First, to be clear, since there's been some confusion about this on past threads discussing a similar issue: We're talking about a 1/16"-1/8" gap between the front band lip that hooks under the ferrule and the ferrule itself, not the distance between the front surface of the ferrule and the back surface of the front band?Looking at my new LRB build I noticed that the tab on the front band is not touching the ferrule, maybe 1/16"-1/8" gap. I'm using a USGI synthetic stock. The lockup at the trigger group is perfect getting tight at the tip of the trigger. The receiver heel is in contact with the stock. The gas cylinder is not unitized or shimmed, but the front band is solid and doesn't move at all. There is the proper vertical clearance at the front band...
There isn't an awful lot of room between the inside top surface at the bottom of the "U" shape of a standard ferrule and the bottom of the gas cylinder sitting just above it. So part of the purpose of any tension is to pull the ferrule down and away from the GC. That much of a gap suggests the ferrule is already either resting against the GC or really close to it.
One way to get both tension and some distance between the ferrule and the GC is to take advantage of the see-saw relationship which can exist between the stock and the barreled action when the trigger group is locked up. You can accomplish that with the shims that NoExpert mentioned (I guess I'm at least one of the guys since I use such shims regularly).
If you look carefully at the stock, you'll notice the area just forward of the bolt lock on both sides is raised slightly. Adding shims there will increase the lockup tension and move the ferrule down towards the front band lip. If lockup tension is already considerable, you might want to use something like a furniture clamp to pull the heel down on the stock rather than making the trigger guard do the work.