In my experience you need to get lucky to get accurate smooth twist 177 barrel. I've owned original FX impact with 177 barrel and it was good shooter. But I've also seen other guys buy new gun and not be able to get the accuracy they expected out of it. Better buy used from someone that got lucky with accurate barrel.
Also matters what is the power level and purpose of rifle. In my experience FX smooth twist barrels achieve better accuracy with more power.
So here's my $.02...
Bought an FX Streamline 2 years ago and found that the accuracy just wasn't what I thought it should be coming from FX with the smooth twist.
Spent the 1st 8 months 2nd guessing my purchase. then came Pyramyd Air. First day I didn't do well, but had a great time as always.
That evening Dennis Himes pulled me aside and asked what the issue was. Explained my problems to him. He asked if I had cleaned the barrel...Well, yeah, of course.
No, did you REALLY clean it. Yes, just two days ago. He pulled out some material that was almost like cigarette filter paper but cut like cloth. Still don't know what the material is.
He pulled out an aluminum rod and started pushing a patch through (with some type of solvent applied to the cloth -- electronic cleaner, brake cleaner maybe -- something that wouldn't hurt the 0-rings) and it came out really black. I was dumbfounded as I had honestly just cleaned it. He proceeded to do this a number of times until the patch came out absolutely clean -- probably 15 patches. Then we ran some dry patches through.
I had never used solvents or metal rods to clean the barrel. I had always used pull-throughs.
Went to the range and I immediately saw an improvement in accuracy to the tune of 55 yards pellet touching pellet or in the same hole. I honestly couldn't believe it. It almost got boring shooting that night. I was hitting everything I aimed at.
2nd day my score went up by 10 points.
So, IMHO, clean that barrel really, really well. And possibly clean it some more.
I just happened to get one of the few 177 x-barrels for my streamline that following winter. I think the x-barrel adds that bit of extra accuracy as well. Ended up winning PCP hunter this past year at Pyramyd.
Had a Royale in .177 with the Smooth Twist barrel. It was exceedingly accurate... for about 40 shots. Then accuracy would go to pot. Cleaned barrel and accuracy would return for about 40 shots. I tried different pellets, lubed, non lubed, different velocities. It didnt matter. Every thing I tried would have great accuracy for about 40 shots.
I've got ST barrels in other calibers that I've had zero issues out of.
