Q. Do M193 and M855 shoot to the same point of impact?
...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.
Fact: 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.
Fact: 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).