Big Bore airgun calibers
Writing as B.B. Pelletier
This report covers:
- The greater problem
- The beginning
- Bullets — not pellets
- .308 caliber
- Bore size
- .357 caliber
- Black powder
- The .45 caliber dilemma
- Shoot soft lead bullets that are slightly larger
- Other big bore calibers
Most shooters are familiar with the smallbore airgun calibers of .177, .20, .22 and .25. Even shooters who don’t consider themselves to be airgunners know at least the .177 and .22 calibers. But in recent years there has been an explosion of big bore airgun calibers, and I am seeing that many shooters have little knowledge about them. If that were the only problem it would fix itself, because over time people always learn.
The greater problem
The bigger problem are the airgun manufacturers that do not know much, if anything, about the larger calibers. This report will address the lesser-known truths about big bore airgun calibers.
Where do the big bore calibers start? Well, they start at anything above .25 caliber. But there is a big bore caliber called .257 that is a legitimate .25 caliber. The difference is the .257 big bore shoots elongated bullets rather than diabolo pellets. Instead of 45 grains they can weigh 100 grains and more. They can reach out hundreds of yards, where the wasp-waisted hollow-tailed diabolo falls off fast after just 100 yards.
Bullets — not pellets
Another thing is big bore airguns shoot bullets, for the most part — not pellets. Yes, there have been some .308, .357 and even .45-caliber diabolo pellets made for certain airguns, but big bore airguns have been around since about the year 1350 and they have always used bullets — round balls until around 1850 and conical lead bullets since then. The diabolo pellet is a 20th-century invention.
Recently the .308-caliber big bore has gained a lot of traction in the marketplace. People hear the caliber size and envision the .308 Winchester cartridge that they know is very powerful. But a .308 caliber pellet driven by air is far less powerful. It gives up the one big advantage of the big bore airgun — size. With a .308 air rifle your shot has to be precise or you risk wounding your quarry. That said, the .308 can do the job for a good shooter.
Now let’s consider bore size for a moment. You don’t have to do that with a pellet. A .22-caliber pellet should work in most .22-caliber airguns. Accuracy will differ from pellet to pellet and we sometimes sort pellets by the diameter of their head, but that’s as far as it goes. Not so for big bore bullets! And this is where shooters new to the shooting sports get confused.
A .308 bullet may be very accurate in a .308 big bore airgun, or it may not even stabilize. That gun may need a bullet that’s .309-inches in diameter to work well. It all depends on the size of the bore! You see, smallbore airgun barrels are closely controlled to fit the pellets of their caliber. But big bore airguns can have barrels of widely varying internal sizes. That’s because there are no set standards for barrels of big bore airguns. They tend to be firearm barrels that have been used on airguns.
There are exceptions, of course. AirForce Airguns, for example, orders hundreds of barrels in each big bore caliber they produce from Lothar Walther. They are such a large customer that they can specify the exact inside dimensions, as well as the rifling twist rate. That’s something that Joe from Podunk, who makes 50 rifles a year, can’t do. He has to select his barrel from the stock items a barrelmaker offers.
The next size up from .308 is .357 — and this caliber is a huge problem! Ten years ago it was called 9mm, which is sized 0.355-0.356-inches in diameter. The Koreans who were the first to make rifles in this caliber made them with barrels sized for 9mm lead bullets. The trouble with that is, unless you know where to look, it’s very hard to find 9mm lead bullets. That’s because 9mm is mostly a pistol caliber. All the bullets that are popular for 9mm pistols are jacketed and don’t do well in airguns — especially not the underpowered Korean ones! It took a full decade for the airgun makers to realize their mistake, and it took dealers and shooters even longer. Even today there are many shooters who think one-thousandth of an inch shouldn’t matter that much with a bullet. But it does!
This is where a background with shooting black powder firearms comes in handy. There are two big reasons for this. First, when black powder explodes upon firing, the instant high pressure upsets the base of lead bullets, obturating (squashing) them into the bore. They are squashed into the rifling where they fit the bore better and also seal against the burning gasses. Airguns cannot do the same, so the fit of the bullet to the bore is critical from the start.
The second reason a black powder background or at least a knowledge of their history is important is because in the past (1250 A.D. to 1900) all non-military firearms (read that as black powder, because that’s all there was for most of that time) came with bullet molds when they were made. The owner had an idea of his gun’s caliber but it didn’t matter as long as he used the mold that came with it to cast his bullets. The military held gun makers to more exacting specifications so they could produce the bullets for their soldiers. Their guns didn’t have to each come with a mold.
Today, though, black powder arms are produced to more exact specifications, so their owners can purchase bullets to go with them. But it still isn’t a smooth road.
The .45 caliber dilemma
Now we come to one of the biggest dilemmas in big bore airguns today — the .45 caliber that exists in no less than three distinct sizes! And they are not interchangeable. Starting with the Koreans again, when they made .45 caliber big bores they made their barrels for bullets of a diameter of .452-inches. That is the modern .45 pistol diameter for the .45 ACP cartridge. And some .45 Colt revolvers also have bores that size.
Bullets made in that size are expected to be fired from .45 caliber handguns at 850 f.p.s. or so. They weigh from 160 grains to 250 grains — AND THAT IS IT! If they are shot in big bore air rifles in the 200-225 foot-pound class, they are fine. The Turks are also making their .45 big bores with bore diameters in this size. I have no idea of what the Chinese who make the big bores for Gamo are doing, but it does bear consideration.
The next popular size of .45-caliber bullet is .458, and it has another problem. Some makers are calling their rifles a .457, but I doubt you will find bullets of that size unless they are custom made. Don’t worry, though, because .458 bullets are what you want to use anyway. They fit the bore, where .457 bullets usually don’t.
These are the air rifles that shoot bullets weighing 350-500 grains. They will also shoot the lighter bullets, as long as they are sized .458 and not .452.
Then there are the big bore rifles that are made by boutique makers who turn out a few guns a year, in caliber .454. These guys don’t last that long and finding bullets for their rifles can be a real chore. This size was for Colt Single Action Army revolvers from decades ago. You’ll have to go to a custom bullet maker to buy them today.
So — they’re all .45 caliber, but in three different sizes! Did you know that? If you didn’t and you shoot your rifle with the wrong size bullet you aren’t going to do very well. A 24-inch group at 100 yards can shrink to a 3-inch group, just by using bullets of the right size!
Shoot soft lead bullets that are slightly larger
So—the lesson today is to shoot lead bullets of the right diameter. The right diameter is one-thousandth over the bore diameter in most instances. But you need to experiment with different sizes to make certain. I have been doing this for over 50 years and in my experience a thousandth larger with a lead bullet is what you want.
They should be soft lead bullets, because hard cast bullets will leave lead deposits in the bore. Air rifle bullets also don’t need to be lubricated — as long as they are soft lead. In fact they shouldn’t be. They are ideal for the velocities at which the most powerful of these air rifles shoot — generally 700-900 f.p.s.
Other big bore calibers
Yes there are big bores in calibers other than the ones mentioned here. A number of rifles in .40 caliber have been made, but they were all made by low-rate or custom makers who probably made them to work with a lead bullet that is commonly available.
That being said, there can be big bores in other calibers, as well. I know of several big bores made to shoot the 12 gauge rifled slug that looks like a diabolo pellet. It’s called the Balle Blondeau.
The Balle Blondeau is a 12-gauge slug that looks like a diabolo pellet. Some smoothbore big bores have been made to shoot it.
Like black powder arms, a big bore airgun can be made in any caliber. If you plan to buy one, make sure you can get the bullets for it first.
This has been a brief but fairly complete look at big bore projectiles. I have concentrated on the bullets rather than the few diabolo pellets that exist, because the bullets are where it’s at for big bores. Stay tuned.