Big bore bullets: Part 2

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

Part 1

This report covers:

• The dumbbell bullet
• Did those bullets make a difference?
• Longer bullets must spin faster!
• Shorter bullets need less spin to stabilize
• Less contact with rifling reduces friction
• Summary
• Pyramyd Air Cup

Let’s look at big bore bullet design. I’ve written about this before, specifically in this report back in May 2007. And I told you about a big bore contest in which a smoothbore shooting special bullets out-shot many rifles. Let’s look at that first.

The dumbbell bullet
At the 1999 Damascus, Maryland, airgun show, we had a big bore shoot in which about 25 shooters competed. Among them was a father-son team of Bob and Mike Chilko, who each had a big bore gun they had made themselves. Bob shot a .398 underlever, and Mike had a front-pumper that had to be pumped 30 times for each shot. Both guns were smoothbores that shot strange-looking dumbbell-shaped slugs. The funny thing was that they outshot most of the other competitors for accuracy, which included hitting a 4-inch target at 40 yards. In 1999, that was a quite a feat for a big bore airgun — especially a smoothbore!

dumbell bullet
The Chilcos shot bullets that looked like this. The shape promotes high drag that stabilizes it in flight.

When I saw those bullets, I knew what they were and why they worked. They were an adaptation of the French Balle Blondeau shotgun slug of the 1960s that revolutionized all shotgun slug shooting. Before the Balle Blondeau, shotgun slugs were doing well to group within a couple feet at 100 yards. They were considered 40-yard projectiles at best. But the Balle Blondeau allowed run-of-the-mill shotguns to put five shots in 6 inches at the same 100 yards. It was as if rifling had just been invented!

Balle Blondeau
The French Balle Blondeau shotgun slug stabilizes in flight by using drag — the same as a smoothbore pellet gun.

I asked the Chilkos if they knew about the Balle Blondeau and they said no. But Mike was an engineer who figured out the same basic aerodynamics independently, and then demonstrated them to all who attended that match!

Bob Chilco
Bob Chilco pumps his .398-caliber homemade underlever big bore airgun.

Did those bullets make a difference?
Within one month of that match, Gary Barnes was casting lead dumbbell-shaped bullets for his own big bore rifles. He called them spools, which is a very descriptive term. Until those bullets came along, shooters were happy to put five shots in 4 inches at 50 yards from a rifle. Several months after he started making his spools, Barnes was grouping better than that at 100 and eventually at 200 yards! So — yes — those bullets made a huge difference!

I’m not advocating this bullet shape as the only kind for big bore airguns. I’m not even saying they’re the best. With today’s rifles able to shoot faster and faster, we can stabilize conical bullets quite well. When it comes to penetration on game, a conical will out-perform these dumbbell-shaped bullets every time. But we learned a valuable lesson in aerodynamics from this bullet shape, and we would be foolish to ignore it.

Longer bullets must spin faster!
If shooters have learned anything in the past 50 years it’s the lesson that longer bullets have to spin faster to stabilize in flight — all things being equal. The American Army learned that lesson in Viet Nam, when our M16 rifles started out with a 1:12-inch twist (one rotation of the bullet for every 12 inches of bore travel). Their bullets were barely stable under ideal conditions, and they quickly lost stability in flight. While they did well at short ranges under 100 yards, you could not hit a man-sized target out at 250 yards and beyond, which put the American troops at a tremendous disadvantage.

The M16 twist-rate debacle came about from a too-short development cycle and a design that was rushed to production for political reasons. Had the developers been shooters with a knowledge of firearms history, they would have known that this path has been traveled before in the 1920s. But if anyone on the development team knew that, no one listened to them and the lesson had to be relearned at great expense.

There are 2 ways to spin a bullet faster. Either speed up the twist rate or increase the velocity. The length of the barrel has nothing to do with how fast the bullet spins, despite what you may have heard or read. A bullet that leaves the muzzle of a 1:12 twist rate barrel at 1,000 f.p.s. is spinning 1,000 times per second (1 turn every 12 inches), and the bullet is moving 1,000 feet (1,000 X 12 inches) per second. Increase the velocity to 2,000 f.p.s., and you increase the bullet spin rate to 2,000X per second. Or you can leave the velocity at 1,000 f.p.s. and change the twist rate to 1:6.

The bullet loses velocity fast as it moves downrange, but the spin doesn’t slow down at the same rate. Even when the bullet is 500 yards away from the muzzle, it’s still spinning fast enough to stabilize (assuming it was stabilized to begin with).

Big bore airguns are generally slow, so their spin rates are also slow. They don’t stabilize long bullets very well. When we test them for accuracy, this is what we discover.

Do you think these 68-grain bullets might be unstable? They have keyholed (gone through the target sideways) because they’re tumbling in flight. Even centerfire rifles that shoot over 3,000 f.p.s. like this .219 Donaldson Wasp need the correct twist rate to stabilize their bullets.

Shorter bullets need less spin to stabilize
If you don’t want to speed up the rifling twist rate, another way to improve accuracy is to shoot shorter bullets. Regardless of the caliber, a shorter bullet needs less spin to stabilize. So, big bore shooters need to avoid those bullets with long noses and concentrate on bullets with shorter noses.

I learned this lesson early on with big bores, but it never ceases to amaze me that other shooters are still discovering it for the first time. It’s reported in the literature time after time, yet some shooters continue to disregard what others have been reporting.

Both bullets are .458 caliber, but the one on the left needs to be spun a lot faster to stabilize it in flight.

Less contact with rifling reduces friction
A bullet doesn’t have to make contact with the bore along its entire body. With the right design, the bullet can have as few as 2 bands that engage the rifling, while the main body is slightly narrower and doesn’t touch anything. The dumbbell bullet at the beginning of this report is one example of that, but the difference in size doesn’t have to be that great. By making the main body of the bullet just a few thousandth of an inch narrower than the top of the rifling, you can get the same result and still have the bullet weight you need.

contact bands
The bands are the only part of the bullet that contact the rifling. This can be reduced to just the top and bottom bands by design.

You can take these lessons and apply them anywhere. For example a bullet that has a deep hollow point or a hollow base will be longer than a bullet of the same weight and caliber without those features. Since length needs a faster spin, shooters want to be careful of those types of bullets. That’s not to say that they can’t be used, but some thought needs to be given to their effect on stability in flight.

That’s a look at some of the most important things we know about big bore bullet design today. They’re simple lessons, but they have a profound impact.

Pyramyd Air Cup: October 24-26

Pyramyd Air Cup
Pyramyd Air has put together a 3-day shooting event called the Pyramyd Air Cup. There are so many airgun events that they couldn’t stuff them all into one day!

The festivities start on Friday, October 24, with an open practice range and the Payday Challenge. For just $5, you can shoot at targets at varying distances and kill-zones. The best shooter will take home $200. Not a bad return for a $5 investment!

On Saturday, October 25, head toward the sight-in range to make your last adjustments before you start the Silhouette Offhand Competition. Read more on this page (scroll down to find the Offhand section). In the afternoon, there’s a field target competition.

The last day of festivities is Sunday, October 26, and a field target match starts it off with a bang. That afternoon, the second silhouette match will be held, and it’s the Gunslinger Competition. Read more about it here.

AAFTA (American Airgun Field Target Association) rules will be followed for both field target matches.

After the competitions are over, there will be an awards ceremony and the big $1,000 Grand Prize will be awarded. Yes, you heard that right: 1,000 big ones!

Register online for the competitions, lunches and a dinner banquet held on the last day. You can attend all events or just a a few. The more you enter, the greater your chance of being the Grand Champion of the day. Even if you don’t win the Grand Prize, you can still win some super runner-up prizes.

There will be door prizes for anyone not participating in a shoot. Some of the items that will be given away are an AirForce Talon SS PCP rifle, a Hatsan Galatian QE PCP rifle, a Walther LGV spring-piston rifle and a Hawke Sport Optics Sidewinder 30 8-32×56 scope.

I’ll be there, too, as will airgun hunter Jim Chapman and Airgun Reporter Paul Capello. Come to shoot. Come to try out guns you’ve always wanted to shoot. Come to chat with Paul, Jim and me. Come to immerse yourself in the world of airguns.

Big bore bullets: Part 1

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.).

Back to airguns
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.