unlikely to cause an issue if just a bit too low. Many barrel nuts have a feature that indexes on the gas tube - it's impossible to unscrew more than 2-3 degrees with a gas tube in place. Most ones that don't have gas-tube indexing have rails with anti-rotation features. Keep this in mind... many many precision competitors that have switch-barrel bolt-gun setups will screw barrels on hand tight and then give a little smack on the wrench just to make sure it's on to make sure it'll stay snug. I can't see this making more than 15-30 ft-lbs on the barrel, and these dudes are making tiny holes or hitting targets a long way off successfully. Too little torque on an AR15 really isn't a problem as long as it's at least just little more than snug - I suppose you might have to be careful worrying about a nut coming loose on a crappy freefloat without anti-rotation anywhere... but it's very easy to hit 20-30 ft lbs with just plain wrenches.
Too much torque and you risk twisting the upper receiver, shearing a pin, or cracking the threads. You need a big bar and some retard strength to do this though. I've had some nuts up to 100ft-lbs (to hit the next index point) and it turned out just fine. Anymore I just screw things on hand tight and then either tighten to the next index notch (if there indexes) or I just aim for 50ft-lbs and call it good.