Machining inside cylinder vs machining thread?

We are working with my kids on CAD project - precisely on a simple pressure valve for DIY confetti launcher. Yesterday question had occured: what is cheaper and easier for hand turner (alternatively cnc machine): to machine a nipple and  then turn a thread on a cylinder or to machine nipple inside cylinder? How much bigger is cost of one of these options? 

I prefer turning inside cylinder (no potential problem with sealing), anyway I'm asking this question to more experienced people.  

PS: Size is showed in mm. We consider using metric thread.

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You can do it internally, but it takes special small, expensive, cranked tools. Why does it have to be "internal" like that ?

It would be millable, but the O ring groove tool would be murderously expensive.

Also, as you've drawn it, its impossible to assemble !
mdt2170m (author)  steveastrouk5 years ago
It's impossible to assemble. I showed this as very simple example of cross section because we were searching for an answer, why it is not common practice to machine complicated nipples inside cylinder. Thank You for an answer!
The last comment refers to your screwed in version.
"Impossible to assemble." Three words any physicist hates to hear from an engineer!
"Ye cannae change the laws of physics", as the great sage Montgomery Scott would have it/