as sold by Fulton has me wondering what specific model rifle would they have been made for?  I think the date code says 8/86(could be '96, stamp is light on one side, hard to read).  Rollmark says "C  MP 5.56 NATO 1/12", and the FSB is marked with an "F" on the left side.

I had been looking off and on for months to locate a chrome lined 1/7 A2 barrel for a build, then realized I mostly shoot M193 spec ammo anyway, and have had awesome results with lighter bullets like win 45 jhp and fed 50 jhp in my 1/12 bolt gun.  A recent build with an M16A1 tube shot well enough, and my range only goes to 200 yards anyway.

Why would Colt have built medium weight A2 tubes in 1/12, after the 1/7 twist had become standard?

Paladin

replacement barrels for the M16-A1?

You sure these are 'Govt Profile' (M16A2) and not the M16A1 profile (narrow the entire length)?  I know Fulton was selling surplus M16A1 barrel (which are 1:12).  And yes they could have been manufactured in '86 as replacement parts for the thousands of M16A1s still in use.

Fulton has a "med. wt." 1/12 for the A2 shown on their web site.  The Fulton p/n is FA-AR-300-050A2 and is described as "Barrel, 20in GI, 1x12, M16A2, Chrome Lined".

And yes, they are skinny under the hand guard, and larger in front of the FSB, i.e. "govt. profile".  The park was very dry, and some shelf wear of the finish is evident, but this is not for a safe queen anyway.  "New" when the item has sat around for 15 or 20 years does not mean "mint, factory perfect" to me, so I feel it was worth the $.

I have a RRA A4 upper to install it on for a "vintage varminter" look with the M16 triangular handguards.  The barrel has a snug fit, and no detectable play once the pin is in the slot.  A compact 4x needs selected, and will sit in Leupold Mk4 rings.

They also now are showing a 1/7 A2 tube by FN on their web site, FWIW.

Paladin

I've seen some of those barrels that Fulton is selling. They are A2 "Govt" profile 1/12 chrome lined barrels. I have heard - but don't hold me to it - that they were part of a production run that Colt made for export.