Cleverly cheesing rules is up to the game master's discretion, while transforming into a whale sounds like a very reasonable, if overpowered, interaction, the peasant rail gun wouldn't fly in most campaigns. Not that it makes it any less hilarious.
Pretty sure the official rules have killed off the idea of the peasant rail gun. Now a barb raging before impact to halve falling damage, or doing the same thing with a Moon druid in 5e, that's just exploiting the rules.