What are the consequences of a barrel nut torqued to low say 25lb.s and or one torqued over 80lb.s? Has this happened to anyway?
Not judging anyones mishap or skill...but say you went back to check down the line and realized your wrench at the time was unwittingly out of spec etc etc

Too low and might loosen when shooting
Too high you could shear the index pin

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.

Had the first happen to me. Urx4 I did not want to mar up.

If it is a thru nut gas tube version,
it really can't back off more than a tad

True, but it'll give you accuracy issues.

On a pistol cal build specifically if the barrel is able to slide forward enough it could give you a dangerous amount of extra headspace.

Tight is tight.  Numbers are for rookies.  You would be surprised how light switch-barrel shooters tighten barrel nuts.

Thank y'all very much. Not having a torque wrench this go around this advice gives me comfort now. Much appreciated

Look on their forums, switch barrel shooters sometimes like going up to 150 ft lbs.   You don't see many happy with 15 as mentioned here.  I like mine snug without going over 80 as spec'd.  I set my torque wrench to 35, when it clicks, I switch to a non clicking wrench (fixed beam) and screw/unscrew until it gets to the next notch for the gas tube.

http://benchrest.com/showthread.php?66643-Switch-Barrel-Torque/page4

And the guys I shoot with like to be able to switch them out on the tailgate of their pickup.

Take the time to do it right.

This is not accurate.  Headspace has nothing to do with how far a barrel is in the reciever.  Headspace is established when the bolt locks into the barrel extension.  You can check headspace without the barrel installed in the reciever.  Reciever has no bearing on headspace.

ETA: Barrel nut torque has zero effect on headspace either.

Most pistol caliber bolts don't lock into the barrel extension. Then what?

You are correct.  I missed the pistol caliber part.

The barrel nut would have to be sloppy loose to allow a barrel to slide forward even a fraction of an inch, and being the handguard is attached to the barrel nut I think you'd notice.  And even if there were all that would likely happen is a fail to fire.  How would you get a "dangerous amount of headspace" with a pistol caliber?  People shoot 45 Colt in 454 and 460 pistols all the time as well as 38 Special in 357 magnum and so on.  Hell, people even shoot 40's in a 10.  There is no such worry.

saint stoner designed a marine resistant gun

Lol

You know you can get a workable 1/2" SAE ft-lb torque wrench at Harbor Freight for around $20?  And you can usually find a 20% off one item coupon that you can use.  Sure, it ain't Snap-On, but it is good enough for this application, and inexpensive peace of mind.

If you really can't afford it, just about any shade tree mechanic should have one you could borrow for 15 minutes.