I recently came into a Contender barrel with one scope mount screw that had the head broken off.
No problemo says I so I proceed to clamp it in the vise (yes protected from the jaws), latch onto the screw with a pair of small vise grips and give a gentle turn. The screw body gives way quite handily. Time to back up and wait/take a break!

OK, is my next move to heat the screw and let it cool a couple of cycles to break away anything that might be in the threads (like "rust", did not see any thread treatment on the other three screws)then heat it again and try turning it out?

Heat it with a 500 watt iron, butane micro torch??

File it flat and drill it out with larger and larger bits and then "pick out" the remaining thread pieces?

Right now I have just mounted a scope mount with 3 screws and shoot low powered 357 Mag loads. It is an "inside" screw (middle of the screw pattern) and likely has less stress on it anyway.

Before I broke out the old torch I would squirt some Kroil on it and let that work for awhile and then try the same gag as you did at first with the pliars. If you don’t have access to Kroil, try some plain old Kerosine.

The small tips on wood burning tools make a pretty good heat source that isn’t quite as “scary†as using a torch flame on your weapon.

Acetone dissolves loctite.

Heating to approx 400F kills loctite as well.

Heating then applying solvent is usually effective.

AllanD

I'd take a piece of copper flashing, drill a tiny hole in it for the screw head to pass through, and then heat it. Then remove heat and apply your kroil, kero, liquid wrench, WD40, etc. keep it on it for a while as it cools down, as it keeps cooling it will continue to draw it into the threads. Wait a day then heat not so hot and try to turn it. If it moves at all let it cool down and back it out when cool.

Oh, I have learned my lesson with the "file flat and drill" method. On anything worth money I use the "file flat and put in a mill" approach! Don't ask me why I changed my approach, but fortunately it was on something that was expensive and not necessarily pretty, otherwise I'd have been really in trouble!