Either a hammered piece of lead, or a strip of leather will do a better job. Oil the leather well, so it doesn't dry out and catch fire, itself. When you clean the lock, just drip some oil on the esposed leather, to renew its soft texture.
Actually, the lock needs to be seated better against the barrrel, but that takes time, patience, and a fine hand using a scraper, along with marking dye, to find the high spots in the lock mortise.
If the gap is uneven, that sometimes indicates that the bolster was not filed parallel to the barrel, and a little filing there can go along way to closing the gap. This is seen often when the barrel is tapered or round.