Field stripping a buck mark?

in the manual that came with my buck mark it says to remove the rear site base screw. easy enough, and it takes the allen key that came with the manual.
Next it says loosen the barrel mounting screw. Anyone know what I use to loosen it? it's not the allen key and i can't see in to tell what it is, and i don't want to start trying different things and mess up the head of whatever it is.

Thanks

It's another hex screw, I forget what size, but it is NOT metric.

Thanks

7/64 hex

Field stripping is easy once you have done it once or twice. Just make sure you hold onto the plastic piece that the recoil rod seats into while removing the spring/rod. I have found that one application of blue loctite on the screws will prevent them from loosening while at the range. I have stripped mine dozens of times without having to reapply the loctite.