At first glance, you might say that it is idiotic to put a "backwards taper" on a barrel, going from skinnier to heavier toward the muzzle end -- and you would be right.
The story behind the "gov't profile" barrel is a big bureaucratic mess, as the name implies. The original M-16 barrel, as designed by Gene Stoner, was a 20" pencil profile all the way, and they handled wonderfully. In the late 70s the DOD wanted to redesign the M-16 to work with the new 62 grain round NATO was switching to, and all the branches started piling on with what other new features the A2 would need to have. The Marines wanted to make it more of a target rifle, because the A1 sucked on their 500 yard qualification course; so they lobbied for fully click adjustable rear sights (which they got) and a heavy profile barrel to cut down on vertical stringing and mitigate the POI shift that you get from sling-supported shooting. However, this conflicted with the Army's desire that the new rifle still be compatible with their old M203 grenade launchers, which clamp to the barrel before the gas block... they wouldn't be able to attach to a heavy barrel. So the DOD bureaucrats tried to "split the baby" on the barrel profile, and made the ridiculous decision to keep the barrel pencil profile before the gas block, and make it heavy profile AFTER, where it is completely useless and adds weight in the worst possible place for handling. Furthermore, since it is still pencil profile under the handguards, it still suffers from all the same issues that the Marines wanted to avoid. A classic government FUBAR committee decision.
Then when they decided to requisition a carbine version (the M4), the DOD had the opportunity to use something other than that idiotic profile... but instead they just made a slight modification to it by "notching" it down to pencil diameter on a spot on the barrel forward of the gas block to allow the M203 to attach (the shorter barrel puts the front attachment point in front of the GB). And of course it is still pencil profile under the handguards.
Of course the civilian market largely wants to imitate whatever the military is doing, so most commercial "M-4geries" also have FUBAR profile barrels. Not me... make mine a pencil please. They have noticeably better handling. Other than cost, I see no reason to go with FUBAR/gov't profile. If you have a gov't profile barrel, it can be milled to pencil profile... Adco mills down barrels after the gas block and re-parks for $65. I have a 20" A2 barrel that I am probably going to send them pretty soon.
"This motto may adorn their tombs
(Let tyrants come and view):
We rather seek these silent rooms
Than live as slaves to you."
Lemuel Haynes, 1775
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