I've seen the Roni's with the folding stock at gun shows, and was always interested until I found out it needed ATF paperwork & tax stamp. Since then, the pistol brace verions came out and you can buy through the mail. I have two CAA MCK's for my Glocks G19 & G20/G21. This is the way I'd go (and did), as far as the options in the article, with the exception of the Sub2000s. I have the first gen in both 9mm & 40.
For our member
@adam sr , get yourself a G23 Glock and a MCK for that model with a pistol brace, and you got your PCC in 40 caliber. Below is an excerpt from the article discussing the Roni and G23
"
Using my stock Glock 23 slide assembly, the Micro Roni shot groups similar to a Kalashnikov at 100 yards. I found that to be quite acceptable for a .40 S&W pistol".
They do make the MCKs for other manufactures such as Springfield Armory, Sig P320, S&W. So if you have a 40 Cal. from one of those manufacturers, you can select the appropriate MCK for that. I searched around online for the best price, I bought my two from Classic Firearms while they were running a sale. I've seen them for around the same price on Optics Planet, and I'm sure they're sold else where as well.
Buy MCK | Micro Conversion Kit and other tactical products such as, Micro-roni, Roni, Glock Conversion Kits, Stabilizers, accessories, optics
caagearup.com