If you look on Brownells and similar sites they share many of the same parts but there are some parts that are not shared.  The burst cam for one you can see the geometry is slightly different.  There is also an M4 burst hammer and an A2 burst hammer but I can't see a difference.  The triggers appear to be the same but some websites list different trigger assemblies (includes trigger and disconnector springs).  Do they use difference disconnector springs?  The M4 appears to use two different disconnector springs and the A2 version just has two of the same disconnector springs.  Are there other difference between the two fire control? 

Can anyone point me to a good document that shows the changes to the burst fire control as it moved from the M16A2 to the M4?

One of the cams is rounded more slightly, not sure which. For what ive seen the color of the A2 cam is black , the M4, silver

They are functionally identical, and should be interchangeable.

Might have some manufacturer or generational differences in part profiles, I guess.

M16A2-A4- M4
all samey same...

They are not.   I know for sure the cams are different.  I am also fairly sure that one of more of the Disconnector Springs (two per assembly) is different in at least the M4 version of the trigger sub-assembly.



M4 burst cam Colt Part # SP64693



M16A2 burst cam Colt Part # SP64559

Notice the difference in finish and the geometer of the teeth especially the point of the deep tooth.

never noticed and we used them interchangeably...like to hear what the difference is.

The M4 trigger body has larger disconnector spring pockets, so requires larger-base disconnector springs. The M4 burst cam is shiny and is referred to as a "cam, control" instead of "cam, burst", and even has a separate part number for its torsion spring.

I can only speculate on why the different springs - maybe different spring rates to improve trigger feel in semi-auto?

The full-auto M4A1 takes a variant of the old M16A1 hammer (hammer, small) and hammer spring, too.

The differences in the burst mechanisms are explained on pages 73-74 of "Black Rifle II."

Long story short the different firing dynamics of the carbine combined with the A2 rifle's burst parts led to occasional cases of "non-conforming"  (4-round) bursts, so the parts were redesigned.

The Burst cam for the M16A2 and M4 are VERY different. They changed the color to identify each one. The M's burst cam has "clipped tips" to allow for faster response when firing. The M4 has an entirely different timing sequence, due to the length of the gas tube being much shorter than for a rifle, hence things happen faster for the carbines than with the rifle.

Its actually the M16A2 version that has the clip points on the larger notches...