anyone know how i can back out or just re-tighten a allen screw that i stripped the head out of on a scope mount?

  Can u take the next size bigger in metric (or whatever your screw isn't) and tap it in with a hammer. I've gotten some to turn that way.

Sears sells screw extractors.  You put them in an electric drill and it digs into the screw and backs them out.

You may also be able to try a spline drive tool.  Tap it into the hex thread and back it out.  

Next step would be a reverse running drill bit followed by a screw extractor if needed.

I'd try this route first.  You could also try a small Torx or Star driver.....I've had decent luck with Torx drivers grabbing what's left of buggered-up Allen-head screws.  Find something that just barely WON'T fit, and gently tap it in.  I've even dabbed a tiny amount of JB Weld on the tip of the Allen wrench, stuck it in, let it harden and unscrewed it.

I wouldn't try re-tightening...you need to get the buggered-up screw out of there.

Epoxy a cheap allen wrench in the stripped head, wait for it to dry and remove.

ETA Beat by seconds.

Pics of the specific configuration would be helpful.

In the WORST case, drill off the head of the booger'd screw using a sharp bit slightly smaller in diameter than the screw head.  Go SLOW, so as to minimize contact with the mount when breaking through; you will feel the screw head come off.

You will then either have enough residual screw sticking out to GENTLY get a hold of it with a pair of small needle-nosed Vise-Grips to rotate it out or not.

If not, break out a tiny drill bit and VERY CAREFULLY drill down into the CENTER of the stub; if it is not a blind hole and you can get all the way through, then bonus points towards eventual success.  Select the appropriate tiny extractor from your tool bin, and cross your fingers.

If it comes out, then chase the threads with a tap, and get a replacement screw.

I went through this taking a rail off a precision bolt gun recently....very nerve wracking, to say the least!

Do you have enough room to tighten the other heads on the mount down a bit to take the pressure off the stripped head?  If you can, it will make it much easier to get the stripped one turning.

find a slightly larger torx bit socket, put it on a extension, hold as straight as possible, strike sharply w/ hammer. Hopefuly this will mesh into the head, then remove with ratchet