Am very interested myself, for my 200-yard 38-55 loads.

But I can tell you with considerable experience that going sub-sonic w/ my flintlock 'roundball' loads really disrupts the load's inherent accuracy, i.e., versus the accuracy as observed at the shorter ranges where still super-sonic.

Transonic turbulence was something the 'old Timers' knew nothing about as it wasn't discovered until the 1940's when aircraft approached supersonic capability.
Scientists knew there were problems when aircraft went out of control, suffered severe vibration or disintegrated in flight.

This aerodynamic effect has an influence on bullets fired from rifles especially those fired by the likes of ASSRA members which are in many cases supersonic when leaving the barrel but enter the transitional velocity between supersonic and subsonic flight downrange. The movement of shockwaves backwards along the bullet sides as it decelerates causes cavitation with grease groove bullets on every single grease groove. It appears that this has much less effect on smooth sided bullets as used by the 
paper patched bullets weight for weight if of exactly the same bullet shape. Cavitation is also caused by the flat or hollow base that also causes deceleration, hence the use of boat-tailed bullets in more modern rounds. What we really need is another Dr Mann with enough money to have a supersonic wind tunnel and the necessary expertise to study this problem in great detail and write about it afterwards. Any rich volunteers. 

Harry.

I think there are those who let it get into their heads to much and it can become detrimental to their overall performance. In looking at the groups out to hundred yards shot by John D. Kelly and others back in the day it did not seem to have any ill effects with their overall performance. There are still those today who shoot in the 1250fps muzzle velocity range out to 200 yds. and have set several National Records by doing so, one being Bernie Connon from Cottage Grove Oregon. 

JLouis

I have no personal expertise nor any basic knowledge in regards to 1000yd. competition so I can't go there.

JLouis

The transition through the transonic range doesn't mean that the bullet will be inaccurate but, with pitch and yaw, it will kill the ballistic coefficient during that time and have more overall wind drift than a fully stabilize bullet that starts and ends on either side of that range. 

If the bullet has enough rotational stibility after going through the transonic range, it will indeed pick up the wind drift advantage of subsonic flight, here the long range guys get the benefit.

Frank

Some interesting background...

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Interesting but bring it down to the practical level

32/40's can stay supersonic to 200 yards just. Most go sub sonic around target impact. How much and when depends on a lot. Bullet Weight Powder Charge Temperature Elevation and is the target actually 200 yards away.  Bullet going sub sonic right at impact has little effect. Even if not stable distance to impact is so short it's not important. I will bump my powder charge if I see oblong holes, but don't need to. just feel better if they are round.

38/55's with plain base bullets can't stay supersonic to 200 yards. If you start them sub sonic they are too slow for 200 yard shooting.

Longer than 200 yards no cast bullet is going to stay sub sonic.

Rim fires most think sub sonic all the way is more accurate.

So the only choice is do you use a 38/55 or 32/40 for 200 yard shooting.  Personally I don't think going under the sound barrier just before impact is a negative.  I do think if the recoil is managed with a heavy rifle 38/55 is the better offhand 200 yard cartridge.  32/40 is the one to use if the rifle is lighter and recoil is an issue. Wind the 38 has the advantage. It's a toss up and I use both. Get a rifle and condition one way or the other will favor one over the other.

And I am sure sub sonic rim fires are more accurate than supersonic.

One mans opinion take it or leave it.

Boats