It occurred to me that if I could invent a machine - a gun - which could by its rapidity of fire, enable one man to do as much battle duty as a hundred, that it would, to a large extent supersede the necessity of large armies, and consequently, exposure to battle and disease [would] be greatly diminished.

Grace is not against good works! It simply does not bless on the basis of good works. We receive blessing from God based solely on the merits of His Son--blessings freely given to us in Christ and nowhere else. The completeness that is in Christ mean deliverance from trying to 'be good' and 'do right' in order to be accepted by God.