PCP Pre-Charged Cartridges for DIY Project?

Hi, after much research I decide to go for a Kral puncher np-04, problem.. it's not on the market yet... so meanwhile I think what about do it yourself thing?



I got a modest workshop and some tools, of course i'm not gonna reach the power or quality of a branded PCP, but I want to try something like this:



https://www.youtube.com/watch?v=iMptz_k4drQ



But instead of arrows, just marbles.





Percutor, canon etc... are pretty simple to do, the trickiest part is the pressured chamber, Slingshot man uses some sort of PCP shells that looks similar to the BACS system, but I was unable to found something like that, a 1 discharge shot, small chamber whit quick release primer, the closest thing es that from airsoft/paintball:

https://www.surplusstore.co.uk/m56-shells.html



But I think it only takes green gas and don't look very powerfull.



Does anyone know where to found a shotgun shell size PCP chamber whit primer or simple mechanism release?



Thanks!
 
I never hear about the Modoc rifle, is quite impressive, but the cartridges are a bit expensive for what I have in mind, also they come whit what it seems a specific charge aparatus, and I don't thing its a good idea to shoot these out of a crappy pvc pipe.

I have also think about a regular pcp tank, I already got a compresor, but really don't know much about valves, I just need a spring powered hammer that strikes the tank valve?