Whiscombe rifles and barrel harmonics - Part 5
More accuracy questions, plus a look at airgun power
by B.B. Pelletier
Well, the next big question everyone wants answered is this: Does high velocity put an end to all hopes for accuracy? Before I give you even more data to support the fact that it does, let me first clear up some confusion. I get comments from readers telling me that their .22/250 centerfire rifle that shoots 50-grain .22 caliber bullets at 3900 f.p.s. can group five shots in a half-inch at 100 yards. How can that not be considered accurate?
It is accurate, of course. But, the people asking the question were not aware that I was talking about diabolo pellets when I made that statement. Streamlined spitzer bullets handle hypersonic flight very well. Diabolo pellets do not. In fact, diabolos are made in such a way that they are not intended to be accurate above the speed of sound, or even in the transsonic region that exists above Mach 0.8.
Do we need better pellets?
This is where the newer airgunners often say, "Well, if diabolos aren't accurate at supersonic speeds, let's shoot solid bullets" without realizing that they are undermining all the potential accuracy an airgun has.
Airguns were never intended for hypersonic speeds. In fact, with the exception of a few extraordinary airguns owned by the government and used for test purposes (don't lust after them, the barrels are 100 feet long in some cases!), only recently have airguns been able to achieve supersonic velocities. And, will all those who claim the speed of an airgun is limited by the speed of sound and who seek to "prove" it to me by directing me to this website or that chat forum PLEASE refrain from responding to this? I don't mean to challenge you, but there are technical means of increasing the speed of airguns beyond what physical laws would seem to permit by changing their environment and thus changing the laws that apply.
I digress. The fact is, as a pellet approaches the transsonic region, a sonic pressure wave forms around the nose of the pellet. This wave generates turbulence that impacts on the skirt of the pellet, causing the pellet to vibrate and flutter as it flies. This causes increasing destabilization, the farther the pellet travels. At 10 yards you might not notice it much, but at 50 yards, it's pretty obvious.
And here's the proof
Immediately after shooting several tight groups like the ones I showed you yesterday and the day before, I removed the transfer port limiter to boost power. This was to test the Crosman Premier hollowpoint, but I also shot the Beeman Kodiaks at this power level. They went 929 f.p.s., which translates to 20.32 foot-pounds of energy. And they grouped poorly.
Boosting the velocity of Beeman Kodiaks from 813 f.p.s. to 929 f.p.s. resulted in this 1.589" group - about one inch larger than it was at the slower speed! It is representative of what was possible at this power level.
Please understand that the HOTS was not tuned for this velocity, so I can't say that the groups I got were the best you can get from this rifle and this pellet. But they were shot through the same barrel and by the same powerplant, and the same shooter on the same day at the same distance. So, the transsonic region is to be avoided, as well, if ultimate accuracy is your goal.
What is the transsonic region?
Read this two-part post. Then, read this post about the transsonic region. Those two postings should clear up the meaning of transsonic.
Another lesson - powerplant potential
The final lesson we'll get from this session is the first step in learning about powerplant potential. The Whiscombe is set up as a .177 for these tests. So, the final thing I did was shoot several pellets to determine the maximum power potential of the powerplant for this caliber. You have read many times that light pellets generate more power than heavy pellets in spring-piston guns, and the reverse in CO2 and pneumatic guns. I fired several pellets to see what kind of power I'd get with the Whiscombe running wide open. The 7.9-grain Crosman Premier hollowpoints gave an average energy of 21.54 foot-pounds. Unfortunately, they were not very accurate at that power level. They really preferred to run at 14.63 foot-pounds.
Beeman Kodiaks were very accurate at 15.56 foot-pounds, but not nearly as accurate at their maximum of 20.32 foot-pounds. The old standby RWS Hobbys screamed out the muzzle at 1159 f.p.s. wide open and generated 23.57 foot-pounds. I didn't bother to test them for accuracy because I know they cannot be driven that fast without scattering. Hobbys want to pull 10 foot-pounds and less in a springer. Only the gentler PCP or CO2 powerplants can drive them faster with accuracy.
Just to keep the heat on Gamo, I did test their Raptors. They scooted out the hole at the front at 1390 f.p.s., for 21.46 foot-pounds. Once again they did not live up to the promise of greater power than lead pellets and I know they can't be accurate at that speed, either. Interestingly, they didn't come close in velocity to the AirForce Condor, which punched them to 1486 f.p.s. I'm still waiting for someone with a 1600 f.p.s. Hunter Extreme to step forward.
What have we learned?
Well, I hope you now see a relationship between a pellet's velocity and accuracy. If you do, you're well on your way to shooting better. I hope you also see that the claims made by airgun manufacturers and carried forward by airgun stores are meaningless without accuracy figures to go with them. Sure, it's neat to say a certain airgun gets 1000 f.p.s., but without any real accuracy, such a number is pretty worthless. Give me a rifle that shoots "only" 750 f.p.s. but can also nail its target every time. Close only counts in horseshoes and hand grenades.