Thanks Jamie for all of your advice! I've already extensively studied your videos on the pneumatics setup of both Shunt and Renegade, so your advice is super helpful. I will definitely be asking a few questions as I go along, if you don't mind?

Having done some measuring, you are unfortunately correct, it is too shallow. You can get 1kg bottles, but mostly for powder and not CO2. Would these still be suitable?

I've had a look and Lancashire Fire Protection are very close to me, so that's great advice. I shall get in contact with them should I pursue pneumatics further.
Ask away anytime. I'm certainly no expert on pneumatics, very much a relative noob, but I'll help wherever I can.

Regarding 1kg bottles, I don't think it matters if they're for CO2 or not, as long as they can handle the pressure. If you were to use a powder one (perhaps fitted with a screw-top valve) you'd probably have to get it pressure tested/certified. The guys at Lancashire Fire Protection may even be able to offer that, or at least advise on suitability or where to get it tested. I'm sure if you mention Robot Wars or combat robots, it would pique their interest.

Nice choice on the motors and gearboxes too, definitely worth it if you've got the budget. Low-profile and ideal since the chassis is designed for them. Look forward to watching the build progress. I seem to have gotten into the habit of keeping my builds secret until they're done now and have to do retrospective diaries, so always good to have another progressive one to peruse.