Got this answer back on another forum....

I prefer a slow twist deep grove rifling for prb , I have a tc hawkins in 54 cal , wiff the factory barrel ( 1-48 twist ) it does not have deep grove rifleing , it shoots a concal bullet good but for prb I get 2 to 3 inch groups at 50 yards , so I replaced the barrel wiff a green mountain barrel 1-72 inch twist deep groove rifleing and all I can say wow , it shoots a one hole group at 50 yards and at a 100 yards I can get 2 to 3 inch groups or better. I use 90 grains of ffg , last fall I shot a doe wiff it using prb, she was 100 yards from me and I drop her using one shot in the head , for me I like a slow twist barrel , others mite not but it works for me and very pleased wiff the groups I can get. Dats my 2 cents