As per my previous posts, I am parts shopping for my first build, a 16" postban RRA A4.  I will need a BUIS for my rifle, and as I would love to get a SIR system for it I believe only the ARMS 40 BUIS will work.  I'm wondering though if I will have any front sight height problems?  Are the two compatabe or will I need to change the front sight post?  Any suggestions on other quality BUIS that I should also consider?  Thanks as always!

As you say, only the #40 will work with the SIR handguard. As for needing a taller sight post....some guys, do, and some don't. My dissipator barrel flattop didn't, but when I switched to an M4 barrel on the same upper receiver, it did. I'd go ahead and get one, just in case; hell, they're only around $5. Even if you don't get a SIR, the ARMS #40 is, IMHO, the best folding BUIS around. I prefer the one with the standard aperture, myself.

If you have a Bushmaster you'll need a taller front sight post.

I ended up using the Trijicon front sight post. It is taller than the stock Bushy post.