...but within 300 yards, they're generally close enough (for combat use) that rezeroing isn't necessary. Obviously, you wouldn't want to switch from one to the other for a match without rezeroing. Consider the graphs below with battle zeros for each round. (250m zero for M193, 300m zero for M855).
Even out to 300 meters there is a mere 4.5 inch difference between the paths of the rounds
The big differences in bullet path are past 400 meters
A: M16A1 firing both M193 and M855, zeroed for M193
B: M16A2 firing both M855 and M193, zeroed for M855
- The Scoop from the Army's Ammunition Information Notice (61-01) "INTERCHANGEABILITY OF 5.56MM BALL, TRACER AND BLANK AMMUNITION."
- Do not zero M16A2, M16A3 rifles or M4 and M4A1 carbines with M193 and then fire M855/M856 as performance will be affected.
- Generally M193 is zeroed out to 250 meters for the flattest trajectory. Using that "battle zero" the round is never more than 4 inches from the point of aim until almost 300 meters.
By contrast M855 is usually "battle zeroed" to 300 meters. With this zero the M855 round is never more than 6 inches from the point of aim until 325 meters.
Comparing the bullet paths with these zeros out to 300 meters, we find that M855 is about 5 inches higher than M193 at 300 meters.
M855 (left) and M193 (right)