I had one of the later models they made which was 1:38 twist and shot the breakaway sabots with 80 - 85 gr of powder real well also. I was wanting a longer barrel for it but Green Mtn only made a drop in for it in 1:28 twist for sabots and it still was only 21" long.
The WMC barrel is 1/2 octagon and 1/2 round so a New Englander barrel won't fit it and the stock is too short for a Hawken barrel to fit it (the barrel keys are in different places). I had asked T/C if I could get a longer barrel for it and they told me while they didn't make one for the WMC the barrel for the PA Hunter which is 1:66 twist will fit it. Fox Ridge sells the PA Hunter barrels in 31" and 21". Fox Ridge also sells the Fox Ridge Carbine which is about the same gun as the WMC but it has a 1:48 twist and a 21" barrel (too bad it doesn't have a 24" barrel or I'd have to get one). Might want to give Fox Ridge a call and see if they still have them. They were advertising the PA Hunter barrels in 1:66 twist without the QLA so I would ask about that.