what are the feeding diferences between m4s and a2s-  the feedramps?  another question is about dissies.   dpms makes theirs with the standard length gas system right?  bushies have a car llength system and a non gas related fsb that brings the sight radius out.  what would be the reliability concerns of the dpms barrel (assuming it was made by someone else)?

The M4 feed ramps reduce the stoppages (failure to feed) that one may experience due to the faster, more violent cyclic action in the M4. There are slight differences in the upper and the barrel extension as the ramps are cut longer to ensure proper feeding.  Simply, the bolt strips the round harder, so the round needs to be guided a little more as it feeds.

I'm not exactly sure what you're asking in the next questions.

16" barrels using the standard rifle gas block placement require the gas port to be enlarged in order to achieve reliable function.

i see...      how does the distance from the gas port to the muzzle figure into the equation?

yes.


the dpms dissipator is the same as bushmaster i believe.

Bushmaster makes two different dissipator barrels, a heavy config (like the DPMS) and an M4 dissipator barrel.

Heavy


M4

Quoted: i see...      how does the distance from the gas port to the muzzle figure into the equation?

High-pressure gas is following the bullet down the bore. After the bullet passes the gas port (but is still in the barrel), the only place for the gas to go is up through the gas port and down the gas tube, which cycles the action. Therefore, the distance between the gas port and muzzle is critical.

If you shorten this distance too much, the bullet will exit the barrel before enough gas gets siphoned back to the bolt carrier. This will cause a short stroke malfunction. One way to rectify this situation is to enlarge the gas port so that more gas makes it to the action. How much the gas port is enlarged is critical, as too large a gas port causes problems as well. Bushmaster has taken a different approach by using a carbine length gas system in order to have ample amount of barrel past the gas port, but putting another FSB at the rifle-length location in order to achieve a longer sight radius. Therefore, it appears to be a rifle barrel that has been cut down to 16", but it has more reliability.