Selvagee;
That's a toughie...I live up in Nederland (the last ungentrified mountain town in Colorado) and shoot on forest land because there are no public ranges in Boulder County or surrounding environs and the Boulder Rifle Club has a 27-year waiting list to get in.
The area I shoot on has obvioulsy been a range forever; it's also a staging area when needed for fire control , so it's hardly what one might call pristine wilderness. Still, there are some trails behind the range area, and I occasionally get some mountain biker from Denver (the locals know better!) who'll give you a hard time.
My policy has been to :
1) First, make sure I was in the right. I talked to the rangers and the local cops before I started shooting, so I knew I was on solid ground.
2) Explain the concept of "mixed use" to the moutain bikers involved. I'm a mountain biker myself, and I don't want to dis' anyone for no reason. Plus, as has been said here, the bikers have been on the bleeding edge of access issues.
3) Offer to tell the bikers where the "real" trails are...out here in the boonies, the trailhead signs are placed well up the trails, not visible from the roads and not necessarily marked on many biking trail maps. You have to know someone local or a biker who has ridden the trails.
4) Offer to instruct the biker on shooting...show them safety, what drills I'm doing, etc.
5) Give them a time frame on when I'm going to be finished and tell them which of the local coffeeshops has the best muffins.
Does this always work? Nope. I've had my ears ripped off by as***les in spandex...usually, they whip out their cellphones--which don't work up here--to try and call the rangers and/or the cops. I encourage them to ride into town and tell the chief of police that I'm shooting up here.
I would like to mention (hesitantly, to be sure) a final point:
6) When target shooting in the backcountry, *always* have a *hot* gun on your person! Usually, this is not the gun you're practicing with. Unless I'm doing USPSA/IDPA draw drills, I might not even have a holster on. And let's not even talk about practicing cowboy! I usually stick a J-frame in my back pocket. I don't much like surprises, especially when I've got hundreds, sometimes thousands of dollars of guns around. By having a second gun on my person, I have the option of clearing the gun I'm practicing with, then setting it on the table, gunbag, whatever, which tends to lower the volume a bit...and still be comfortably armed.
mb
PS: Buy my book, TRAIL SAFE, for more backcountry tips! And watch my show! How's that for blatant commercialism??