What do we really know about the transonic zone?

[Distant Thunder]

Or think we know even?

The purpose of opening up this discussion is to lay out what is known or thought to be know on the subject and hopefully add to the understanding among BPCR and BPTR shooters with regards to the TRANSONIC ZONE (1346 to 897 fps) and bullet "flight".

I certainly hope I am not the only one but I do spend a fair mount of time about what happens to our plain lead bullets as they travel from the muzzle to the target. Now I am NOT a ballistic scientist or engineer or anything close, I'm a retired tool and die maker, so anything I say has to be seen in that light and I welcome other thoughts and corrections from those who have a better understanding of the subject.

I also welcome any information or thoughts from other shooters who like me have read or in some other way have come upon information that may be useful in any way. Please do be afraid to chime in and add to the discussion. Please.

Ok, so a few of the basics as I understand them. BPCR has two very limiting components, those being the plain lead bullets and black powder. I also think we can add to the mix the use of rather large caliber rifles, at least by todays standards. By large caliber I mean .38 to .50 caliber, though the lower end of the spectrum may be in less common use it still has a place and must be considered.

Even in the smaller calibers bullets of 400 or grains more are generally considered necessary to carry to 500 yards and beyond. With rifles in the 12 to 15 pound range launching bullets at over 1450 fps, while possible, will bring heavier recoil which can be detrimental over the number of shots required for most matches. Shooting 45 to 60+ rounds of 550 grain bullets over 120 grains of black powder and do it with accuracy is not something many of can do or would want to attempt.

Black powder is a very limiting propellent with the upper limit 1500 fps pretty much. I don't think there are many shooter using loads on a regular basis that reach the 1500 fps mark. There are always exceptions and for this discussion I would rather stay with that which is in common use and that I think would be bullets of 400 to 550 grains at velocities of 1200 to 1400 fps.

A couple of examples of velocities for two of the most commonly used calibers would be as follows:

A .45 caliber bullet weighing 550 grains launched at 1400 fps would enter the transonic zone at about 50 yards, would hit the speed of sound (1126 fps) at about 300 yards, and would drop below the transonic zone at about 800 yards.

A .40 caliber bullet weighing 410 grains launched at 1400 fps would enter the transonic zone at about 50 yards, would hit the speed of sound at about 300 yards, and would drop below the transonic zone at about 800 yards.

I know there will be some variations in velocity, weight and BC from bullet to bullet and cartridge to cartridge, but it's pretty clear that there isn't much difference in the two calibers at the same velocity. Both are spending the majority of their flight in the transonic zone.

Modern high power shooters do all they can to avoid transonic flight and with good reason, there are many unknow or at least not understood phenomena that lurk in the transonic zone and they can destroy bullet stability and with it any resemblance to accuracy.

The only reason the BPCR shooter plays within the transonic zone is because he has to, it can not be avoided with our BPCR sports.

It seems to be hard to find much "good" information about shooting in the transonic zone by searching the internet. What I have found in searching is information about transonic aircraft flight and that is certainly because more studies have been done on that. Unlike transonic shooting which everyone in the right mind avoids, transonic flight has to be dealt with if only briefly as an aircraft accelerates through it as quickly as it can.

The whole effort in transonic flight is to avoid the sonic shock wave produced by the aircraft. That sonic wave can cause destructive vibrations and unstable flight which apparently can be very negative for not just the aircraft but also the pilot and passengers. The problem with that information is that it always deals with an aircraft that is accelerating through the transonic zone whereas our bullets are decelerating through the zone. So the events are reversed.

The sonic wave, which can be photographed and videoed, actually is slower than our bullets at the muzzle and lags behind it, but unlike our bullets the sonic wave does not slow down. As a bullet does slow down the sonic wave catches up to it first coming in contact with it at the base. As a bullet slows even more the sonic wave, in relation to the bullet, moves forward and across the length of the bullet from the base toward the tip.

I'm guessing that while the sonic wave and the bullet are in contact with each other that the wave has some serious effect on the bullet just as it does with an aircraft and that is when it would have the most destabilizing effect.

What I don't know is at what point during a bullet's flight and while it is near or in contact with the sonic wave does the wave has the greatest effect on the bullet's stability? Is there a specific velocity where the wave affects the bullet the most?

Is it at the point where the shock wave is affecting the bullet to a higher degree that we some times have trouble with and otherwise accurate load. I think we have all either heard of or have experienced problems at 900 yards with a load that seems to shoot better at 800 and 1000.

That is the discussion we had briefly at the end of the Wisconsin Rapids gong match yesterday. I know that for years I struggled at 900 yards but had very good scores at 800 and 1000 yards with the same load in my .45-70. I fixed that problem by switching over to paper patch bullets!

I don't believe for a minute that it was paper patch bullets that fixed the problem, well okay maybe a little I believe that, but rather that it was a change in the velocity with the .45-70 brought about by an increase in powder capacity using the shallow seated bore diameter paper patch bullets.

Could it be that with grease groove bullets seated necessarily deep in the case that my bullets were at the velocity that was most affected by the sonic wave at or just before hitting the 900 yard mark? Then by switching to paper patch bullets and upping my powder charge and velocity that I moved the specific velocity where the problem was to some place in the flight where the target wasn't?

Looking at the velocity charts it seems that most bullets are just nearing the bottom end of the transonic zone just after 600 yards to just before 1000 yards. At that time in flight the sonic wave would be either somewhere on the tip of the bullet or just in front of the bullet. That would be a destabilizing force on the nose of the bullet that could make a difference.

Any thoughts or idea on this subject?