NEWS | Feb. 11, 2020

M16-Series Rifle and M4/M4A1 Carbine: Leave Rail, BUIS Alone

Sometimes it’s better just to leave things alone. Certainly that’s the case with the M16 and M4/M4A1’s mounting rail and backup iron sight (BUIS). 
Mounting Rail         

Soldiers, don’t take off the upper portion of the mounting rail for cleaning. There’s no need for cleaning it and only small arms repairmen are authorized to take off the upper portion of the mounting rail. That’s because only they have the tools to do it.
M4 rail
Only small arms repairers should remove this screw
You can remove the mounting rail’s lower portion and the rail covers for cleaning, lubing or to attach accessories like sights.
Backup Iron Sight
Soldiers sometimes remove the BUIS so it doesn’t interfere with the other sights. That’s not a good idea! 

It’s called backup for a good reason: if the fancy sights fail, the BUIS is there as a backup.  In addition, the recoil screw must be replaced every time it’s removed. 
BUIS recoil screw

Remember, your weapon is NMC if the BUIS is missing. Install it in the rear slot on the mounting rail. 
Keep the BUIS in the down position unless it’s in use. Otherwise, it eventually snaps off. If the BUIS pops up during firing, get it replaced. It’ll interfere with the other sights. 
Small arms repairmen can save units money by not replacing the entire BUIS at a cost of around $60. Just replace the sight aperture, NSN 1005-01-497-2592, for $10. The instructions are in WP 0010 in TM 9-1005-319-23&P (Apr 19). 

BUIS broken? Order just the sight
aperture, not entire sight