The need for speed.
With traditional firearms, the faster that bullet flies, the harder it hits.
It’s a simple calculation of kinetic energy.
An object doubling its speed has four times as much kinetic energy aka “SMACK”.
A good effect of speed is the trajectory is flatter.
It gets there sooner so there is less time for gravity to work its magic. That and less time in the air over a given distance means the object is not so affected by wind drift.
Woo Hoo, Pile on the muzzle velocity!
Except for air weapons.
Approaching the speed of sound (round 1100 fps) a pressure wave builds up called the sound barrier. Once the pellet crosses that the turbulence of the sonic boom totally destabilizes the common diabolo (waisted) shaped pellet and as a result, most barn doors breath a sigh of relief as you probably won’t hit them at range.
But why should it destabilize? What’s going on?
The diabola (pinched waist) style pellet is designed like the old shuttle cocks of badminton.
(Number 7 is a slug and I’ll explain later how useless that is in most air weapons.)
Yet does it need that spin? Does a shuttle cock?
I’ve played with expert badminton players who can “whack” the shuttle cock within an inch of a given mark, every time and they don’t need “spin”. The same is true of diabolo air pellets. It helps, as does all spin stabilization, BUT unlike a traditional bullet, the spin rate is pretty lame.
So fast is good just keep it out of the range of supersonic i.e, under 900 fps is usually considered safe.
OK, if speed is the worry, let’s go for weight.
And a lot of high power air rifle owners do just that.
Problem is the diabolo shape sort of maxes out at round 30 grains.
Thus 900 fps and 30 grains (assuming little drag) gives you approximately 54 ft lbs of smack!
OK, change to slugs. Bullet shapes, like the 22lr.
Now we can generate WEIGHT, a solid blob of lead!
A number of problems come into play now.
First and foremost is you lose stabilization as few air gun barrels can spin a slug fast enough to achieve the optimal rotation speed.
How is spin speed calculated?
Bullet spin rate (RPM) is found by the (muzzle velocity X 720)/ Twist rate in inches.
Wow, that one I had to look up from an accurate shooter website as I haven’t thought in these terms for years..
Anyway some experts say that the usual rpm is around 150,000 rpm to properly stabilize a bullet over average ranges. The ideal rate for spin stabilizing a diabola pellet is found using a twist of somewhere between 1: 25 and 1:16. That gives a RPM of 26,920 to 40,500 rpm
So will a firearm shaped bullet be stabilized at those rotational speeds? Nope, not a cat in hells chance.
Without the drag of that flared skirt it will go all tumbly and unstable in a very short distance.
As for achieving that spin i.e. the rifling?
Nearly all air weapons use a sort of micro rifling. They don’t need more than that as the classic “skirt” design lends itself to a more gentle grip on the pellet.
Not so a bullet aka slug, lead or not!
It needs a really coarse “thread” to engage with.
The problem is the more power you apply, the less the micro rifling works and theoretically all it does is shave the slug down a few thousands of an inch.
You end up with a VERY lead fouled barrel and every bunny in the world feels safe.
Whilst firearms is all about DAMAGE and KINETIC and HYDRO-SHOCK, they have an excess of power to employ to achieve accuracy and smack down.
With an air gun it’s all about pellet placement i.e. putting the shot EXACTLY where it is going to do the most damage. For that you need a CALM, smooth discharge, and a well designed stable pellet working fast but no where near the speed of sound. Finally never use a damaged (deformed) pellet. It’s the shape that gives it stability!
A word about synthetic shrouded pellets and sabot rounds.
A sabot round starts off leaving the barrel and the “skirt” drops away leaving the spin stabilized core to fly on. A shrouded type uses a plastic “slug format” with a heavy core.
Usually lighter than lead, they boast ultra high velocities. To some that sounds brilliant and as explained above is a total waste of time.
So my personal advice is don’t waste your time upping the energy to it’s max plus, pushing for more velocity, and playing with slugs in air guns. As a minimum fit an air stripper or shoot with a moderator to stop the air blast of the excess air tumbling the pellet. That an concentrate on accuracy.
Quality of shot as opposed to a high cyclic rate.
One shot, one kill is the norm for air gun hunters with spray and pray for shotguns and safety shots the norm for most “gunners”.
Think about that as an air gunner goes out with 500 rounds in a little tin and you need a day pack to carry the same and all those magazines.