What is accuracy?: Part 1
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- The big question
- Epic experiments!
- Shooting Gibraltar
- Use the best barrels
- Rotate the barrel
- Damage the bullets
- Are bullets balanced?
- Did Mann learn anything?
Chris USA suggested today’s topic last Friday, when he asked my about the tantalizing results I got from the Webley Mark II Service air rifle. Among his comments, he said this, “Perhaps the biggest thing to consider,… and maybe more accurate,…. it is just plain LUCK. Maybe, it is just the plain and simple fact as to how the pellets flew and just happened to be where they landed. So yea,… I would be interested hear your thoughts on what would appear to be sporadic anomalies.”
Among the “sporadic anomalies” he referred to was the last group, where 3 shots went into a tight cloverleaf and the other 2 were wild. What causes a rifle to do that — to put several shots so close together and then throw the others far away?
This is the target Chris referred to. Why are 3 pellets so close and the other 2 so far away?
The big question
I told Chris he had just asked the big question — the one people have been discussing since the firearm was invented. It all boils down to the title of today’s report, What is accuracy?. I told Chris that Dr. Franklin W. Mann spent 37 years of his life experimenting to discover the answer. He wrote a book about his findings titled, The Bullet’s Flight from Powder to Target, whose subtitle tells more about what is inside — A STUDY OF RIFLE SHOOTING WITH THE PERSONAL ELEMENT EXCLUDED, DISCLOSING THE CAUSE OF THE ERROR AT TARGET. This book was first published in 1909 and contains detailed descriptions of the fascinating experiments that Dr. Mann conducted to test his many theories.
I would suggest a different title for this book — Why don’t all bullets go through the same hole? That is the underlying question we all have and it’s what keeps our sport so fascinating.
Dr. Mann spent a fortune testing every concept he could dream up to find why bullets did not all go to the same place. One notable one was his benchrest.
I’ve been told hundreds of times that I need to test airguns in a vise. I “just don’t get it” that a vise is the only way you can be certain the gun is completely out of human hands (true) and can therefore be most accurate (doubtful). Dr. Mann built such a contraption. It was a 3,500-lb concrete pillar he called the “Shooting Gibraltar.” It stood 26-inches above ground and extended 40 inches below ground to rest on gravel that showed no sign of being disturbed with the last glaciation. On top of this he installed his v-rest and machine rest to hold his barreled actions in complete rigidity. He then constructed a 100-yard fabric tunnel from the bench to the target, so his bullets flew in dead-calm air. This tunnel was curved to allow for the drop of the bullet along the flight path.
Mann shot through a 100-yard long fabric tunnel to cancel the wind.
Surely what he did exceeds the requests of even the most particular shooter. And what did he discover? That shooting from a rigid vise made very little difference. His bullets still struck the target with separation that was random and inconsistent. I try to explain that to readers, but if they haven’t read Mann they don’t understand this has all been thoroughly tested before.
Use the best barrels
Franklin Man was a contemporary of Harry Pope, who is acknowledged to be one of the finest barrel makers of all time. Pope made many barrels for Mann, including specialty barrels that were one-of-a-kind masterpieces. One such barrel had 8 screw holes drilled and tapped into the side of the barrel at the muzzle that went all the way into the bore. Screws were put in each hole and then removed to test the effects of exhausting gas before the bullet exited the muzzle. What is special about this barrel is the fact that Pope built it so the holes were exactly between the rifling lands and had no burrs on their inner surface. He bored and threaded them before he reamed and rifled the barrel. They were just holes that in no way damaged the bullet — until Mann wanted to! Pope also provided him with 8 screws that had points, so Mann could purposely distort the bullet before it left the muzzle! He tried numerous experiments with this rig.
Rotate the barrel
Mann also built a special circular action, so he could rotate the barrel in his rest. He then tested it in 4 different angles (0, 90, 180, 270 degrees) of rotation, to see where the bullets landed on target. His best barrel put 4 bullets in 0.56-inches at 100 yards. The worst barrel put 4 bullets into 16 inches at 100 yards. I have been telling people for years that barrels are never drilled straight. Dr. Mann proved it with this test.
Four shots from his straightest Pope barrel at 90 degrees rotation with each shot produced this 0.56-inch group at 100 yards.
Damage the bullets
Mann also experimented with bullets that were intentionally damaged. He did hundreds of experiments, but they all come down to this — the nose of the bullet is not critical to accuracy. The base is! When he shot bullets with damaged noses, they grouped as tight as did the same bullets undamaged, but bullets with damaged bases became very erratic.
Are bullets balanced?
We worry about the head size and the weight of the pellets we shoot. Dr. Mann worried about the balance of his bullets. To test that he built a machine to spin the bullet up to high speed so it would balance like a top inside a glass Petri dish. Lead bullets were impossible to spin up to speed, so he machined steel bullets. At this point he was testing not the stability of cast bullets but the inherent stability of the bullet’s shape and design. He then photographed the bullet as it was spinning in a darkened room for several minutes. He left the shutter open for up to two minutes to see if any wobble was detectable.
Each steel bullet is photographed for two straight minutes in a dark room while spinning on its nose. These bullets show remarkable stability.
But he also actual photographed lead bullets that were spun to a slower number of revolutions per minute. With these he discovered inherent instability that could be seen on paper targets when the bullets passed through.
These two spinning lead bullets from a certain mold show that one bullet (left) is more stable than the other when both were cast in the same mold.
Did Mann learn anything?
Dr. Mann’s experiments were gathered over a lifetime of work. The question is — was anything learned? And the answer is yes, a number of things were learned from these experiments.
1. No barrel is ever drilled perfectly straight
2. Shooting in a vise does not increase accuracy.
3. Damage to a bullet’s base affects accuracy negatively.
4. Accuracy is as dependent on each bullet as upon any other factor.
What Mann did not learn, however, was the single set of rules that guarantees accuracy. He learned that in a batch of barrels all made to the same specification, some will be more accurate than others. He learned that there are imponderables that cannot be named that affect accuracy. In short, more research remains.
This is Part 1 because there will be a Part 2. Some of the material in Part 2 will cover some of the same ground Part 1 has covered, but there will be new material, as well. If there is room I will address airgun accuracy concerns in Part 2. If not, there will be a Part 3.