Just put the arms mount on my M1A.  I have ordered a scope with a 30mm tube and 42mm objective.  Will the arms 22 low rings give enough clearance or do I have to go to med height rings?

That will be a VERY tight fit, if it works at all.  It might be OK as long as you don't mind having to lower your rear peep all the way down everytime you mount the scope.

I used the #22 medium rings to mount my Springfield 6X40 scope and it works real well - without having to mess with the rear peep at all.

Fulton Armory has a page that gives the height numbers for fitting the different #22 rings - but to use it you will need to have the actual dimensions of your scope on-hand.

Thanks, I'll check their site.