Bullet stabilizing, accuracy after dropping below supersonic

[jeener]

Hi guys,

Another theoretical here just for the learning...

Over very long range a bullet which is supersonic slows below supersonic and goes through a bit of turbulence (transonic?)... then continues.

Two questions:

1) After the transonic stage does the bullet stabilize again and remain accurate along that new path? Even if the new path is different after the turbulence?

2) Is there any repeatable accuracy after dropping below subsonic or does this just fling the bullet off wherever?

[cavok]

A projectile travelling at 1125f/s at 68 degrees at sea level is at the speed of sound, hence eg: a rifle projectile starts it's journey @ 2750f/s at some distance goes sub sonic. By the time it reaches 1125f/s it would be nearing its dying stages, however in my experience the trajectory/flight path drops but there is no instability. Hence all the projectile will do is drip as a result of gravity, it's flight path remains same. Tis can be seen with pistol projectiles which with mine begin at 1350f/s and shooting at 100/200 metres the hit the bulls eye, or an area of 6" square, point of aim, all they do is drop 2 - 3" or more 6" @ 200.

[jeener]

I was thinking of a lot further than 200m pistol with my question.

e.g. 1,500m rifle or however far so the bullet has enough distance to destabilize then still have a few hundred metres to travel to test change in path and decline of accuracy.

Any ideas on this?

[cavok]

I Have only shot rifle to 500 metres, and I will stay with what I wrote, the transmission from super sonic to sub sonic SHOULD NOT destabilise the projectile from a high calibre streamline case. Hence even the .50cal rifles shoot 1.5 km, by the time they are reaching this distance they have lost a lot off grunt, I'm sure there are charts somewhere, that show f/s at various distances, accuracy I do not believe they loss, till near the end. As always I also would love to get facts from paper, proof positive required.

[Manimal]

Lots of stuff on trajectory over distance when looking at stuff like the .50 cal and 1,500 metre shooting. Nothing like accuracy in MOA over distance as far as I can find.

I was hoping there might be chart done by someone where you could see the accuracy change significantly at/after the transonic point. That's what you'd expect on paper anyway.

Couldn't find anything I'd call evidence

[cavok]

Possibly because a projectile does not loose accuracy when it goes from super sonic, or speed of sound and higher to sub sonic, under 1125f/s.

[cruze82]

I think you will find it depend on the projectile you are using some handle the transonic transition much better than others
I have had a lot of luck with some Berger projectiles that are made for the job

If you find a projectile that will go transonic well the results will be predictable and repeatable

[Jessie]

the transmission from super sonic to sub sonic SHOULD NOT destabilise the projectile from a high calibre streamline case

I realise we're just doing this 'on paper' but I'm curious how you got to that conclusion.

It's straight physics that going from super sonic, through transonic and into subsonic there will be some turbulence, instability, whatever you want to call it. I can't think how that can be anything other than a negative?

Are you suggesting with a high calibre streamline case it should buck the turbulence evenly so there is effectively no change in the projectile flight?

What about a low calibre bullet then?

Less streamline case?

[cavok]

Ok, I'll play. My pistol, will stay with this as I only shoot .223 a few times a year, usually at 500 metres.

My pistol, the projectile has been very often chrono at 1350f/s. This is a 9mm, projectile, not copper wash. Hawksburry river, 125 conical. It is fired at the metallic silhouette target, at 200 yards, Pig.

By the time my projectile gets to the 200 yards mark it is travelling I am informed at somewhere near 1000 f/s. It projectile hits bang on, so accuracy is maintained.

No I have no proof, yes any object decelerating from super sonic to sub sonic creates turbulence, that is why on all supersonic planes they have a very pointed shape to cause the least amount of turbulence when going into the speed of sound.

However gradual deceleration does not cause more than a ripple.

[Timb0]

I think you will find a .50BMG 750gr match projectile @2800fps will stay supersonic beyond 1500m.

[cavok]

I think you will find a .50BMG 750gr match projectile @2800fps will stay supersonic beyond 1500m.

Hence what happens then, I goes where, I'm talking about the projectile, it misses it's target, or as it is slowly decelerating very slowly in real term it hits its mark, bullseye. You will find the .50 goes to over 2500 metres. So at that distance, it hits.

[Warrigul]

Ok, I'll play...

To be honest you probably would not notice the difference in accuracy at short little distances and on those sort of targets. It is a proven fact that at 1000 with a 155 grain old style dyer or sierra .308(I haven't shot at 1000 with the newer HBC Dyers) you still need to be supersonic if you wish to get the best possible group.

With my Omark to increase barrel, and my shoulder, life I ran slower loads out to 600m as they still stayed supersonic and loaded hard for the longer distances.

A couple of years ago my groups opened up randomly at 1000, at first I thought it was me then after I checked a few other things I finally did what I should have in the first place when changing powder and chronographed the load, it was 60fps(avg) slower at the muzzle than when last checked, it seems that a change of powder batch resulted in a slower load, increasing velocity resulted in the groups tightening back up. Were the sound barrier not having an effect on accuracy only elevation would have changed (yes there would have been an infinitesimal difference in windage to go with the lower velocity but it wouldn't have been much).

There is always turbulence, the .22 benchrest rifle guys know this hence the reason they use standard or subsonic velocity loads. There is insufficient barrel length in most pistols to make the full use of a high velocity .22lr so most of them are standard velocity by default, even my TC with its 10" barrel doesn't quite get there so I just use standard velocity. If using a bloop tube is enough to be worth doing for them then avoiding the sound barrier is definately up there on the to do list. (I respect what they do but am not anally retentive enough to shoot benchrest .22 anymore).

[Timb0]

I don't know that much about this as I usually try to avoid shooting beyond the piont when a projectile goes trasnsonic.

However some people say transonic effects can start at 1200fps ( before the speed of sound is reached).

The center of pressure shifts forward when a bullet goes transonic and can cause some bullets to basically get the death wobbles.

I didn't have much luck with .308 168gr SMK's going transonic, they left oval holes and larger groups. The same load out of a longer barrelled .308 didnt have the same problem. Both rifles shot similar 200yds back. So I chaged to a higher BC bullet that stayed supersonic longer.

I have heard some bullets have been designed to go through transonic flight and not destabilise.

[Warrigul]

Rule of thumb is each inch is worth 30fps out to 30" or 30fps less for each inch less.

[Timb0]

Yes, there was almost 200fps difference between the rifles. Enough for the shorter barrel to go transonic at around 750-800 yards

[Jack V]

Generally, Yes and Yes.

How well a bullet gets through the trans sonic zone is governed by many things but if it's stable going in then it will settle again coming out.

It is possible for the trajectory to divert slightly going through the zone, cross winds are a factor also. You can see this loss of stability going through the zone in the BC changes in ballistic calculations. it lowers in the zone.

Bullets tend to shed a fair bit extra velocity after the zone and the trajectory does tend to plunge somewhat but high BC bullets work better than low BC ones.

Hits can still be made after the trans sonic zone, snipers do it all the time.

[inventurkey]

I didn't have much luck with .308 168gr SMK's going transonic, they left oval holes and larger groups. The same load out of a longer barrelled .308 didnt have the same problem.

Too much velocity out of the shorter barrel then causing them to tumble?

And the longer barrel slowed it down enough I guess?

[Jack V]

It's a huge subject and the speed of sound changes depending on altitude and temperature . Rifling twist , barrel condition , load velocity , can all play a part in wether a bullet stabilises properly or not . If the barrel is starting to wear out then long boat tail bullets can start to keyhole . If twist rate is too low dito. If load pressure is too low dito. Also if velocity is too high , barrel condition too rough and jacket material to thin a bullet can melt the core or tear the jacket causing tumbling or blow up . For long range accuracy you do need good barrels in top condition , of the correct twist for the bullet length and drive them fairly hard to stay supersonic as far as possible. Moly coating helps to reduce jacket deformation and tearing from the rifling engraving although it takes a few grains more powder to maintain status quo velocity . First thing is you need to check is that the twist rate is adequate for the bullet . Modelling on JBM ballistics site should tell you that .

[Timb0]

I didn't have much luck with .308 168gr SMK's going transonic, they left oval holes and larger groups. The same load out of a longer barrelled .308 didnt have the same problem.

Too much velocity out of the shorter barrel then causing them to tumble?

And the longer barrel slowed it down enough I guess?

What I mean is the longer barrel was giving higher velocities and the projectiles were not destabilising due to going trasnsonic as the others were.
The shorter barrel was giving much better groups at shorter ranges (600yrds) as I developed that load for that rifle.

[Jack V]

Comparing two barrels is impossible without all the specs of both barrels and some indication of wear on each . Even a perfect barrel wears out eventually and starts key holing bullets . Short barrels tend to be stiffer for their length because of the better ratio of diameter to length . This can aid grouping at shorter ranges but velocity loss can reduce it at longer ranges but sometimes it's not so bad. I figure that for hunting at longer ranges you want all the residual velocity you can get as long as accuracy is still good. No point in missing with higher velocity . Generally to get the long heavy high BC bullets moving fast you need slow burning powders so that also means longer barrels to use that powder effectively or it just burns in the air .

[inventurkey]

What I mean is the longer barrel was giving higher velocities and the projectiles were not destabilising due to going trasnsonic as the others were.

I understand. I had the wrong line of thinking to start.