Some benchrest/smallbore shooters use 1:16.5 and 1:17 twist barrels, and slower. On top of that, some Suhl 150s were made with 1:19 twist barrels and I had one of them- it shot poorly in temps below 60 degrees but was decent otherwise. On the flip side I've read of some Remington rifles (40x? 541? 540x?) with faster twist rates.

Another reason "SV" ammo is preferred over "HV" for accuracy is HV ammo has less resistance to wind drift than SV ammo, due to the lower velocity. Check this out: http://www.thewindisnotyourfriend.com/read/long-rifle-wind.html A relatively recent thread on the subject: http://www.rimfirecentral.com/forums/showthread.php?p=4548839#post4548839

 

I've shot .22's out of ar15 upper with 1/7, 1/9/ twists with conversion kits. They are not particularly accurate and tend to lead around the beginning of the rifling. I've also shot an ar15 upper, an SP-1 with a 1/12 twist that shot a lot better and tended to lead less.

My only experiences with .22's and different than factory .22 rifle bore twists.

 

22short uses a 1:20 twist, iirc. There have been special barrels made w/ a 1:18 twist for CCI's Stinger [32gr bobo.]

However, spinning a bullet faster will have no effect on the degraded accuracy from the transition from super- to sub-sonic flight regimes, unless the bullet was only marginally stabilized to begin with. There are problems encountered w/ spinning a bullet too fast, as well: that's why Green Mtn. stopped making their 1:9 twist barrel that was created specifically for the Aguila 22SSS for a time: it never realized their accuracy objective for normal 22lr ammo. They only brought it back due to ongoing demand from those who really wanted to shoot the Aguila 22SSS...

 

Very interesting. I'm new to the finer points of .22s and have never heard that SV drifts less than HV. Weird.

It would seem to me that the ultimate .22 ammo would be subsonic but heavier than 40 grains. Maybe 45 or 50 grains. The SSS ammo goes too far as the velocity is very slow. The heaviest bullet that would just barely be subsonic would seem to be best. I'm surprised bench rest shooters have not pushed for this. This would probably require a faster twist.