Before getting more machine tools and building a marker:
Get a paintball gun, any paintball gun, and make ALL the parts in AutoCAD. Then do another, different gun. Get a broken gun and fix it. Fix allot of guns. Get a gun or 3 that you can tinker with and not care about.
See, even if I handed you design to build a gun and all the tools, you need to know how to tune it. You could build everything to spec, but then you still would need to know how it works, need to know how the o-ring is suppose to feel when the parts go together, need to be familiar with the screws and action and weights.
After that, to build a gun, even from plans, means you need to understand the o-ring crush limits, the dimension of an air chamber, the tolerance of a bolt to breech fit. You need to know why parts are sized the way they are, or at least know what dictates the sizes of parts (often it is ANSI o-ring sizes.)
Then you need to weight hammers (if you use them) understand the operation of a solenoid and bolt, be able to tune a gun and figure out why it is leaking.
There are a tons a variables, and you need to feel comfortable knowing which is which.
So, I would get a paintball gun, even just a spyder, and build parts for it. New bolt, new hammer, new valve. Machine the body, make it lighter, polish up a few bits.
I would give this advice to anybody of any age - unless you have allot of experience teching paintball guns and allot of experience machining then starting right out and building a gun is going to be very frustrating. But designing parts, testing them, looking at variables, finding out what fits and what feels right will go a long way to setting you up to begin to build your own.
And I say this as a guy who has 8 prototypes of a gun in my garage, over 30 guns and years of CAD work.
That being said, simple is better. But you need to understand what the different guns are, what makes them simple to machine and simple to build. Just having random strangers on the internets dictating what gun to build means you are not ready yet to build a gun. When you have the experience you need to go forward, you will have a gun in mind.
"If you build it, they will run"
Colin @ Deadly wind: “We need to stay away from each other. You scare me.”
Owner of Patent #7610907: Paintball marker action assembly