Rifling query

**treeman** · 07-10-2022, 14:49

Do not think BC or SD was ever the question, PSI at start up of ignition is the worrying question being bandied about.

**A-R** · 08-10-2022, 08:43

I don't think the effect on BC, SD etc is measurable.

What is significant is the difference in friction, shot start pressure, peak pressure and gas leakage.

While I understand the concept of bullets being "bumped up" to a larger diameter than they started off with, it a known phenomena with relatively soft cast lead bullets. Whether this actually happens to any significant degree in jacketed bullets is debatable. If it does, it will likely only be when the peak pressure is reached, which happens when the bullet is already +/-100mm down the bore. Remembering that copper is quite elastic (springy), we can safely assume that said bullet might shrink back a little when the pressure drops as the bullet glides down the barrel. Should this happen, accuracy (precision actually) will suffer. So, my guess is that if a jacketed bullet does indeed obturate under pressure in an oversized barrel, it could shrink again and precision will suffer, but if it stays the same diameter (and not fit the barrel perfectly), precision may be better.

**AJR** · 08-10-2022, 10:49

I would think a shotstart pressure differential between the two type barrels is a given. It is initially dependant on barrel shape/diameter, because there is engraving required to make the bullet fit the barrel. (Whether the subsequent bearing surface friction between the engraved projectile and its barrel differs between the two examples, would probably be dependant on the twist rate, generating torque.)
We can see that a bullet shapes to the shape and diameter of the barrel, by simply shoving a bullet down there. Add 4000 bar pressure under frightening acceleration to that equation, and the projectile has no other option but to obturate, and fill the barrel / seal it. Once obturated the projectile allows for max pressure to be reached because it is now 'sealed'. A larger volume barrel would have to 'shorten' its projectile because the bullets' volumes are the same.
It would be improbable for similar projectiles expelled from different diameters and volumes of barrel to remain the same? The barrel effectively becomes a mandrell shaping the bullet, rendering a differing bc & sd from its expelled, obturated product.

Accordingly, for the same amount of propellant american barrels might produce a projectile a bit faster with better bc/sd, but german barrels (if made from the same material) might last /retain 'accuracy' a couple of hundred shots longer? Because of an average .02mm difference between them. Splitting hairs, if you will.

**A-R** · 08-10-2022, 22:24

Originally Posted by AJR

I would think a shotstart pressure differential between the two type barrels is a given. It is initially dependant on barrel shape/diameter, because there is engraving required to make the bullet fit the barrel. (Whether the subsequent bearing surface friction between the engraved projectile and its barrel differs between the two examples, would probably be dependant on the twist rate, generating torque.)
We can see that a bullet shapes to the shape and diameter of the barrel, by simply shoving a bullet down there. Add 4000 bar pressure under frightening acceleration to that equation, and the projectile has no other option but to obturate, and fill the barrel / seal it. Once obturated the projectile allows for max pressure to be reached because it is now 'sealed'. A larger volume barrel would have to 'shorten' its projectile because the bullets' volumes are the same.
It would be improbable for similar projectiles expelled from different diameters and volumes of barrel to remain the same? The barrel effectively becomes a mandrell shaping the bullet, rendering a differing bc & sd from its expelled, obturated product.

Accordingly, for the same amount of propellant american barrels might produce a projectile a bit faster with better bc/sd, but german barrels (if made from the same material) might last /retain 'accuracy' a couple of hundred shots longer? Because of an average .02mm difference between them. Splitting hairs, if you will.

Lots of assumptions in here. So far, nobody has claimed to be able to see what exactly is going on in the barrel, after the firing pin hits the primer. All the experts are admitting that their explanations are just guesswork, although mostly based on good observations of what can be seen externally.

So to prove or disprove your theory, you may need to catch and measure a few bullets from the different barrels.

What has been measured is the effect of twist rate on MV and pressure, and the difference is so small that it is negligible.