What makes a pellet accurate?
by B.B. Pelletier
A question came in to Pyramyd Air's owner a few days ago. "What's the fastest speed for the most accurate pellet I can shoot?"
I say in my About ME Blogger profile that there are no stupid questions, but there ARE situations where the person doesn't know what he has asked. Therefore, an answer is either difficult to give or he wouldn't understand it when he hears it. This question is one of those.
The things not asked
A few parameters were missing from this question.
Caliber of airgun in question
Range at which accuracy is desired
Powerplant selected for shooting
Have the pellets been sorted by weight?
Pellet shape (wadcutter, domed, pointed, novelty shape). Or, does the question also include solid "pellets" that are really bullets, and do not perform the same under similar aerodynamic situations?
All these things affect accuracy. Velocity, itself, is a variable for most of them. Without knowing it, this person has asked a question that is impossible to answer without a lecture on pellets, velocity and all the variables that affect pellet flight. In fact, this question requires a book to give a complete answer; even this blog is not enough. So let me take a very small slice of the question and try to explain the variables.
For a .177 domed diabolo pellet, what is the maximum velocity possible for accuracy at a given range of 50 yards? I ask that question because it seems to be the No. 1 concern of readers. I excluded hollowpoints because, though they may group acceptably at 25 yards, by the time they go 50 yards, they're hopelessly inaccurate.
The magic of 900 f.p.s.
No doubt you know that diabolo pellets destabilize when they get up to transonic velocities. Transonic speed is defined as a speed just below and just above the speed of sound. That's Mach 0.8 to Mach 1.2, or about 880 to 1320 f.p.s., if you accept 1100 f.p.s. as the speed of sound. Because the sound barrier is a variable itself, it is impossible to give an exact speed that will always be correct.
Look at the lower velocity - 880 f.p.s. That's the lower limit of transonic flight. Transonic flight is a speed at which the airflow over the body of a projectile is moving at supersonic speeds at some places and subsonic speeds at others. A barrier wave is created at transonic speed, and that wave causes flutter on a shape not designed to handle it. That flutter causes the projectile to wobble, which causes destabilization. Like it or not, the transonic range is the worst range at which to shoot a diabolo pellet. And because of its shape, a diabolo pellet slows down so rapidly when accelerated to supersonic speed that it's into the transonic region soon after leaving the muzzle - even when the muzzle velocity exceeds 1400 f.p.s.! So, shooting faster than 900 f.p.s., is bad with domed diabolos all the time, and worse on days when the sound barrier is below 1100 f.p.s., because the pellet remains transonic for a longer period of time.
Food for thought
If you're with me to this point, here is how changing one other variable changes the entire question. Substitute a solid pellet - a bullet - for a domed diabolo and everything I have said changes. A solid bullet can go faster than the sound barrier without a problem. In fact, the new Piledriver pellets are shaped correctly to do just that. Their boattail base and rebated skirt lower drag significantly. Theoretically, solid pellets could be accurate at supersonic speeds.
This is the .177 caliber Piledriver pellet, looking at the tail end. It weighs 20.7 grains. This pellet is too heavy and too low-drag to properly stabilize at normal air rifle speeds, and no commonly available rifle can shoot it fast enough for accuracy at long range.
HOWEVER - and this is a BIG however - no pellet rifle currently made has a twist rate fast enough to properly stabilize a Piledriver pellet at the velocities it can be shot, so at 50 yards the accuracy isn't there! Unless the Piledriver is accelerated to around 1100 f.p.s., it will wobble on its axis and the groups will open, the farther it moves from the muzzle. Shooting this "pellet" at 600-800 f.p.s. would be futile, if the goal is accuracy at 50 yards. The proof of this, if you don't have any Piledrivers to shoot, is to try to group a .22 CB cap at 50 yards. The 29-grain lead bullet that groups well at 25 yards will spread out at 50 yards, because the standard 1:16" twist of a .22 rimfire barrel (which is identical to the twist rate of most airgun barrels) is too slow to stabilize a 29-grain bullet when it moves that slow.
Like I said at the start of this post, some questions ask more than was intended. This has been an attempt to explain a portion of the answer to the question we looked at here.