can anyone tell why m16 is now restricted for only 3 round burst fire eventhough it performs well in automatic also?
This is a very important question.
M-16 is not a very robust design, has gone through many design modifications since the Vietnam War days and today has a decent degree of reliability although accuracy is extremely good. The earlier
M-16s that the US Army used in Vietnam had full auto. However, it is not correct to state that
M-16s perform well in full automatic. Actually
they do not.
With 5.56 mm bullets, they are too difficult to control. In Vietnam War, US servicemen ended up wasting too many bullets, and many even used to use
AK-47s if they could get their hand on one, although it was officially disallowed.
See the video below. You can see from
3:02 onwards, the person is shooting an
M-16 Shorty, that has full auto capability but shoots 9 mm Luger bullets, that are less powerful than the regular 5.56 mm bullets. Even then they are difficult to control in full auto mode. You can imagine how difficult they will be with 5.56 mm bullets. Hence, the US Army, correctly, decided to remove the full-auto capability in
M-16s and that is the same reason with the Indian
INSAS Assault Rifle.