So I'm trying to piece together a .22 pistol. I have a TacSol upper and plan to get a 1911 style Volquartsen frame. Question is some of the other parts I need such as the bolt and main spring housing have the option for MK II/III or 22/45. So what is the difference? What parts do I need? Thanks!

I did this very thing.  PacLite upper on the Volqs lower.  I also used a ruger 22/45 bolt and mainspring.  Other than that you'll only need grips.

I also put some three-dot sights so it would approximate my carry guns.  

I have a Vortex Venom on order.  I really built it for open sight practice but red dots are SO much fun.

I'll be screwing a can on the business end as soon as they get that bill through to do away with the tax stamp.  Hoping....

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You can use any of the Ruger bolts as well as the Volquartsen bolt. I have always used the Ruger 22/45 mainspring housing
or the Volquartsen housing which I have found no advantage over either. I own 3 of the Volquartsen 1911 style Target Frames
that I swap 6 different uppers on. My favorite is this PacLite upper that has been shortened to 3 1/2". I have the barrel lugs no the
frames adjusted to different positions so I can swap uppers by using hand pressure.