I'm pretty sure you couldn't, certainly not without about ten years of practice and $100K worth of equipment.
An actual clone of a Bushmaster .223, you have a fair point. Not going to just pop out a frame and barrel, file down a working action, and call it good.
A crude-but-functional high(er than .380)-caliber magazine fed semiautomatic pistol, though? Lookin' at machinery that costs less than current low-end 3d printers (an entry metal lathe will set you back around $600, and the rest you can rough out with a Dremel and finish by hand with a sub-$100 set of metal files).
That said, yes, you would still need to invest a good amount of time to learn how to do it right. A few months to a year, perhaps (talking about the basics to produce something functional, not designing your own new-and-improved custom actions)? Sure. Ten years, though? By then you've already mastered the art, gotten bored, and started looking into changing careers.
Also, FWIW, you can buy all the "hard" parts to make on a gun without it counting as a gun. Only the part stamped with the serial number (the frame or lower receiver in most cases) has any controls around the purchase, and that part amounts to nothing more than a passive hunk of metal with holes in the right places (YouTube has a video of a guy making a fully functional lower receiver for an AR-15 out of... A shovel. A few folds and a few holes, and bam, it goes "blam").