Yes, for personal use it is legal. It cannot be fully automatic (without a $200 Tax Stamp), and if you choose to sell it you must obtain a serial number from ATF.
If you however... go into production... you must have a Federal Firearms Manufacturer’s license which is $500, and pay a tax to ATF on each firearm you produce.
I may be incorrect on the sale part... if so I apologize, but if appropriate taxes are paid it may suffice... not an expert.