NEWS | Sept. 30, 2019

M16-Series Rifle & M4/M4A1 Carbine: Fitting Rack Solution

Dear Half-Mast,

I’ve had trouble getting our M4 carbines to fit securely in the M12 racks. Of course, that’s critical to security. Has the Army come up with any fixes for this?

Dear Staff Sergeant,

Yes, there are solutions……but which one you use depends on why your M4s won’t fit.

Older M12 racks need an adapter bar so that M4s can’t be removed while the rack is locked. The instructions for fabricating and installing the adapter bar are in WP 0032 of TM 9-1005-319-23&P.

But if the problem is caused by the new fire control selector that was installed on M4s and M16 rifles several years ago, the fix is different. The slots for older M12 racks aren’t wide enough to accommodate the selector.

There are two approved methods to widen the slots: 

  1. Open the plier grips fully. Position the welding pliers 0.5 inches back from the face of the rack. Set the jaws to a size smaller than the sheet divider when the jaws are closed.

  2. Squeeze the pliers in place on the rack until they lock and then remove them.

  3. Test to see if the weapon fits in the rack slot.
  4. If necessary, adjust the diameter of the pliers and go back to Step 2.


Use a 1 to 1 1/2-in diameter steel bar and a 5-lb mini sledge hammer. Insert the bar in each rack slot and pound the bar with the hammer. That rounds out the flat portion of the slot, making it longer and narrower as well.

Test each slot to make sure it securely holds the weapon.

Any time you modify a weapons rack, you must have it certified by your local small arms LAR or security officer.

Fortunately, all M12 racks manufactured after Sep 2013 don’t need these modifications.