Subsonic or supersonic ammo?

[trg]

Yes, but does it all really matter with a handgun round? We know it does with a long thin projectile, but what about a short stubby one?

That is a good point and I have been thinking the same thing.

There is really only one way to find out.

Range test at 25, 50, 75 and 100yds with the following three loads:

1. 124gr. load that stays supersonic all the way (and it will do that )

2. 124gr. load that is transitioning at 50 yds or so.

3. 147gr. subsonic load

I do not have access to a Ransom rest so all shots will be fired from a sand bag rest.

If I should bet on the result I would say that there is a difference but shooter error will be even more and you cannot tell the groups from each other.

I will post the results here but I have to wait a couple of months until we start practicing at our outdoor range

[Allgoodhits]

People called me a nut job for thinking a few years ago but here it is:

Speed of sound at 65 degrees = 1122 fps

85 " " = 1143 fps

Light bullets loose velocity faster than heavy bullets. A 115 gr XTP loses about 125 fps from the muzzle out to 50 yards. A 124gr XTP loses about 85 fps at the same distance. So..a 115 with a muzzle velocity of 1150 will experience "transonic buffering" or adverse yaw very early.

At the time, I was shootnig NRA AP and was wrestling with shooting 115's or 124's. I went with 124's at 1050 fps for this reason. And EVERYONE said I had to shoot 115's to be competitive because of the decreased mover lead.

Then someone told me Doug K shoots 115's at about 1150 or close. I thought a velocity of 1100 was a better bet. I scratched my head and shot 124's since 115's at 1100 was a little too close. Doug K keeps winning so maybe it doesn't matter and I'm a nut job.

Hope this helps.

Imagine how good Doug could shoot if his bullets were not wobbling across the finish line!

MJ

[safenate]

Any results on the loads above for supersonic, transonic, & subsonic ammo? i'm curious of these results after the incredible accuracy of 115 FMJs at 100 yd with a muzzle vel of 1150ish. My MG JHP aren't that accurate. Didn't know if the HP had anything to do with it or ?

According to QuickTarget, the 115's will be supersonic at 100 yd if they leave the muzzle over 1400 fps (1450 would be safer for testing though). May try this with +P brass & PowerPistol some time. Would be pretty hot (164 PF @ 1425).

Edited July 31, 2008 by safenate

[fiddler]

Any updates?

[Mark Bellon]

Physics background here. I'm going to do a bit of hand waving over the math to summarize the results.

Perhaps people remember the "sound barrier"? We're so used to supersonic speeds now-a-days that we forget how hard it was to even get close to the sound barrier (in airplanes) and then exceed it. What happened? The airplanes shook themselves apart or the control surfaces began working in unusual ways. The old idea was that the compressibility of air was at it maximum - there was no way for it to get out of the way so one couldn't go faster than it. The maximum speed, the compressibility point, is the speed of sound.

There are many bullet factors - size, mass, deformations from the ideal/normative shape (dents?), overall shape, construction, rifling spin, air temperature, air moisture content, air friction, altitude (and more). Bullets are manufactured and have natural variations and this adds (more) to the unpredictability; so does powder charge variability and uniformity and brass factors (e.g. hardness, ductility). All of these affect a bullet's flight at any speed.

At any speed there are a non-trivial number of force components, some of which push the front/middle/back/side of the bullet in highly complex and somewhat chaotic ways. These smaller, highly variable and rapidly changing components usually (or largely) cancel out - they don't dominate the path of the bullet - at low speeds or high speeds.

At low speeds - not too close to the speed of sound - things largely behave (largely) as they do at common human speeds. Motion is not too complex and can generally be understood - things move (mostly or closely to) as expected. Work-arounds for problems are fairly easy to find and can be overcome fairly easily.

In the trans-sonic range the forces that affect the bullet are coupled with highly complex and/or chaotic air flow around the bullet - forces of air motion unique to the trans-sonic range. Complexity loves complexity, chaos loves chaos. You got it! Things add up or even multiply. Now the bullet is subject to highly complex or chaotic motions that are non-trivial in magnitude (the summation of (many of) the force components no longer (usually) cancel out) - oscillations (resonances) and/or random movements become major factors in how the bullet moves. Wobble, tumbles, apparent curves off trajectory and any number of (scientifically) cool but (shooting) unhappy things are much more likely to happen and often do.

Above the speed of sound things go (largely) free flowing again albeit with a somewhat different set of conditions (same rules, different conditions). Things behave in a non-complex (or lower complexity) manner again - one can make sense of it "simply".

This is why it is good to stay below the trans-sonic range or to go above it. Below things move smoothly and above it things move smoothly - it's easy (easier) to predict what's going to happen. In the trans-sonic range things are understood but the number of factors is more complex and the outcome is complex or chaotic - it doesn't go where you expect it.

One the things that does change dramatically below and above the speed of sound is air friction. This is why we see subsonic bullets fall off much faster than supersonic bullets. Above the speed of sound the air isn't ("sort of") as much of a frictional problem as below it.

Hope this helps,

mark

[goneracin]

Here is a cool pic that shows a bit of what happens to anything at the SS threshold. This was at an airshow, and the plane was going right under SS speeds, but very very close. You can see the standing waves around the wings and tail section of the plane. the bullet will see these same waves at speeds around the threshold.

In an answer to the problem, why not have 2 different loads, your short range, not quite as snappy load, and a few mags filled with hotter loads that may stay supersonic for the longer distance? Im not sure of the rules, so if this is not legal, sorry.

[Mark Bellon]

Way Cool!

In particular one can see the standing waves breaking and going chaotic as one scans from right to left. The "trailing edge" is often entirely embedded in chaotic air flow. Sometimes the waves couple with bullet - which has it's own issues - and the resonance can do amazing things. I've seen the math and in simulations it is jaw dropping just what can happen.

Thanks!

mark