issue is to leave no evidence such as yr dna/fingerprints behind? hmm..

1. use water gun, but instead of normal water u use deadly poison which as soon as it touches the skin, dead!

2. use metal bullet. but instead of it going in a piece, make it :

2.1 so hot that it penetrate the skin
2.2 make it disperse .. but the bullet disperse as soon as it touches object..and the bullet better be sharp..
got it?

to avoid tracking, use the bullet that dispersed .. into smaller bits that's impssible to track.