If you want to use a flip front sight, you can use any 16" mid-length barrel and the Yankee Hill clamp-on flip front sight/gas block. Because the barrel in front of the "real" gas block is usually smaller than .750, not all gas blocks/front sights work in the "secondary" rifle position. They YHM clamping front sight works like a scope ring and therefore compensates more for this variance.
Even Bushmaster's Dissipator is .740 in front of the GB and uses a special front sight with a slightly smaller diameter hole. However, since the M4 profile is slightly larger than an HBAR in front of the gas block, you're still more likely to have success with other gas block front sights than a standard HBAR or A2 profile barrel. (.730 I believe)
CMMG medcon barrels have the .750 step in the rifle position so you could use any clamp on front sight/gas block, but they are 1:7 twist. They are currently the only place I know of making these as a regular item.
JTAC did make a barrel that had the correct .750 step and was 1:9, (ESR Barrel) but I think he discontinued them. I know I bought one on sale for $200, and they're stainless steel, Mike Rock(maybe?) barrels with a wylde chamber and are very accurate.
ADCO also offered the "dissipator style" barrels once upon a time, but I think he discontinued them as well.