From here I will probably have another look at cartridge designs and hope that some new thoughts unveil themselves.
Self contained cartridges are good because you don't have to worry about a separate gas supply, and a loading mechanism can be easily made analogous to a firearm - bolt action, break action, lever action, straight pull, all these are viable options. With enough power, straight blowback is also an option.
The downside is if you want your launcher to be compact and powerful, you're going to need some very high pressure cartridges with very good valves.
The "valveless" design is therefore a compromise - you need a separate gas supply, but the cartridges are simply sealed tubes with a seal and perforations.
You should therefore first evaluate what you want in terms of the following parameters and build around them:
- availability of tools and materials