Big bore bullets: Part 2
by Tom Gaylord, The Godfather of Airguns™
Writing as B.B. Pelletier
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
• 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!
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!
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!
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.
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 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.