I would think: rocks, slingshot, sling, Bow & arrows, BB gun & knife, pellet rifle & sword, .22cal rifle/.22cal pistol & start Martial Arts, .410ga shotgun, in that order.

THEN start with a M-79 40mm grenade launcher(grenade, willy petes, and buckshot), M-60, BMG .50cal, 20mm Oerlikon, followed by the 25mm Hughes, 40mm single, twin, or quad Bofors, 3".50cal Dahlgren up to the 16".50cal Dahlgren would be the correct order of training.

Ask me and I can tell you where you can find the large calibre stuff to play on.