ZommyGun,
I know the theory about sonic to sub-sonic transition and I am sure it is true generally...so no need to re-hash that...
Despite what you read on the Internet, this is not true.
Back many years ago when I was living in the Phoenix, AZ area, I became good friends with Gale McMillan. He was famous for making match grade barrels and was also a world class bench rest shooter. This man knew more about rifle barrels and ballistics than any person in the USA ... maybe the world ... and proved it by shooting amazing record breaking scores in competition. The supersonic to subsonic transition became the topic of discussion with many serious competitive shooters at the time and appeared to have the proof to back up the theory. At the distance where bullets dropped below the speed of sound, paper targets would indicate an "oval hole" instead of a "round hole" where the bullet had become unstable. In many shooter's opinion, this was proof enough to support the subsonic transition theory.
One day when Gale and I were having lunch, I asked him what he thought about the theory. A single word answer "bull****". He then proceeded to prove it. Turns out ... a combination of muzzle velocity and barrel twist rate generate the bullet's spin rate and affect down range stability of all bullets. At some point down range, spin rate decays from air friction to a point where bullets become unstable. With the typical bullets that bench rest shooter's use, the point of instability occurred at about the same distance as when the bullets went subsonic, therefore they concluded it must have been the subsonic transition. By increasing the velocity, the bullet would travel farther down range before it dropped to subsonic, further supporting the theory. Fact is, higher velocity also increased the bullet's spin rate, which was the real reason for instability. This theory grew like wild fire and since the Internet became popular, it has been accepted as fact, when indeed it has been proven not to be true (another one of those pesky gun myths).
To prove his point, Gale took me to his shop and showed me data that had been collected from testing a variety of different twist rates in his premium 223 Rem barrels. Using a 24" barrel with a conventional 1:12 twist rate, shooting a factory load 55gr bullet at 3240 fps, the bullet would chronograph subsonic at about 300 yards. At about the same distance, the bullet would also loose stability due to spin rate decay. Same exact ammo, same barrel length, ... the only thing different was a faster spin rate of 1:10. This time, the bullet chronographed the same velocity at 300 yards but maintained stability to 425 yards and still maintained sub-MOA groups. So .... the bullet still went subsonic at 300 yards but accuracy maintained for another 125 yards until the faster spin rate decayed. This proved subsonic transition had nothing to do with bullet instability, it was spin rate decay ... just a coincidence that many people still accept as fact.
Further, bullets used in 22 LR ammo have very poor aerodynamics. A CCI Min-Mag 40 gr solid point bullet has a Ballistic Coefficient (BC) of .136 ... pretty grim compared to a typical 7mm bullet with a BC of .500. Using factory data, a CCI Mini-Mag produces a muzzle velocity of 1255 fps from a 24" barrel. The speed of sound at sea level (68 deg F) is 1,126 ft/s. Because of the poor BC, the bullet drops from 1255 fps to 1126 fps at 44 yards.
If the subsonic transition theory were true, any shot past 45 yards would turn to crap ... it doesn't.
Some other statistics of interest: by the time a CCI Mini-Max bullet reaches 100 yards, it has slowed down to 1017 fps and the time-to-target is .266 seconds.
A Remington hyper velocity "Viper" (32 gr HP) has a BC of .117, MV of 1410 fps, drops to SOS at 78 yards, drops to 1077 fps at 100 yds T-T-T=.246 sec.
A standard velocity CCI Green Tag (40 gr bullet) has a BC of 161, MV of 1138, drops to SOS at 8 yards, drops to 992 fps at 100 yds, T-T-T=.284 sec.
The higher the BC the better the bullet will resist air friction and the less winds will affect bullet drift. The higher the velocity, the less time it takes for the bullet to reach the target (T-T-T) so the less time it is exposed to air friction and wind. The heavier the bullet, the better it will maintain velocity down range. The closer the bullet is to optimum weight for the rifling twist rate, the better it will stabilize. Most 22 LR barrels have a 1:16 twist rate that optimizes with a 40 gr bullet.
So the best combination of the highest velocity (least important), best bullet weight-to-twist rate match, and highest BC will statistically make the best long range cartridge. One additional very important factor is the overall quality of the cartridge ... most uniform powder charges, most uniform bullet weights, most uniform crimps, etc. If these factors vary, so will MV, which will cause groups to open. Of the few cartridges I listed, the CCI Mini-Mag has a good combination and is very uniform quality. Expensive match grade cartridges are rated at standard velocity (1140 fps) and tend to be very accurate, despite a lower MV .... because the BC and overall quality is much higher in all respects.
I have a 20" bull Green Mountain bull barrel on my 10/22 build. It likes CCI Mini-Mags better than other brands too, however I never tested it with any hyper velocity ammo. I have tested it with Wolff Target Match ammo ($7.99/box of 50) and found accuracy was better than CCI Mini-Mags but not enough better to warrant the cost.
