You already have a cheapish 22 plinker. If you have an hk sear, get a knockoff 91 and the G3 22 conversion kit. It appears HK parts still has some kits https://hkparts.net/product/hk-german-g3-22-caliber-conversion-kit-p1147.htm/ . I bought one years ago when they first came in and they are pretty slick. They are very well made.
Other than that the AC should be your last choice for a 22 machinegun. Ciener was the only maker for the 22 kits and as others have said, they are spotty in quality.
If you don't want to go the hk route, start with a dedicated 22 machinegun. Your typically encountered choices will be (in order of price from high to low) the Norrell/S&H 10/22, the American 180, and the Voere. The Norell while the best choice is the most expensive. The American 180 can be finicky and is pretty fast. The Voeres are pretty reliable but I believe the biggest mag is 15 rounds.
As far as nondedicated guns excluding the M-16, your best choice at this time is the Uzi. 22 kits for the Uzi are still in production and with tuning can be very reliable. I own one and from a clean gun I can shoot 3-500 rounds with nearly 100% reliability. As the gun gets dirty, reliability decreases ass is the case with any other 22 machinegun. The M11/9 is also a good choice but the kits are not in production and Richard Lage has said unequivocally that he will not be resuming manufacturing.
I would heartily recommend trying full auto 22 before buying. All full auto 22 options are not going to be as reliable as centerfire. Every machinegun owner at some time lusts for a 22 conversion. I personally own two machineguns with 22 conversions. My opinion is that full auto 22 is overrated and truthfully shooting full auto 22 is fucking boring. Sure its fun briefly for the humor of it all but it lacks that recoil and noise that truly makes a machinegun fun.