Are the carry handle rear sights different for the M16A4 and the M4/M4A1?
They should be- these rifles have very different sight lengths. The sight radius on a carbine length gas system is about 14 inches. On a rifle length system, it's about 20 inches- nearly 50% longer. So the Ballistic Drop Compensator (BDC) drum on the rear sight cannot work on both systems.
Carry handles all seem to be marked "6/3" (as opposed to the "8/3" on fixed A2 rifles). But are they all created equal? Are some made for the 20" sight length, and others for the 14" length? Are they marked in some way I haven't yet found?
The 6/3 is made for the carbine, the 8/3 is the rifle size. Note that this is sight radius, like you say,
for gas system, not the buffer size and stock. I believe the rifle system is zeroed on 8/3 and the
carbine system is zeroed on Z, one click above 6/3.
Have you seen a removable handle marked 8/3? I'm under the impression that they don't exist.
I believe that the only sights marked 8/3 are on fixed handle uppers, like on the M16A2.
And the BDC is also designed for one specific ammunition.
If you're looking for absolute accuracy, you're not likely going to use the factory numbers anyway.
If you're looking for hits on a man-sized target, as the sighting system was designed, it's likely good enough.
At 300 yds the zero is the same for both. With the shorter sight radius, each click has a greater moa value. So, all other things being equal, one could assume that the using the preset values would lead to a high POI on the carbine sight radius. But all other things are not equal.
Keep in mind, the velocity difference between a carbine and a rifle. All other things being equal, the carbine will have a lower velocity - thereby increasing the drop.
Does this extra drop compensate for the elevated come-ups? I'm not sure, I don't have charts in front of me.
But, I've had no problem using the standard BDC to make hits out to 600yrds with my 16in carbine w/ detachable carry handle.
That was on steel, so I have no idea where the actual POI was.
At Appleseed, we teach the standard come-ups as being 3-3-3-4. That's because they are good enough for most rifles.
The reality is, that the trajectory IS different for each rifle/ammo combination.
Therefor, a prudent Rifleman will confirm his come-ups at known distance and learn whether his/her BDC devise accurate or at the very least - good enough.
"I believe the rifle system is zeroed on 8/3 and the
carbine system is zeroed on Z, one click above 6/3."
Actually, to zero the rifle, move the 8/3 one click down. When transitioning from the zero range to the qual (50-300 m.) REMEMBER TO MOVE YOUR SIGHT BACK TO THE 8/3. The Z was added to the 6/3 sight because of soldiers like, well, me, who forgot.
Rifle sights are zeroed one click "up" from 8/3 (toward the 4) and set back down one click "below" 8/3 AFTER zeroing so they'll be sighted in for 275m or 300yds. At Appleseeds I typically tell people to put it all the way down so they're not monkeying around with it the rest of the day. They should "set it and forget it" during the event. (If they can put it where they need it, cool, otherwise put it all the way down. Only use the front sight for elevation adjustments. Also make sure that peep is on the little hole and NOT the 0-2 "because we're only shooting at 25 so I thought...")
There is a difference between the handles because the M-16A3/A4 has a detachable handle with rifle sights so it extends all the way to 8/3. They're harder to find though.