Anyone ever made one? I saw SG had .22 mag ammo 2000 rds. / $180 and thought this would be a more potent cal. than the .22 LR and still much cheaper than .223 at current prices.

It can be done but most people are a little turned off by the cost of the 22 magnum ammo -- the same goes for the 17 HMR. It really shines when used for hunting where few shots are needed and the extra power and range (over the 22lr) are really valuable.
This is a 17 HMR upper I built a few years ago and it has a twin 22 magnum upper. The 22 magnum is very accurate and the 17 HMR is super accurate. The bigest problem is the magazines. I went for quantity in my magazine sizes at first but found that I needed only a few shots and the bigger magazines just got in the way. The magazine in the rifle is actually a Marlin bolt action magazine inside of a plastic magazine shell.

I free floated the handguards by using one of those DCM FF tubes -- the handguard cap is part of the tube and the gas block was just put there so the upper kinda looked normal.

This is my newer magazine loaded with the 17 HMR ammo.