He would be richer than Roosevelt!
I am not sure it would have to be an agreed % of each rifle or a one off sum?
He might have done an Alec Guinness who agreed to play Obi Wan in Star wars as a favour and said oh sod it just give me 2% of the profits (seriously expecting it to be a flop) - he made a %^$& ton of greenbacks
So John G might have asked for a paltry amount say 1% per rifle expecting a few hundred thousand to be made for the relatively small peacetime US Army only to get 1% of the 5.5 million or so guns made?