Building a custom or bespoke gun from an idea through to something you can use can be very time consuming, more costly than going out and buying an off the peg high end gun and frustrating as hell at times.
You also have to figure in that when there are problems or niggles, its down to you to be fault finder and fixer.
This is from experience, not hearsay or guesswork
On the other side, when everything goes right, its really satisfying.
Dont do it if you cant do the engineering yourself, or you dont have sponsors, it will cost too much.