.625...no way. When we increased the end diameter of the A2 barrel in 1980-1983, we brought the front end's larger (read: stiffer) diameter to about 1/2" behind where the handguard cap is "trapped" between a shoulder on the barrel and the rear face of the gas block/front sight tower.
A1 Rifle and old XM177E2's carbine barrels were this smaller .625 diameter. Colt 9mm barrels remain this smaller diameter as well. Thyat is why our RIS is our recommended rail forend for the 9mm, vs the M4 RAs which needs the .840 diameter up front to stabilize properly.
Real Colt-H-Bar, as I remember, justs continues the .840 behing the front sight tower, fully to the chamber area (actually tapering to an even larger diameter. If you need one measured, I can pull one from the storeroom at work and measure it for you.
ColdBlue sends from home...