Hmmm..
Seems like You forgot to connect the cable that comes from the little switch at the back of the gun unit to “CN3 M.G.Fire” on the RX. Older tanks need that.
The gen4 controllers have some wiring shenanigans going-on to get separate gun raising and lowering. To make that function, they switched-around many wires and connectors, and made a completely new mini-board for the turret (the connector board where the thick multi-coloured cable comes from)
Just check the tiny switch at the back of the breech, and connect the wire that comes from there directly to pin 4 and 5 of port cn3. That will generate the “bang” sound.
Buuut.. that’s just the start. You also need the new connector with the 4 + 1 setup for the turret, and then find-out which connector/wire goes where. A rather infuriating task. It is do-able, but quite maddening. I gave-up and kept at my IBU’s as an upgrade for older tanks.
Getting the “old” tanks working with the new 4th gen (dual sound) RX, and the “newly wired” tanks with older IBU’s or Clark controllers is no mean feat. I stìll haven’t fully managed that. But, then again, I prefer IBU and Clark over the RX18 and rather take-out all those infernally finicky new boards and wires and stuff, put back the older connector boards, and go from there.
A bit of searching on this site, will get You enough information to start converting the “right way” But be forewarned. There’s no “simple solution” no escaping tracing ALL wiring. Fun...
I have recently had the electrics upgraded to the current standard HL system, but the BB gun has stopped working. Ineed to know, 1/ what do I check to determine whats wrong, and 2/ how much work is it to convert it to the recoil system? All answers in basic English as I get confused very easily! 
