Easy enough to find out. Take your cleaning rod and jag, with a tight fitting patch and ease it into the rifling . Choose a place on the cleaning rod and make a reference mark. 12" back from that mark, make another, and then again at 14". Make the all the marks on top of the cleaning rod. I put masking tape on the rod to make the marks easy to see and use a Sharpie. Advance the rod with the tight fitting patch into the bore watching the marks you made. Has the rod made a full rotation at 12"? At 14"? Just make sure the patch you choose isn't too tight to pass through the bore and that the handle on the rod has good bearings so the rod rotates with the rifling. When you can't seem to find written information about twist rate, this is how many find out!
My 1st Swift was a 77v ,26 inch bbl 1/14 twist it shot 50,52,53,55,60,& 70 grn Speer semi-sptzers very well , never tryed any 40 grainers , that rifles sold but it's replacement shoots even better ,a Semi/custom useing a older Ruger action mated to a ss/26/1/14 twist Chanlynn cut rifled bbl about a # 3.5 conture , with a bloked mag well to make it a single shot, Canjar set trigger groups at 200 yds hover 3/8 to less then 1/2 , again this rifle shoots all the same weight bullets that the above rifle did, long live the 220 Swift !!!
This has to be one of the all time Classic Rifles and yours is in a Classic Cartridge, congratulations to you Sir.
Just recently I received the latest Norma catalog and was horrified to see that 220 Swift ammunition is no longer listed. They do still supply brass at this stage. I would suggest if you want good Norma Swift brass you may want to get it sooner rather than later. My 220 Swift is a Ruger M77 MkI and from day one (1978) it has been a tack driver. Enjoy your new project.