Pellet velocity versus accuracy test: Part 1
by B.B. Pelletier
Welcome to the test that blog reader Mel inspired last week when he made the following comment about whether pellet guns can be overbore:
I live in Germany, where all airguns are limited to 7.5 joules (5.6 fpe). This is very annoying for long-range shooters and also limits the choice of airguns, as many models are not offered in low-powered versions. But the big advantage is that the beginners here get an airgun they can actually shoot precisely, while so many Americans buy one of these $200, 1600 fps bangers just to become disappointed because it acts like a supersonic water hose.
Ask yourself how much power you really need and have a look at the Brits that hunt anything up to rabbits with 12 fpe. I personally would never sacrifice accuracy or comfort to exceed these 12fpe, unless I had a really good reason for it.
That got me thinking about something that’s been banging around in my head for a long time. Pellet accuracy versus velocity. Today, I’ll begin a long test to show if there is such a relationship.
I really wanted to write this as if I were a new airgunner who just bought his first airgun, but then I thought about all the confusion Orson Wells caused with his 1939 radio broadcast, despite continuous disclaimers that it was just a theatrical portrayal. It’s doubtful that my experiments with pellet velocities will cause a panic on the East Coast, but a new reader who finds this series a year from now might become very confused. For that reason, I’ll remain in character.
I’m testing the purchase of a .177 screamer breakbarrel spring rifle to see what kind of accuracy we might expect to get. But instead of any of the possible guns that could be selected, I’ve substituted my Whiscombe JW 75 in its place. So let’s clear the air about all the differences right now.
This JW 75 will serve as a testbed because the power can be changed while leaving all other factors the same.
All the differences
First, the Whiscombe has a premium barrel made by Anschütz. So that will be different.
Next, the rifle has the Harmonic Optimized Tuning System on the barrel. The HOTS allows me to move a weight to “tune” the barrel’s vibrations to the best place for each pellet I use.
Third, the Whiscombe is recoilless. Even though it generates almost 30 foot-pounds in .25 caliber (it has four interchangeable barrels, too), it does not vibrate much more than an FWB 300 match rifle.
Fourth, the rifle is both an underlever and a breakbarrel. The underlever requires three pulls to cock the rifle one time, with a combined force requirement of around 100 lbs. Two powerful mainsprings are pulled back so they oppose each other with a 75mm space between the two piston seals. The barrel also breaks open to load the pellet into the breech.
Fifth, the trigger is in the match-rifle class.
And, finally (I hope!), the rifle has an air transfer port limiter that lets me adjust the power as needed. It’s this final feature that caused me to choose this rifle as my testbed.
So, I’m shooting an air rifle with accuracy potential way beyond that of a $350 mega-blaster. But that won’t matter because of how the test will be conducted.
First, I’ll shoot the rifle at its full power potential. That starts today. Then, I’ll shoot the same pellets I test today for accuracy at 25 yards. I will not adjust the HOTS at any time. It’ll just be wherever it is when I installed the .177 barrel, and there it will remain. That will give us a performance baseline.
Next, I’ll install an air transfer port limiter and test the rifle with the same four pellets shooting through the exact same rifle at a lower power level. That second test will be conducted in the same way — velocity of the four pellets first, followed by their accuracy at 25 yards.
Then, I’ll install a different air transfer port limiter to further reduce the rifle’s power. Another set of tests will follow. By that time, we should know if a final round at the lowest possible velocity would be required or not.
What we’ll get out of this is a view of how velocity affects accuracy, with all other variables remaining the same. That will probably suggest other tests that will further advance our understanding of the accuracy equation.
Let’s start with what I’ll call Phase One — the first test of accuracy and power and test four pellets for power.
I selected the Beeman Devastator pellet for several reasons. At 7.1 grains, it represents the kind of pellet a new shooter would select in the hopes that his rifle will shoot absolutely flat, and he won’t have to worry about ballistics because the pellet never drops. You see the same thing among handloaders who pack dynamite in their cartridges behind finishing-nail bullets in the hopes that they will be shooting a laser. The fact that they’ll really be shooting a chaff gun never dawns on them until they try what they have been envisioning for so long. So, for all those armchair sportsmen out there, I picked this one for you!
The other reason I picked the Devastator is because of the neat design. The pellet looks like it wants to blow up on target. We’ll see about that, won’t we?
In my “Forget the dilithium crystals, Mr. Scott, give her all she’s got” tune, the Whiscombe averages 1,216 f.p.s. with 7.1-grain Devastators. The spread went from a low of 1,205 f.p.s. to a high of 1,233 f.p.s., so the spread is 28 f.p.s. That’s actually pretty stable for a spring rifle shooting at this level. The average muzzle energy is 23.32 foot-pounds, which is pretty hot for a spring-piston rifle in .177 caliber. Okay, we’ll store that velocity away for later, when we test the accuracy of this pellet.
Crosman Premier lites
The second pellet I tested was the Crosman Premier lite. I included them because they’re a classic domed pellet, and I wanted them to be in this test. Why didn’t I test the 10.5-grain Premier heavies? Because they’re more often best in PCPs and not in spring rifles. This was just a judgement call and not necessarily the right thing to do. But that’s the way I went.
Premier 7.9-grain pellets averaged 1,134 f.p.s. in the test rifle. The range went from 1,128 to 1,140, for a 12 f.p.s. spread. This is where the good manners of the Whiscombe show through, because at that velocity a 12 foot-second spread isn’t usually seen in a spring rifle. The average muzzle energy was 22.56 foot-pounds.
Beeman Kodiaks weighing 10.20 grains were an obvious choice, given the power expected in this rifle. They averaged 992 f.p.s. and ranged from 979 to 1,001 f.p.s., for a 22 foot-second spread. At the average velocity, they generated 22.29 foot-pounds of muzzle energy. According to my experience and beliefs, that’s still too fast for the best accuracy, but we’ll see.
The fourth pellet I chose (Buy three — get one free) was the 16.1-grain Eun Jin. I figured that they would be heavy enough to lower the velocity into the accurate zone. And they were! They averaged 726 f.p.s. with a spread from 719 to 732 f.p.s. They averaged 18.85 foot-pounds of energy at the muzzle. We’ll see how well they do on paper in the next test, but I must say this — of all the Eun Jins I’ve ever shot, these fit the breech the best. They slid in like any other pellet.
These four will be the pellets I’ll use throughout this test. We’ll compare them for power and accuracy as I adjust the power of the rifle downward in steps from today’s baseline.
Just looking at the results of today’s test, one thing stands out. The classic spring-piston power relationship popped out exactly as expected. Spring-piston guns are usually the most efficient (the most powerful) when the lightest pellets are used, and that’s exactly the way these four pellets are distributed. That relationship will probably remain the same throughout the test, though the close ones may switch back and forth a little.
I’m going to run the second part of this Phase One test very soon so we can keep the data straight in our minds.