My Encore makes 1.5" three shot groups at 100 yards using [B]100 grains of 2F Pinnacle and the 250 grain SST.
Ditto this with the exception of shooting 90grns of American Pioneer (AP)
I'm thinking like thallub
posted. Don't think it's the bullets. Either sabots your using or rifle.
Have you tried any other bullets? Maybe a full size conical like a PowerBelt.
Have you miked your bore? Is your bore a bit oversized? You would then need to get oversized sabots if you wanted to shoot the SST bullet.
Too, FWIW, your actually shooting 115grns. equivalent of the powders others here are using (ooops, didn't see Doyle's load). Anyway, is it possible the sabots you're using are deforming with that amount of T-7? Have you tried backing down to say 80-85grns. of T-7?
Maybe you just haven't found the rifle/bullet/sabot/charge sweet spot.
Also, what's your loading sequence? How much seating pressure? Are you using bore butter?
As you can tell, there are a lot of factors to consider before coming out and making a blanket statement that something isn't worth a damn.
I'll say this, I know a LOT of people using the SST's or the T/C Shockwaves(same difference)and have never heard of any modern inline brand that won't shoot them respectfully. Yours would be the first.