The following is an excerpt from the updated US Army "Sniper Training" Field Manual 23-10, chapter 4, section 11: Occupation of Sniper Firing Positions.
Knowing how to shit in an MRE bag while prone is essential to the modern sniper’s mission. Contamination of a hide site with the olfactory cataclysm that is an MRE shit is unacceptable and should be avoided at all cost. Fortunately, the source of the problem is often part of the solution. MRE bags are great for shitting in.
There are many times over the course of your career as a sniper when you will be laying in a hide site, leg interlocked with your spotter on the 32nd hour of a 48 hour overwatch, wondering how you’re going to poop. It’s sleeting, freezing, and you haven’t slept since before the 40-minute ROE brief your platoon leader forced you to endure before the 11-mile infil. Quitting is not a word that any part of your body understands — except your colon.
In order to tactically and efficiently take a shit, move through the following list point-by-point:
Remaining in a good, prone, supported firing position, remove and unfold MRE packaging from your cargo pocket using your non-firing hand.
Bring MRE packaging into your work space and blow a little air into it like a balloon. Mentally note that your poop is not to exceed the size of the inflated packaging. Place packaging beneath rifle butt stock so it doesn’t blow away.
Keeping eye on glass and one leg in line with your rifle, slowly move your other leg over the support leg so your weight is on your hip and your anus is kind of pointed at the ground. If concealment foliage is disturbed, correct it.
With your spotter's assistance, pull your pants down 2 – 4” below your buttocks.
Still using your support hand, move bag to your butthole. Press one end of the opening into the taint (USMC Sniper parlance: “fun bridge”) and try to clench it in place. Hold the bag open around the butthole.
Poop in the bag.
Use re-sealing strips to close bag. Fold edge as many times as space allows and tie package closed with 550-cord.
Place package back in cargo pocket for use back at the bay. Leave pants in place. No unnecessary movement.
Note that the above practice is difficult and requires repetition in order to perfect. Sniper section leaders must ensure its placement in the training schedule while at garrison.