Gil;
If the tigger guard may be taken off the gun.....create a wooden template of the arc you wish to impart. Leave the front and back area a little short so you can get into the bow without closing up the loop. Use a rubber mallet to stike the metal onto the shape of the loop. Be firm but gentle. Oh, roll the edge of the side edges corners just a little to keep from putting creases inside the bow. If the trigger may not be taken off....make a curved hard wooden arced punch with half of the width of the tiggerguard as your guide. Stike the bow from the inside of the loop downward at an angle from both sides using again the rubber mallet. Metal hammer seems to put too much force straight down and the bow will not move out in an arc.
