Transonic and BC at muzzle versus mid flight

Few corrections, its Diabolo not Diablo

Trans sonic speed / speed of sound changes a bit based on the temperature, so if you're out shooting in the winter, it'll be much lower than what you experience in the summer. For almost every degree of temperature change, the speed of sound increases roughly 1 fps.

Speed of sound is referred to as Mach1. Trans-sonic speed is generally seen as between .8-1.2, many variables effect this. Critical Mach numbers can be influenced by shape of projectile, or aerodynamically speaking, the flow around said object. Which is why (I think?) slugs are more stable at higher velocities than diabolo pellets, likewise wadcutters are far less stable than diabolo.

There was a recent video of a gent doing very well with Diabolo pellets at 1000 fps, both at 50y and 100y.

I can't 100% give a good range that covers ALL diabolo pellets to which they experience unstable flight, but, staying below 1050 fps on most pellets is optimal, where as lighter pellets that may be 950.

In terms of actually exceeding a specific projectiles subsonic speed and entering into the trans sonic region momentarily without any adverse effects, only guy I can think of that will give a very appropriate response would be @Ballisticboy, and heck, he might correct me on something mentioned above in my post, to which I welcome, always love learning from the likes of him.

-Matt

june and july issues of hard air magazine have some good info on this

All the work on finding ideal speeds for reducing errors at the target tend to be based on the muzzle velocity, not the average speed to the target. Bob Sterne has shown that for pellets, the effects of cross winds tend to be a minimum at around 850-900 ft/sec. For slugs, based on the modern reference drag laws, the speeds are higher, around 1000 ft/sec. These numbers are only approximate, as the exact speeds will depend on the projectiles shape. The other thing to remember is that the error at the target is affected mostly by what happens in the first part of the flight due to simple geometric vector changes, unless something happens later on to change the projectile behaviour.

As speeds approach Mach 1 (1116.5 ft/sec in ICAO standard atmosphere) pellets actually seem to get more stable due to the aerodynamic centre moving forward towards the CG until at some point the pellet has infinite gyroscopic stability. This is in reality very bad as the pellet cannot change its orientation and will thus have an aerodynamic side force in one direction which will change its direction of flight and thus give large errors. Speeds slightly below Mach 1 will still give stabilities which are too high, making the pellets sluggish to change orientation, particularly as they slow down making the ratio between spin speed and forward velocity worse. The sluggish behaviour of the pellets will eventually lead to spiralling if ranges are long enough.

Slugs will be the same, it is just that they will lose less forward velocity and their spirals are a lot smaller. All high spin rate projectiles spiral, it is known as heave and swerve. If you have dynamic stability the spirals will get smaller, but if you have dynamic instability as many small projectiles seem to have, then the spirals can get bigger if the gyroscopic stability gets big enough giving long yaw wave lengths. Pellets never become gyroscopically unstable down range, in common with every other spin stabilized projectile.

So in a nutshell, to answer OP if I am reading correctly, the velocity of your projectile in the first 10-15 yards, or more importantly at the muzzle, is paramount in maintaining ballistic integrity through the common target range between 50-100 yards, thus pushing velocities towards Mach 1 will result in undesirable errors presented at the target, however the exact velocity at which errors begin to increase can vastly change based on projectile shape.


-Matt

Velocity can identify as trans if it wants.