So...

no. the cost and labor time is expensive.

actually attainable.

The alternative is buying them, you fucking fed.

Not for slam-fire shotguns. Or zip-guns if you have an angle grinder/dremel.

Rifling isn't the most important thing. Would you rather have a cheap, unrifled gun or no gun at all? If you're creative enough and have a chart of dimensions (and tensile strengths of the material), you can easily make a barrel out of pretty much any small steel tube/pipe. For example, seamless ⅛" NPT/BSP normally pipe works as a barrel for .22lr.

The fact that that no one has designed a decent repeating firearm that doesn't require more than (optionally) cheap hand tools disproves that

op this is so stupid. what do you expect to discover "new" about gun design? you won't improve on the basic design of the gun it's been optimal for 400 years.

if you wanted to do this to find a bullet design with higher ballistic coefficient, one less prone to tumbling through transsonic, find an optimal rate of twist for your rifling, optimal lens design for a scope - i would agree with you.

Obviously the biggest problem is not the gun itself, but the fuel. I've seen video of bullet manufacturing and that shit looks hard to DIY

What you want to use is OpenFOAM along with your basic AutoDesk/CAD tools.
OpenFOAM is very robust, I used it to design foundries/furnaces, but even their 'demo' has things like engines, etc. it's really a good simulator.

What I do for my projects is this:
paper sketch/"rough draft", this will be completely out of scale, crudely drawn, etc. Figure out what you want to do, then determine what is physically required to accomplish this. E.g. don't use thin wall pipe for a barrel.

Next move onto CAD, your design will change a bit as you figure out the scale of everything.

Anytime I come across a major 'flaw' in a design, I go back to the paper and make my changes there before making changes to the CAD design, I just find it 100x easier to do complex calculations on paper than with any program/text editor.

Finally, once you have a design you think is good, import it into OpenFOAM and start running your tests, determine failure points, etc.

Repeat as much as you feel is necessary. It's also not a bad idea to make a simple cardboard cut out to determine the size you'd like your gun to be, with large industrial furnaces it was a bit easier as I just needed to know how many tonnes of metal to melt.

Gun barrels have been made for centuries now. Anyone with a basic forge and a mandrel or mandrel-like device can make a barrel.
Also, buy a lathe and a gun drill, problem solved.

Ammunition is a bit harder, particularly the brass case, primers, gunpowder, lead, etc. is all fairly easy to procure. Could always use a really stiff cardboard or cast plastic or something.

what is the Luty? numerous Sten clones? you can clone basically any pistol with hand tools and time. there are a few more advanced designs in the pinned file sharing thread.

eh? that's easy. if you're super ghetto about it you can even get by with match heads for powder.

It's easy. All you need is sulfuric acid (plenty of ways to sythesize it), a nitrate (most 2-part ice packs), water, cotton (printer paper is way better, everybody just uses cotton) a means of cooling it and add some ammonium nitrate to your nitrated cellulose. The ammonium nitrate reduces the nitrocellulose when fired.

I know about Luty's SMGs. I don't know if you noticed it, but I used one as the thread picture. His BSP SMG (not the one I posted, the pipe one) requires straight thread pipe and fittings. Here in America, we use tapered thread pipe and fittings. As does pretty much every country other than Britain. That's not the biggest problem as this can be rectified by buying from the right sources or having taps and dies on hand to straighten the threads or just buying them from somewhere else. The big problem is that many parts have to be cut out of ⅛" and ¼" steel plate. This isn't even mentioning the other oddball Brit parts.

Lutys basic design can use a simple piece of square tubing, its great on how adaptable it is. Its how the emus Australians do it.