I have never seen a 6721 barrel; But basically, foward of the front sight tower its identical to the light weight M-4 barrel. Under the handguards, its almost identical to a Bushmaster (only one I have seen) 16" HBAR. Only real difference is that there is the flat sided cut for the 203.
As for the weight, its noticable, but not really. By the time you put an optic, RAS, PAQ-4, the thing weighs a ton anyway. My soldier was happy, as he had an M-16 before.
In the 7 years or so that I have had an M-4A1, with shooting it 3 or 4 times a month, 600 or so rounds per session, I have only put it on auto maybe 10 times. Thats not counting just ammo burning. The only time I have ever been taught to use auto is during break contact drills, "mad minute" base defense, stuff like that.