Don’t know how you would do it but 120 PSI sounds a little high when it finally let’s go.
Whether using the C02 or an air compressor, if muzzle is pointed in a safe direction when used, it doesn't matter.
Far as how to do it? Somewhere around here is an air valve I made up years ago that would thread into my Hawkins. Never had to use it.
I have a rubber tipped blow gun that I've always had success with. Remove the nipple, press the rubber tipped blow gun down in nipple port and use short burst of air. On really jammed loads, pour TC13 or soapy water down the bbl. and let set for 5-10 mins.
Have had a couple over the years people have brought to me that I had to leave the TC13/soapy water set in for a couple hrs or so before any success but from the looks of lack of maintenance on the entire rifle, it was hard telling how long the thing set loaded.
On one CVA I remember doing many years ago, I had to end up pouring automatic trans. fluid into the breech, plugging the breech, then pouring some down the bore as well and letting it set for a couple days.
After dumping the ATF out, the load, with the jag of the busted off ball puller left there by when the owner was previously tryin to pull load, pushed out rather easily.