There is a lot company that sell HK clones and sell a stripped down: basically a treat Like a Brownells stripped AR15!
Most people buy a complete PTR 91 or MP5 clone or other Hk models.
Then they upgrade the stock and then the buffer and wide forearms ,bipod, and AMBI trigger housing
and pack(with a trigger job). Then changing out your cocking handle.
Then most firearm owners kept the parts for back up or sell them on the internet.
Black Market part does this you can buy MP5K stripped down, and have it sent to your local FFL. ( This company website is down, I wonder if there still in business??)
Black market do not sell HK91 or HK91K !
OR.......Next you can have a company build you a rifle/SBR/ pistol or find you host gun and and buy F/A sear pack.
I like building rifle, I think this would sell more than a standard complete rifle/pistol/SBR.
Thanks ,from rainy Oregon.