>On another note, what about pneumatic guns? ...
They trialled some in about 1800 with the Girandoni air rifles, so called Windbuechsen back then. You may have heard of them.
One problem was reservoirs needed to be refilled in the field, either exchange empty for filled (like with mags), or send someone refilling behind the line of fire. You have no time to pump 'em up right there and then. They made handcranked compressors on carts to refill a handfull tanks at once iirc. The empties are brought back, full ones given to the troopers. The tanks need to fit well for this to work, like magazines one could use on another, similar gun. You cannot have tanks to yourself/for your gun only. Imagine the nightmare.
Tanks could start leaking, which will render them unusable till you can fix them. Somewhere to the back. Also the rifles were manufactured, literally handmade. Lots of handfitting, something we can do smarter now I say. And they were finnicky in general, not very robust. Could be solved today (also if SHTF).
We have advanced from 1800s to nowadays, and I believe we could overcome some of the challenges. You need e.g. to compensate for power decline as the air reservoir is emptied. To be able to hit targets until you run out of air, or bullets, which comes first. The tank needs being protected, so fit into the rifle, the best way. Exchangable buttstocks. Also a rugged simple twist-lock mechanism, lugs like how you mount bayonets. So yeah, is a thing, can be done, but as a first choice I'd rather prefer other proven solutions tbh.
Range was over 100 metres/110 yards, and they didn't use true pointed projectiles, only the literal bullets or 'musket balls', not even Minié types. Reminder, 1800. When Napoléon ran out of chewing gum. So there is wiggle room for improvement too. Perhaps up to 150 metres/165 yards, seems feasable. Similar to shotgun (with slug, rifled barrel, in it's role as a musket - how ironic). But definitely more silent I think. Should be a plus, shouldn't it?
(vid related is review of 'portable' compressors, the smallest is still huge as fuck.)
To me it boils down to is it uncomplicated/reliable to use? Similar to the question of battery driven car versus fuel driven car. We got used to liquid fuels, they are great to store for a while, easy to transport, and it just takes minutes to refill even a big car tank. Who wants to literally stand by, waiting for half an hour and more to charge a car battery? Even if you only had to do it every few hundred kilometres/miles? Me not. Is why alcohol and fuel cell is the right thing imho.
>Fill tank, produce electricity on the way, use for traction motor(s) and whatnot.
Do not get me wrong pls: For fixed routes, battery-electric cars/busses/trucks and water/air/railway vehicles are alright. Like it's done with public transport of goods and passengers. They did use some in the past (e.g. German battery-electric railcar, the Wittfeld-Akkumulatortriebwagen of 1907, in service for half a century(!)), and are using boats reliably for a time now, too (a modern one is the MV Ampere catamaran ferry in Norway, since 2014 afaik).
>Charge, takes a time, but you do it once a day.
>Then you run the vehicle all day long.
>During evening/in the morning charge again.
This can work fine, but those vehicles in use are rather not extended-range ones.
Same goes btw for compressed-air driven vehicles (little motorbikes for transporting pizza a.o., compressed-air 'fireless' locos, as shunters - awesome, no kidding). You need a compressor placed somewhere, nothing I want to take with me all the time. Or how about classic steam-engined ones, which you need to bring to temperature first - and not let them cool down else you wait. Sounds familiar?
For an 'if you need to, move on - quickly, to anywhere' style of travelling it's a no-go, literally. Sorry for the long rant...