fireman 9731 observed:
Hmmm, very interesting. I have a Norinco mini-mauser 22lr Training rifle that I have been trying to do some accuracy testing with. I have no idea what the twist rate is, but it will shoot 29 grain CCI 22 short target ammo(830 fps) just as well as the CCI Standard velocity 40 grain ammo at 1070 fps. All of this is at about 75 yards. It doesn't like any of the super-sonic stuff at all. I haven't tried the AR Tactical yet though. So am I lucky and my rifle just has two sweet spots or am I crazy?
No.
Look at it this way, in terms of the bullet RPM for the different weights of bullets (and hence their different velocities):
29 gr @ 830 ft/sec 16" twist
(830 / 1000) X (12 / 16) X 60,000 = 37350 RPM
40 gr @ 1070 ft/sec 16" twist
(1070 / 1000) X (12 / 16) X 60,000 = 48150 RPM
But assuming a supersonic velocity of, say, 1350 f/s with a lighter bullet:
(1350 / 1000) X (12 / 16) X 60,000 = 60,750 RPM
Note that the ultra-high-velocity bullet is being spun about 25% faster than the 40-grain bullet, which is "comfortable" at between 49,000 RPM and 54,000 rpm. That's not the rifle's sweet spot, that's the bullets' "sweet range."
And you may be referring to even higher-velocity bullets in the 29- grain range, which somebody mentioned, which will have an even higher velocity (and hence RPM) than my assumed velocity of 1350 f/s above. My opinion is that your hypervelocity bullets are being overspun in your rifle with my assumed 1 in 16" twist. (See below.)
I point out that over many many decades (like about ten of them) the best rifling twist for .22 cartridges has been 1 in 16", which has been confirmed empirically for the normal range of .22 cartridges (.22 Short, .22 Long, and .22 Long RIfle). It is in almost universal use, even though it's kind of a compromise for those three cartridges and bullet weights and normal .22 velocities.
Ah, but comes now the craze for even higher velocity, and someone decided that they could advertise even
more velocity, whoop-de-doo, and we'll sell a billion of them.
Except that with the "standard" twist of 1:16, accuracy falls off at the higher resulting RPM of the higher velocity lighter-weight bullets.
Hey, velocity isn't everything, folks.
Terry, 230RN
P.S. You may determine your rifle's twist by running a tight patch down the bore with a mark on the upper part of the rod so you can tell when it's made one rotation as it passes through. Place another mark right at the muzzle. Push the rod down until it has made one rotation. Mark the rod again and withdraw it. Measure the distance between the two marks you made at the muzzle, and that's your twist.