What is wrong?

[gbflyer]

Heat should not appreciably effect a properly stress relieved barrel. If heat is causing vertical, your choices are to hope the first couple rounds count, or get a new barrel from a quality manufacturer. It’s a frustrating thing if you are shooting groups, competition, or sustained fire at varmints. Not a huge deal for game, usually only need one or two from a cold bore.

[CFJunkie]

prdatr,

The following might help clarifying what I was referring to.
I submitted this text in another thread several years ago in response to a similar question ion an attempt to explain the process.

"If you're referring to exit time tuning, then I can provide some information.

The bullet exit time is the time it takes for a bullet to proceed down the barrel and exit the rifling.
It is both powder charge and barrel length dependent because those two factors along with bullet weight determine how far and fast the bullet moves down the barrel.

When the round fires, there is a shock wave that tries to expand the barrel and it proceeds down the barrel towards the muzzle and then reflects back to the chamber. It does that again and again until the bullet exits.
The concept is that barrel steel has a reflectionspeed that is far faster than a bullet can progress down the barrel and leave the muzzle.
The reflectionspeed is dependent on barrel length too, but it is also dependent on the amount of carbon in the steel.
Old barrel steel, pre-WWII vintage barrels, usually had a reflectionspeed of 18,916 fps.
Savage barrels use .3% carbon steel and have a reflectionspeed of 19,107 fps.
4140 and 4150 vanadium steels have a speed of 19,969 fps.
416R stainless steel has a reflectionspeed of 20,014 fps.

To do anything with the theory, you need to know what your barrel length from breech to muzzle and the type of barrel steel.
It doesn't matter what caliber because the reflection travels in the steel not the hole in the barrel.

I find that for most calibers 6.5mm and larger, with barrel lengths of 24-inches, the bullet exits somewhere around 1.15 to 1.3 msec. depending upon what charge you use.

For a Savage 24-inch barrel, the reflectionspeed is about 0.104 msec. to go from chamber to muzzle.
It takes another 0.104 msec. to get back to the chamber. (the 2nd reflection).
That means that you have to set your load to get the 10th or 12th reflection, depending on how close you are getting to Pmax.
At the 12th reflection in a 24-inch Savage barrel, the exit time would be 1.256 msec.
If you set up your load to get your bullet to exit the muzzle at that exit time, that reflected shock wave is at the chamber and the vibrations at the muzzle are minimum and the crown is the tightest.
The theory is that it should yield the best accuracy.

A 26-inch Savage barrel has a 12th reflection time of 1.361 msec.
A 22-inch Savage barrel has a 12the reflection time of 1.154 msec.
It you use a muzzle brake, a flash hider or a suppressor you need to include that length because the reflection travels all the way to the end before it reflects.
The complication is that the steel in those devices has to be considered because it most likely isn't the same as the steel in the barrel.
Most of mine are 4150 steel. (Colt AR flash hiders also use 4150 steel)

I have tested this effect for about 3 years now and, after minimizing my shooter induced variations and taking temperature into consideration, I have found that my 7 most accurate rifles actually do shoot the best when I load for exit time and get it within 0.002 msec.
I have one rifle (maybe because the steel batch was slightly different or my barrel measurement was a bit off) that likes to be 0.005 faster than I calculated.
QuickLOAD gives you exit time for loads that seems to be accurate enough for my testing.
There is a difference of about 0.060 to 0.090 in average group accuracy between having the exit time with the reflection at the chamber or the reflection at the muzzle.

I have found that with a new rifle, and I have used the approach on three new rifles, I get great results right from the first groups.
Then all I have to do is find out what bullets, powders, bullet weights, and jump the rifle likes the best. They all seem to be different but I don't have to hunt for nodes."

I hope this helped explain the concept and the process I use to reload using exit time.

[charlie b]

The "reflection speed" is the speed of sound in the material. That's why different alloys have different characteristic speeds.

Why they call it 'reflection' is probably because they are looking for the reflection of the shock wave from the ends of the barrel. Kinda like a tuning fork.

There is a lot of confusion about this in places. Some folks think it is the same as barrel whip (it isn't). The theory is that the barrel is expanding and contracting as the shock wave moves through the steel of the barrel. The barrel does not move laterally or vertically (unless maybe there is a really bad stress point in the barrel that causes it to react in an asymmetrical manner).

[Robinhood]

The "reflection speed" is the speed of sound in the material. That's why different alloys have different characteristic speeds. Why they call it 'reflection' is probably because they are looking for the reflection of the shock wave from the ends of the barrel. Kinda like a tuning fork. There is a lot of confusion about this in places. Some folks think it is the same as barrel whip (it isn't). The theory is that the barrel is expanding and contracting as the shock wave moves through the steel of the barrel. The barrel does not move laterally or vertically (unless maybe there is a really bad stress point in the barrel that causes it to react in an asymmetrical manner).

Very interesting.

[CFJunkie]

Technically it is the speed of transmission of a shock wave in the barrel steel (since the s1ound waves of the blast propagate through the air) but the analogy of transmission is a good one.
The speed varies with the medium that the shockwave propagates through (the steel alloy in the barrel steel) while the speed of sound varies with the medium that sound wave propagates through - air, water, etc.
The caliber doesn't make a difference to the reflection time, only the steel of the barrel (and of the flash hider, muzzle brake, or suppressor if they are attached to the barrel).
The shock wave propagation is in the steel not the bore.

The propagation velocity in steel is between just under 19,000 fps and just over 20,000 fps (depending upon the alloy) while the nominal velocity of sound is from around 1100 to 1150 fps depending upon the temperature.
Air is a lot less dense than steel.
A 24-inch Savage barrel with 0.3% barrel-steel has a shock wave reflection time to go from chamber to muzzle of 0.1046 msec. It would take another 0.1046 to get back to the chamber (2nd reflection).
In that case, the bullet would accelerate from 0 to 2,500 fps in the length of the barrel, let's say 2 feet or 24 inches.
The muzzle velocity of the bullet and time in the barrel depends on the type of powder, powder charge, bullet seating depth and trim length of the load.
With a difference of almost 17x in velocity between wave propagation in steel and sound wave propagation in air, the reflection would go back and forth from 5 to 6 times from chamber to muzzle and back to chamber before the sound reached the muzzle or the bullet left the muzzle.
It would take about 1.1 to 1.3 ms for the bullet to exit the muzzle, depending on the barrel length and the steel alloy, assuming the absence of a muzzle brake.
It would take about 1.74ms for the sound to reach the muzzle at the end of a 24-inch barrel.
The bullet would be well out of the barrel and on its way down range before the sound arrived at the end of the barrel.

[charlie b]

Yep, speed of sound in steel is the technical term. Common term used in explosives testing (and high power laser effects testing). And, yes, it varies with the density of the material, even in different alloys of steel or in other metals, or in liquids or gasses. Higher density means faster travel of the shock wave, which is why it is so much faster in steel than it is in air.

[Dave Hoback]

Exactly why I shoot down the idea of people talking about “loading for the bullet to exit on the number “x” reflection at 1.02m/s” -blah. I started picking up on this several years ago. Then I started wondering, are these people using filming equipment that can accurately capture 20K FPS reactions? And overlap software to correlate the top/bottom of reflection to the actual timing in M/S? The answer is, of course they don’t. It simply finding the published data of such & such a metal and the reflection numbers exhibited when a certain load is fired through it. It’s already established numbers we can easily find online. Basically it’s no different than simply saying “I’m load “x” grains of “y” powder and hoping for “z” velocity.

So why then do some continue to use the loading for reflection speak? Because it makes you sound really, really, ​SMART! LOL!