Can a pellet gun go “overbore”?
by B.B. Pelletier
There’s a new instructional video on Airgun Academy. It’s all about dot sights. Click to see it.
Mary Kulesa Geraci wins this week’s Big Shot of the Week.
Last week at a gun show, I learned something that eluded me all my life. I learned what overbore means and why it matters so much.
I was introduced to the .250 Savage cartridge, which is also called the .250/3000. It is one of the smaller .25-caliber cartridges today, but when it was first introduced back in the early part of the 20th century it was a screamer — the fastest .25-caliber cartridge around. It wasn’t the biggest, mind you. That honor probably went to the .25 Krag, which was a .30/40 Krag necked down to .25 caliber. It held more powder, but its bullets went no faster.
In fact, if you look at the reliable reloading books today — not online, where you can find anything — the .250 Savage is still faster than the .257 Roberts with an identical 100-grain bullet. I couldn’t believe that, but according to my Lyman manual, 48th edition, the top safe loads for the .250 Savage go faster and use less powder than the .257 Roberts.
I don’t want to start an argument about this — it could probably go either way, depending on the manual you read. The point is that the .250 Savage is a mild-mannered and small centerfire that delivers the same punch as a well-respected middleweight round.
It does so, according to Harvey Donaldson because the cartridge case volume is optimum for the caliber. A deer hit by a 100-grain spitzer soft-point traveling 2,700 f.p.s. will not pause to inquire what cartridge case launched it. Donaldson designed his .219 Wasp to do the same thing. It delivers the ballistics of a .22/250 with far less powder and pressure. That leads me to my real discussion point for today.
Is there such a thing as an optimum caliber/velocity ratio for an airgun, along the same lines that I have just described for a centerfire rifle? I think there may be, though I’m pretty sure I don’t know what it is.
I suspect there’s a velocity range that’s optimum for each caliber; and within that range, most guns and most pellets will do their best. This range may even be defined by the type of powerplant that generates it. If I knew this for sure, my job of teaching new airgunners would be so much easier.
Well, I may not know the optimum, but here’s what I do know.
1. If you shoot a .177 pellet at 650-750 f.p.s. in a spring rifle with a good barrel, it’ll be as accurate as that rifle will ever get. But if you try to increase that by as little as 100 f.p.s., you run the risk of creating a nasty monster. The HW 35 comes to mind when I say that. Left at its factory velocity of 725-750, the .177-caliber HW 35 is a gentle giant. But try to push it up to 850 f.p.s., and the rifle becomes hard to cock, buzzy and a real pain to shoot.
The reverse is also true. Take a .177-caliber Beeman R1 that delivers 1,000 f.p.s. right out of the box and detune it to 850 f.p.s., and it settles down. That isn’t quite the low range I stated, but it’s going in the right direction.
2. A spring rifle that delivers over 1,000 f.p.s. will almost certainly be extremely hold-sensitive. I dread testing the mega-magnums when it comes time for the accuracy test, because I’ve seen it go bad so many times.
3. A .22-caliber spring rifle gets twitchy at around 900 f.p.s. and above. As long as I keep the velocity below that speed, I seem to get the best performance from the gun. You may have wondered why I have spent so much extra time on some airguns but not others. This is part of the reason why.
4. Precharged airguns are entirely different than springers. They handle velocity better, but even they have upper limits. I have seen some PCPs do well at slightly above 1,000 f.p.s., but that’s about the point at which they start to fall apart.
5. Big bore airguns haven’t yet reached their full potential in any caliber I’ve tested to this time. I think a big bore PCP could tolerate 1,200 f.p.s., if it could get the bullet up to that speed. And that statement only goes for real bullets — not for hollow-based “pellet” designs.
But back to my question. Do you think there might be an optimum velocity/caliber relationship, and would it hold across models and pellets types? And a corollary: If there’s such a relationship, is there also a pretty good chance that when a pellet is driven too fast that it becomes increasingly harder to shoot accurately?
Is there a way to test this? I have the Whiscombe that I can adjust to change velocities for the same pellets. But how would we test for this relationship? What should be taken into account? And would the test be with a single pellet, or would it be with several pellets?
One final thought. Above, I said that pellets in the 650 to 750 f.p.s. range are usually the most accurate, but what about when those pellets go even slower? What happens then? I know something happens, because I have air rifles that shoot slower than 650 f.p.s. that are very accurate. Yesterday’s test of the HW 55CM is a good example. Should we also put some kind of distance requirement into this experiment?
What are you smoking, B.B.?
What brought this on is that I’ve been eyeing my Beeman R8 recently, but I don’t have a reason to use it in a test. But it’s such a nice, accurate air rifle that I just want to shoot it a lot. I was hoping to find some justification for shooting the R8 more, even if it’s only to cleanse my pallet after the test of some 1,300 f.p.s. hyper-blaster.
But I’m serious. If there’s some sort of velocity/caliber relationship that’s optimum, then we’ll have discovered a powerful tool for the new airgunner who is looking for that first gun. “Sure, you can get the .177 Bow of Hercules if you want, but just remember that the immutable laws of airgunning dictate that the most accurate air rifle will always be….”
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