I *KNOW* I remember reading about the M4 front site base being a different height than the rest of the ARs. SOOOOOOOO does this also apply BUIS like there'd be a certain one you could and alot that you couldn't use?

yet again maybe a dumb question... Hell my NSN has backups on it as is but i'm just wondering

buis are all the same height .

and the nsn buis is worthless.

yea i pretty much realized that seocnd part

ARMS #40 is designed for the "F" front sight post, so it is slightly taller than some others.  Some people have to resort to a .040" taller front sight post when using the #40 without the "F" FSB.

I have personally noticed slight height differences between different BUIS, with the DPMS fixed detachable being very low compared to a Bushmaster chopped carry handle, and the ARMS #40A being slightly taller than both.

I think the VERY few issues regarding a BUIS is with folks who have the LOWER FRONT.  You should be fine regardless.

Quoted: I *KNOW* I remember reading about the M4 front site base being a different height than the rest of the ARs. SOOOOOOOO does this also apply BUIS like there'd be a certain one you could and alot that you couldn't use? yet again maybe a dumb question... Hell my NSN has backups on it as is but i'm just wondering

Keep in mind- there are TWO different heights of carry handles.