My first post here at Benchrest.com
We recently had this discussion on rimfirecentral. To clear a bit of possible confusion over the long vs short barrel accuracy question, the long average of 25" for benchrest appears to work nicely in bolt-actioned rifles like turbos and anschutz, etc. The short barrels tend to have the accuracy edge in blow-back actions, like the 10/22.
In fact, a couple of us plan on doing an experiment to see if 12" is, in fact, the best in accuracy in a 10/22 rifle as was suggested by a respected member and specialty parts manufacturer/designer.
Why the big difference? First, check out the graph, half-way down this page titled "22LR Pressure Curves from Strain Gauge Data"
http://varmintal.com/a22lr.htm The example is of a Benchmark 24.75" barrel on a bolt gun shooting a standard velocity bullet.
Peak pressure of 7500 lbs/sq.in. is reached between 5 to 6 ten thousanths of a second after primer ignition, then drops quickly and levels off at about 500 psi at 16 "tenths".
The bullet departs at 26 tenths from that 24.75" barrel with still nearly 500 psi behind it. Because the pressure remains quite steady after 16 tenths, the bullet continues to accelerate somewhat and generally in bolt guns to the end of the barrel. I noted comments from those who submitted chrony data on bolt guns indicating speeds continued to increase slightly up to his longest barrel in the 26 inch range.
Something different happens in 10/22s, persumably because of the blow back of the bolt during the firing sequence. I was informed that the 10/22 bolt does not even begin to move until the bullet is 2" down the barrel at which point it has already accelerated to 60% of its maximum velocity. The bolt never travels faster than about 80 mph, which is 0.13" per ten thousandth of a second ("tenth" for shorthand). As data is scarce, let us assume for rough figuring, that the bullet breaks free of its case just before maximum pressure is attained, say at 3 tenths after ignition and reaches 2" down the barrel at say 8 tenths. It is still accelerating at this point at which the bolt begins to blow back. The gas release at the rear might then occur "starting" at about 14 tenths after primer ignition based on accerating bolt speed average travel at this time-frame of 0.1" per tenth.
OK, so what happens when the breech is unblocked at 14 tenths? You now have a "balloon" which is more or less opened at both ends now and more open at the breech end.
The air pressure would drop MUCH faster than we see in the above-referenced graph of the bolt gun which only loses pressure by pushing the bullet out. I would surmise that the pressure drop would "reach" the bullet (perhaps even felt as a slightly destabilizing wave pattern on the bullet) at about 16 tenths after primer ignition. Now, take that information and add it to the fact the bullet will travel no more than 1.4" per tenth at 1170fps and something interesting happens at 16 tenths which happens to coincide with with approximately 12" of bullet travel. The bullet now starts to decelerate at approximately this point as the combined resistive forces of air and bore drag exceed what little remains of pushing pressure, so the bullet goes from accelerating to decelerating in the flash of an instant. This "instant" may be termed the moment of dynamic stability. At this instant and only then, are the forces of the expanding gases behind the bullet "pushing" equal to the forces of drag on the bullet -- the bullet is then and only then "floating" in an equilibrium of tension. If the bullet is released from the barrel at precisely this instant, I would predict it would not only be at maximum velocity, but also poised for maximum accuracy. That is a theory which has yet to be proven in testing. It could be that we haven't seen it proven because of the hassle of class III BATFE requirements and the fact that such a short, light barrel would need a super-stable platform to perform at its accuracy potential.
Anyway, that's one explanation of why 10/22s often do well with short barrels and bolt guns do better with long barrels. I have proven that shortening a 10/22 barrel from 20" to 17.5 inches increased average velocities of all bullets by about 30 feet per second. CCI SV went from 1080 to 1110 fps, for instance. Bolt guns can expect just the opposite trend -- shorten a 20" to 17.5" should decrease its bullet velocity.
I expect over the course of the upcoming year, I may have more data to share on the 12" barreled 10/22 test -- we'll see. Anyone here interested to know the results?
