Powerplant, Pellets, and Footpounds

Monday, October 11, 2010

For years, I have heard things like “spring-pistons don’t like heavy pellets” and “CO2, pre-charged pneumatics and pump guns are more efficient with heavy pellets.” I guess I just accepted these truths as an article of faith and never really thought much more about them.

But recently I have been testing several .25 caliber – quarterbore – air rifles, and the tale that the chronograph tells is interesting. Put simply, when it comes to power generation – that is, foot-pounds measured at the muzzle – springers tend to like light pellets and pneumatics prefer heavier pellets. Of course, it isn’t always a straight linear function, because there are other variables, such as how tightly the pellet fits in the bore.

It all started when I was chronographing a trio of break barrel .25 cal. springers. I was using Gamo Pro Magnum 21.91 gr. pellets to chronograph them, because I had a plentiful supply of those pellets. One of the rifles was slinging the Gamo pellets at 565 fps average, which works out to 15.53 foot-pounds average. I mentioned this to the importer, and he suggested trying JSB Kings (25.4 gr.). Somewhat counter to the “law” about springers, the heavier pellet did better in terms of power but slightly worse in velocity: the JSB Kings averaged 555 fps for 17.37 foot-pounds at the muzzle. But the real surprise came with the lightest pellet. 19 gr. Milbro Rhino pellets rocketed through the traps at 667 fps for a sparkling 18.7 foot-pounds. In this case, the law about springers proved right: the lightest pellet did generate the most power in this .25 cal. spring-piston powerplant.

Okay, I thought, but what about the pneumatic airguns, do they obey the “rules” or not? It was raining when I thought about answering this question, and I usually need to do my chronographing outdoors, so I turned to the respected varminter Cliff Tharpe. Cliff, whose online handle is VarmintAir, is producer of the Airgun Hunting the California Ground Squirrel DVD. He has deep experience in hunting and clobbering vermin with air rifles.

Cliff has a factory stock .25 Benjamin Marauder that he routinely uses to pop prairie dogs at 50-100 yards. He sent me some data on his experience chronographing different weight pellets through the Marauder, with the following note: “These were all shot at the factory settings, whatever those may be.  All velocities were taken with the start screen 12 inches from the muzzle.  I use a CED M2 Chronograph set up indoors, with the infrared screens.  Two mags, for sixteen shots with each pellet.  All pellets were weight sorted.  This is with a 3000 psi fill. “

And here’s the data:

  • JSB .25 Quarter Bore, 25.4 grain – avg. vel. – 881 fps – fpe 43.8
  • Benjamin .25 dome head, 27.8 grain – avg. vel. – 845 fps – fpe 44
  • Beeman Kodiak .25, 30.8 grain – avg. vel. – 821 fps – fpe 46.1

Here we have a straight linear relationship – the heavier the pellet, the lower the velocity, and the greater the power that is generated.

Now, having said all that, what’s the most important thing?

Accuracy, of course. A firearms expert once said, “A hit with a .22 beats a clean miss with a .45.” And he was right. If you can’t reliably hit what you’re aiming at, it doesn’t matter how much power you are generating. The first thing you need is sufficient accuracy to hit your intended target at the range at which you plan to shoot. If you are planning to hunt, once you have the accuracy, then you need sufficient power to humanely take whatever game you are after.

Til next time, aim true and shoot straight.

– Jock Elliott

6 Comments

  1. Pete Wareing says:

    Jock, I have read all I can find on pellets of every shape size & weight, but have not seen much blogging on the lead free pellets such as the Skencos or Prometheus pellets. Are these pellets not accurate, or too inaccurate to bother with? Please let me know your take on these. Thanks

    1. Jock Elliott says:

      Pete,

      I don’t have any experience with the Skencos or Prometheus pellets, but I can recommend the Dynamic non-lead pellets.

      Check out these two blogs: http://www.airgunsofarizona-temp.com/blog/2008/06/beeman-r7-genuine-classic-some.html and http://www.airgunsofarizona-temp.com/blog/2008/08/additional-experiments-with-dynamic-non.html

  2. Conor K. says:

    Mr. Elliot,

    I think I’ll have to agree with “spring-pistons don’t like heavy pellets” and “CO2, pre-charged pneumatics and pump guns are more efficient with heavy pellets.”

    I have a RWS 34p in the .22 cal, and my brother has a Benjamin 392. So the other day, we were shooting at an arch to see how far the pellets would go,(at the present time we had Crosman Premier Hollowpoints). So we aimed at the exact same place in the sky, and his Benjamin went roughly 50 yards farther. His went 214 yards while mine went 164 yards. The Crosman Hollowpoints do fit a lot more tight in my barrel then RWS Superdomes, so I’m guessing it is that because my gun is supposed to go 800 fps and his is supposed to go 685.

    Anyways, just thought I’d share.
    -Conor

    p.s. I play banjo too! 🙂

    1. Jock Elliott says:

      Conor,

      Thanks for your comments; it’s good to hear from a fellow banjo picker. I have to ask: how did you find the impact points of the pellets at that range?

  3. Conor K. says:

    Mr. Elliott,

    No, we weren’t shooting at targets, we were just shooting on one of our 300 yard dirt driveways to see where and how far it would go.

    But at 15 yards at some old metal refill blue glue containers, (we were shooting the Crosman Premier Hollowpoints) my pellet went through the first side and made a big dent on the other side but did not go through. The Benjamin however, at 8 pumps, went through both sides! We did some more testing by shooting at the containers and found that the Benjamin with 6 pumps was the same as the hitting power as my RWS 34p, they both made the same dent on the other side of the container.

    1. Jock Elliott says:

      Conor,

      Thanks for the info. It’s fun doing testing with airguns!

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