots.  The sandbags are important since you don't want your gun wobbling around when you are sighting in, and the actually shooting part...well, some ARs hit high (due to the FSB being a tad short), but some do not.  So, before you go spending anymore money, just shoot the darn thing.  

Edited to say: after reading your post more closely, it seems you don't have a rear sight so you might need to get one first.  The ARMS #40 is an excellent BUIS.  Regardless of the height of the front sight post, the ARMS will zero.  It just might hit a bit high due to the front sight post issue.  If it does, call Bushmaster and ask for a .040" taller front sight post ($4.95) and it'll bring the POI down to the POA.

My ARMS#40 zero'd just fine with my standard FSB.  You gotta pick your equipment, then make adjustments.

If you want the ARMS#40, get it. It is an excellent sight and the +0.040" post only costs $5 from BM. In fact you may not even need the taller FP. I have the same setup as you and didn't need one.

Thanks for the replys.  Worst case scenario, if I DO need to get a new FS, how long would it take for a gunsmith to do the presto-change-o?

Quoted: Thanks for the replys.  Worst case scenario, if I DO need to get a new FS, how long would it take for a gunsmith to do the presto-change-o?

You WONT need a whole front sight BASE.  If you DO need a *front sight tip* then thats just an unscrew/screw in..... same as making an elevation/sight-in adjustment.  Better not need a gunsmith for that.  

Quoted: Quoted: Thanks for the replys.  Worst case scenario, if I DO need to get a new FS, how long would it take for a gunsmith to do the presto-change-o? You WONT need a whole front sight BASE.  If you DO need a *front sight tip* then thats just an unscrew/screw in..... same as making an elevation/sight-in adjustment.  Better not need a gunsmith for that.  hr

Oh, Jesus Christ - I was thinking I'd have to replace the whole damn FSB just to get the extra .040".  I was cussing at the retarded engineering that was!    That's quite a relief.  I was thinking this was going to be a real pain in the ass.  WHEW!  Thanks for the correction.