The Marine Corps does generally doesn't conduct zero a rifle zero at 25 meters
From the FM 3-22.9 Rifle Marksmanship, page 2-5
NOTE: The “Z” marking on the elevation knob (employed when using the M4-series weapon's detachable carrying handle) should be ignored. The “Z” marking is only used when the M16A4 is being zeroed.
From MCRP 3-01A Marine Rifle Marksmanship
9005. Initial Sight Settings
Initial sight settings are those settings that serve as the starting point for initial zeroing from which all sight adjustments are made. (If the Marine already has a BZO established on his rifle, he may begin the zeroing process by using the previously established BZO sight settings.) To set the sights to initial sight settings:
Front Sight Post
To set the front sight post to initial sight setting, depress the front sight detent and rotate the front sight post until the base of the front sight post is flush with the front sight housing.
Rear Sight Elevation Knob
To set the elevation knob at the initial sight setting, perform the following:
- Rotate the rear sight elevation knob counterclockwise until the moveable rear sight housing is bottomed out on the upper receiver.
Once bottomed out, the rear sight elevation knob should be three clicks counterclockwise from 8/3. If the sight fails to move three clicks counterclockwise from 8/3, it
must be adjusted by a qualified armorer.
- Rotate rear sight elevation knob clockwise until the number 8/3 aligns with the index mark located on the left side of the upper receiver
To set the windage knob to initial sight setting, rotate windage knob until the index line located on the top of the large rear sight aperture aligns with the centering
on the windage index scale located on the moveable base of the rear sight assembly.
9006. Zeroing Process
During the zeroing process, all elevation adjustments are made on the front sight post. Once a BZO is established, the front sight post should never be moved, except when rezeroing the rifle. (The rear sight elevation knob is used for dialing in the range to the target.) Zeroing is conducted at a range of 300 yards/meters. To prepare a rifle for zeroing, the rifle sights must be adjusted to the initial sight settings as outlined in paragraph 9005. Perform the following steps to zero the rifle:
-Fire a 3-shot group.
-Triangulate the shot group to find the center.
-Determine the vertical distance in inches from the center of the shot group to the center of the target.
-Make elevation adjustments on the front sight post to move the center of the shot group to the center of the target.
-Determine the horizontal distance from the center of the shot group to the center of the target.
-Make lateral adjustments on the windage knob to move the center of the shot group to the center of the target. Repeat preceding steps until shot group is centered.
-Fire a 4-round shot group to confirm sight setting.
Once the sight setting is confirmed, determine the value and direction of the wind and remove the number of clicks added to the windage knob (if necessary) to
compensate for current wind conditions. This becomes the BZO setting for the rifle.
9007. Battlesight Zero
Zeroing is conducted at a range of 300 yards/meters. If a 300-yard/-meter range is not available, a field expedient BZO can be established at a reduced range of 36 yards/30 meters. When a rifle is zeroed for 300 yards/ meters, the bullet crosses the line of sight twice. It first crosses the line of sight on its upward path of trajectory at 36 yards/30 meters, and again farther down range at 300 yards/meters. Therefore a rifle’s BZO may be established at a distance of 36 yards/30 meters and the same BZO will be effective at 300 yards/meters. To establish a field expedient BZO at 36 yards or 30 meters when a 300-yard/-meter range is not available, a Marine performs the same steps as the zeroing process outlined in paragraph 9006. However, since wind does not affect the round at 36 yards/30 meters, windage is not added nor is it removed from the windage knob after confirming the BZO. To be accurate, the target must be placed exactly
And from the new version (draft) of MCRP 3-01A Marine Rifle Marksmanship
A universal BZO target (see fig. A-1) is used to establish a prezero sight setting at 36 yards/33 meters with the service
rifle/carbine. The target can be used when employing the RCO or iron sights.