by Tom Gaylord, The Godfather of Airguns™
Writing as B.B. Pelletier

This report covers:

• What’s a big bore airgun?
• Hydrostatic shock
• Back to airguns
• Why, then, the great debate?
• What’s ahead?

What’s a big bore airgun?
While there’s no official definition, those of us who talk about airguns call .177, .20, .22 and .25 calibers the smallbore airgun calibers. From that, you can deduce that anything larger than .25 caliber is a big bore. A few years ago, there was actually a heated debate over this threshold, when a rifle made by the late Jack Haley in .257 caliber competed in and won the last LASSO big bore competition held in Texas. I hope to show in this report why that debate was so heated.

pellets and bullets
From the left are the 4 smallbore calibers — .177, .20, .22 and .25. The jacketed bullet is approximately what the .257 Haley rifle shot, so even though it’s also a .25, it’s quite a bit larger than any pellet. Next to that, a 405-grain .458 bullet dwarfs everything.

To understand why the definition of a big bore is so important, you first have to understand how big bore airguns differ from most firearms that you may be familiar with. When firearms used black powder, there was substantially no great difference between a large bore airgun and a firearm. Sure, firearm bullets travelled more than twice as fast as airgun bullets when they left the muzzle, but all that did was shorten the range at which airguns were effective. In those days, all a bullet did to an animal was penetrate and create a wound channel through which blood was lost. It was important, therefore, to hit a vital organ to dispatch the animal with certainty.

And animals did not fall over when hit with bullets in those days. They usually stood their ground for several minutes until blood loss took its toll. It was much like hunting with arrows, only more effective because the bullets penetrated deeper and also went much farther with accuracy. A bowman might take a deer at 50 yards if he was a good shot — a rifleman could take one out to 200 yards if he was so inclined.

Once hit, it did not matter what gun sent the bullet, as long as the penetration was adequate and the vital organs were hit. What I’m saying is that a 45-70 buffalo rifle bullet is no more effective on a deer or bison than the same lead bullet fired from a .458 air rifle that develops only 500 foot-pounds at the muzzle. True, the hunter can shoot farther with the buffalo rifle, but their bullets are equally effective as airgun bullets.

Hydrostatic shock
Hunting with firearms changed forever at the beginning of the 20th century. Savage’s 250/3000 (also called the .250 Savage), created in 1915, was the first commercial bullet to leave the muzzle at 3,000 f.p.s. That was an 87-grain .257-caliber bullet. When that happened, the .25-caliber centerfire rifle went from being adequate for squirrels through fox to taking deer at 250 yards with certainty. It changed everything, because the new high-velocity bullets produced hydrostatic shock in the game.

You have probably seen slow-motion videos of a high-velocity bullet expanding in ballistic gelatin. As the bullet impacts the soft substance, it transmits a large portion of its energy in the form of a shock wave that travels through the liquid inside the target. In animal tissue, this shock wave hits nerves and causes them to transmit disruptive signals to the brain that shut down the animal’s life support. For this reason, a 50-grain .22-caliber bullet that impacts a deer-sized animal at 2,500 f.p.s. can actually knock that animal down on the spot, while a 400-grain bullet from a .45-70 that impacts at 1,000 f.p.s. will slip right through and exit the animal, leaving less than a quarter of its energy behind. The slow-moving bullet has to connect with vitals to do its job, while the lighter high-velocity bullet gets a boost from the shock it creates. Hunters of old were aware of this and knew they had to hit certain places on each animal to have an effect. They never gave it a second thought. But once hydrostatic shock entered the equation, the game changed forever.

Now, don’t get confused and think that faster pellets can do the same thing! Airguns top out at less than 1,500 f.p.s., so they can never produce hydrostatic shock in game. You need centerfire rifle velocities for that (above 2,000 f.p.s.).

Big Bore Air Rifle Ammo

No airgun will ever get a bullet or pellet going fast enough to create hydrostatic shock. That is a fact of the physical world that we have to come to grips with. So, all airgun hunting, and especially big bore hunting, has to be done exactly as hunters did it in the 1870s. Shoot for the vitals and be prepared to wait for the bullet to do its job.

In this vein, we’re interested in the bullets that are shot by big bore airguns. What we need are 2 things — accuracy and penetration. Accuracy good enough to hit the vital areas on the game we’re hunting, and penetration adequate to go deep enough to pass through those vitals. Now — prepare to be shocked.

A 405-grain bullet from a Quackenbush .458 rifle will pass entirely through a 1,500-lb. American bison when it hits from the side. I say that because it’s been done — several times, in fact. I’ve seen several medium-sized animals (250-400 lbs.) hit with smaller big bore bullets that completely exited the animal. So, the thing you want to do is try to match the bullet to the intended target. A bullet that’s sized .458 is the same diameter regardless of whether it weighs 193 grains or 510 grains. The hole it leaves will be identical. But the depth of penetration won’t!

All these bullets are .458 caliber, but they vary greatly in weight. From the left they are: 193 grains, 350 grains, 405 grain and 510 grains.

Stephan Bowles (right) dropped this bison with a .458 Quackenbush rifle. The bullets went completely through the animal from over 50 yards. He was guided by Eric Henderson. Henderson photo.

Why, then, the great debate?
This is why there was such a debate over the Haley rifle in .257 caliber. It was certainly capable of hitting the half-sized goat silhouettes out at 300 yards (the Haley hit them every time), but was it able to deliver a knockout punch on a goat-sized animal at that distance? Ballistics suggest, no — it wasn’t. While the 90-grain bullet might hit and even penetrate a game animal at that range, the entrance hole would be too small and the penetration too shallow to matter. Don’t get me wrong — I would not want to be hit by such a bullet at that distance! But it would have less energy than a .22 long rifle bullet has at 50 yards. So, it would not be a good choice for a reliable bullet on 150-200 lb. game.

What’s ahead?
In this series, we’ll discuss the various aspects of big bore bullets and the guns that shoot them. We’ll look at accuracy, bullet weight and length, lead alloys and other pieces of the big bore puzzle. This would be a wonderful time for you to ask any questions you may have.