I double checked the twist rates, the M16A2 introduced the 1/7 to deal with the heavy M855 vs the M193 in the M16A1 with a 1/12.

As I understand it the heavy load needs better spin to stabalize, the M855 would start to tumble in the 1/12 at 100 yds.

As far as length, I was speaking of the gas system. You can get a 16 in carbin or mid length (http://www.del-ton.com/AR_15_Barrel_...y_p/dt1023.htm)

Seems the weight would be the same or close and the portability and easy of use would be the same.