What causes a barrel to not easily unlock when the trigger guard is pulled fully to the rear?
Barrel lug binding on the trigger housing up front under the hinge pin hole on old style frames. Spring in the locking bolt hole too long..... compressed fully by not enough to let the bolts clear the end of the lug, but you checked that. The end of the sear arm too high, or the overtravel set too close to permit the sear to set the striker. The trigger has to travel a certain amount in order for the trigger housing to move up and cam the locking bolts out of engagement. If you have a combination of the overtravel set too close (back it out and see if the problem changes any) and the surface of the sear arm that pushes the striker down extends down too far ( too high in reverse), the trigger housing cannot travel far enough. If you need to keep the present overtravel setting, remove the sear and grind some off of it where it contacts the striker. Where is this? Whip out the felt tip pen and ink the bottom side of the sear arm. Put the trigger group back in, and work it a few times to get a signature on the sear.