I played with a group recently and the topic came up about weapon trading. No one could find anything in the official rule book about this and we collectively made a decision that you can trade weapons among players by being adjacent and spending a movement point to do so.

The invader fragged the marine who was going to trade his next turn and then he forgot about the trade as their strategy changed so we never got to actually execute it and see how it changed the dynamic of the game.

After thinking more about this more I feel like there would need to be contingency rules involved in trading like:

1) all weapon set cards need to be in the action deck or hand
2) one movement needs to be spent to find all the action cards of that weapon set - then re-shuffle the action deck
3) one movement spent to trade to an adjacent player
4) receiving player shuffles received cards and places them on the top of his action deck (or shuffle into the deck?)

Is there any official rules on trading?

I can’t remember where I read it, but I’m pretty sure you can’t - the example given was cycling the BFG.

Maybe do a trawl through BGG as I probably read it there. Normally if there’s no official rule to cover it, it’s because there’s no intention for it to be an option.

I found a post that doesn't allow weapon trading in the doom 2004 game, but nothing related to the 2016 version. I never thought about weapon cycling in the same round. I think maybe it's probably a better idea to not allow any weapon trading.

Officially, no. There are no rules for trading weapons likely for the purpose of excessive deck rebuilding. Take Zombiecide for an example; it allows players to swap items amongst one another and use weapons and spells designed to be one use per round but because it's a new character in possession they get to use it again.

Taking DOOM's rules picking up a new weapon takes the cards associated with it and shuffles the three cards before putting them on the top of the action deck. Allowing players to pass around a BFG, chainsaw or the Gauss Cannon will tilt the game heavily in favour for the Marines.

If you want to house rule it though go right ahead.