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Affordable under barrel air "cartridges"

Hi there. Apologies in advance and I hope no-one minds but I'm not after air-guns themselves at all, instead I'm an airsofter.

However... I could use your help and tech info. 

Long story short: some airsoft guns use HPA as a power source, airguns have neat looking "tubes" that hold some, while airsoft guns only use bottles. 
Those cartridges look lot nicer and could change the looks of the gun I'm planning. 

(external HPA is not an option. Me no likey) 

Questions:
1: are there any affordable ones out there? Ones I've seen start at 150'ish while a HPA bottle is mere 50 with in-bottle regulator. 
2: how many "ci" of air do some of the larger ones hold? My smaller bottle holds 13ci. 
3: are they "raw pressure" or is there a regulator on the tip? 

I've handcrafted several guns before and HPA is quite familiar to me, the airgun cartridges are not and I can't find much info on em. 

I'm hoping to find a as long as possible cartridge that is biiiiit more narrow than a bottle. Plenty of room length-wise but not so much sideways. 

In the side I'm interested in airgun valve-systems. Airsoft valves are inaccurate and unreliable. 
I'm interested in adapting an existing setup for airsoft use.