At one time US small arms ammunition was classified by its intended use, rifle, machine gun and aircraft. Here is an excerpt from TM 9-1901-1, dated 1957.
"7. Grade of Ammunition
a. Ammunition for 20-mm aircraft guns is not graded; all accepted
lots thereof are serviceable for issue and use in all applicable- weapons.
b. Due to the varying characteristics of the several types of caliber .50
weapons, applicability of immediate action, and the location of the gunner
relative to the weapon, caliber .50 ammunition is classifieds either grade
AC or grade MG for purposes of identification, issue, and use.
Caliber .50 ammunition lots indicating minimum or no probabilities of misfires or
other malfunctions which may cause stoppages in remote 'weapons is
classified as grade AC for use in aircraft and antiaircraft machine guns.
All other ammunition lots within allowable specification limits are classified as grade MG
for use in ground machine guns under the immediate, direct control of the gunner."
The rifle classification was by this time semi obsolete as manually operated bolt action rifles (most tolerant of inconsistent ammo)were no longer in general use.