by B.B. Pelletier
Why don’t we wise up?
This discussion seems to beg the question: Why not abandon diabolo pellets and go to bullet-shaped projectiles? Bullets have been around for small-caliber airguns for a decade or more. I’m not advocating their use in this article, but I am making a case for testing the accuracy potential of subsonic versus supersonic diabolo pellets for long-range shooting–before I begin actually shooting.
Before the test is over, I’ll also shoot regular bullets, but that limits the selection of airguns to only the most powerful because they’re the only ones that can stabilize heavy bullets properly. I don’t want to rule out the other air rifles in my quest for accuracy. Besides, a Career 707 shooting a 30-grain bullet at 900 f.p.s. sounds suspiciously like a .22 short standard speed. If I want a gun to shoot like a rimfire, that’s what I’ll shoot. This test is to see what PELLET guns can do!
The Whiscombe JW 75 is the perfect testbed rifle for the “velocity destroys accuracy” theory. Transfer port restrictor screws allow me to vary the velocity of any pellet from barely coming out to as fast as it will go. For the pellet we finally selected, that was supersonic. Scope size is limited on this rifle because the barrel tips up for loading, even though it’s cocked by an underlever. Easy access to the breech allows pellet indexing.
The perfect gun for this test
I designed the following test to see if there’s anything to this velocity/accuracy barrier. We own a Whiscombe JW 75 breakbarrel rifle, which seems like the perfect testbed because it’s so easy to adjust the velocity. The air transfer port is threaded to receive a small Allen screw through which a hole is drilled. A small hole passes less air, resulting in lower velocity. A bigger hole equals higher velocity. Take the screw out altogether and you get all the speed the rifle is capable of with that particular pellet. It’s a simple solution to power adjustability, but it’s very effective and you can adjust that hole to within 10 f.p.s. of a particular desired speed for any given pellet.
Our Whiscombe has barrels in all four popular calibers, and they can be changed in just a few minutes. For this test, however, only the .177 will do, because of the tremendous velocity I’m after. At its peak, the Whiscombe is a 20-30 foot-pound rifle, depending on the caliber and pellet used, so I figured it would be perfect for testing a pellet at two speeds in the same barrel. One special restrictor screw is adjusted to deliver 900 f.p.s. or a bit less, while the open port with no restrictor permits as much velocity over 1,000 f.p.s. as the rifle can muster.
Another advantage you get from a Whiscombe powerplant is stability. The extreme variation in velocity is quite low, as long as the pellets are all prepared and handled the same. Some folks think I place too much emphasis on extreme variation in a shot string, but I feel it’s a great indicator of how consistently the rifle is performing. And consistency is what I’m after when I go for tiny groups.
One potential fly in the ointment is the Whiscombe’s Harmonic Optimized Tuning System (HOTS). Several years ago, Whiscombe started offering barrels with an adjustable muzzle weight built in. We ordered the HOTS on all four of our barrels. But what should I do? Adjust the weight for low velocity and again for high velocity? While that sounds easy in theory, it takes a lot of time to accomplish–as much as a whole day per pellet and velocity. I initially decided not to adjust the HOTS at all and let the barrel vibrate where it wanted. You’ll discover what happened–just as I did.
This view is looking down past the raised barrel at the adjustable transfer port, which is in the center of the picture. The port is partially unscrewed for better identification. But while shooting, it must be flush with the rifle so the barrel will clear when it is swung closed. When the screw is removed completely, the entire hole becomes the transfer port. With this system, it’s possible to adjust velocity to within 10 f.p.s.
My test guidelines–making sure we don’t compare apples to oranges
I decided that all pellets should be lubed before shooting, because I didn’t want anyone saying the high-speed pellets were leading the bore. The rifle’s inventor, John Whiscombe, gave us the formula for an oil–Whiscombe honey–to keep his air rifle barrels free from leading, so I decided to use it exclusively in this test.
These two transfer port restrictor screws have been adjusted to different velocities. The screw on the left shoots .177 Crosman 10.5-grain Premiers at 905 f.p.s. and 9.1-grain Chinese domes at 980 f.p.s. The screw on the right shoots 9.1-grain Chinese domes at 869 f.p.s.–perfect for the test! Each screw was hand-adjusted with a diamond needle file and tested over a chronograph. After the first day at the range, I made a third port for 7.9-grain Crosman Premiers.
Another guideline I adopted for this test is one I have wanted to do for a long time–10-shot groups. Back in the 1960s and even earlier, 10-shot groups were the norm in gun reports. Five-shot groups were sometimes shown, but there was a lot of talk about how they weren’t as representative of true accuracy potential at 10 shots. If you go back to the writings of the 19th century, you’ll find 20- and even 50-shot groups being used to back up accuracy claims. From a statistical standpoint, though, 10 shots will give you a pretty good picture of potential. And it is certainly much easier to shoot 10 than 20.
Ever since the first days of the Airgun Letter, I’ve had a running dialog with champion airgun shooter Rodney Boyce [who recently passed away] about the number of shots per group. Never one to complain, Rodney told me he is amazed at the difference in size between 5-shot and 10-shot groups from the same gun on the same day. I agreed with him that 10 are better than 5, but I’ve done nothing about it until now. For this test, I resolved that all test groups would have 10 shots and let the chips fall where they may.
That said, I must impress on those who have never shot at 50 yards with an air rifle that it is not easy! Any puff of wind WILL open your group. That lightweight, relatively slow-moving pellet is just as sensitive as it can be to the movement of air. Why don’t I shoot this indoors? I would like to, but the facilities aren’t available. So, I’ll shoot outdoors and take my lumps. If that means a half-inch rifle starts shooting an inch or more, so be it. There will be no editing here. Because, if you think 50 yards is hard, you ain’t seen nothin’ till you back up to 100! Your “groups” start looking like open shotgun patterns at that range. And 100 yards is where we’re headed. [Note: That 100-yard, long-range test was one I was working toward in The Airgun Letter. I never did the final test.]
I’m not conducting this test to find out how accurate a certain air rifle can be. Who cares what a Whiscombe can do, if everybody doesn’t have one? I’m doing this to learn about general long-range airgun accuracy potential, as it is affected by velocity. Undoubtedly, I will get lucky a time or two, but that should be more than offset by the vagaries of shooting 10 times at the same target. No one is that lucky.