I listened to one of the podcasts today (Episode 171, I think) and a couple questions popped up in my head about LH-GT barrels.
What benefit is provided by gain twist if you are already are adequately stabilizing your preferred bullet and have no need to twist anything faster? I know Frank mentioned something about the barrels being really tolerant of different bullets/loads, but I wondered if that was a common assessment across multiple barrels and users?
Secondly, left hand twist and gain twist are used by Frank as a pairing. There is almost no time where he says one and doesn't reference the other with it. I do know that they are mutually exclusive, but I want to make sure I'm getting it right. The left hand twist gives the cancelling spin drift/Coriolis and gives the modified recoil pulse or does the gain twist also contribute to the different recoil pulse?
Thanks for the clarification!
LH twist mitigates spin drift, not coriolis (which is generally over calculated by most BC calculators). Remember, (to be succinct) coriolis is the effect of the planet moving, and the direction you're shooting, and where the bullet will land/strike because of perception of where the target is when sent, and where the target will be, due to planetary mechanics, when it lands/strikes; LH or GT rifling has no direct effect on those variables, spin drift does (or the negation of the spin drift to be accurate). LH twist also allows the torque that occurs, as the rifle recoils, to torque
into your body (assuming right handed shooter here) versus torquing the rifle
away from the shooter with a RH twist. Pope did a lot of this experimentation over a hundred years ago, using anecdotal evidence, without truly understanding why. Regardless, he was on the right track.
The Gain Twist allows better use of bullets across a range because it limits the deformation of the bullet as it enters the lands and grooves. A static twist rate is optimized for one length of bullet (not weight, but length) so that it stabilizes the bullet, but also doesn't deform it. The GT allows different bullets (of different lengths) to be used so that deformation is limited, which equates to better accuracy down range (at distance). Deformation occurs, and is more prominent, when long for caliber bullets are used (which is the trend these days in our sport), and when the front of the bullet begins to turn upon hitting the lands/grooves, but the rear of the bullet wants to keep going straight forward ("An object in motion tends to stay in motion until affected by an outside force"; this is the opposite of that, in that a "An object at rest will stay at rest until acted upon by an outside force). As the rear of the bullet remains (from a rotation perspective) at rest, the front beings to turn, and thus deforms the bullet (jacket separation, core deformation, etc.), so when it leaves the barrel, it is no longer a uniform projectile. This non-uniform bullet then tends to oscillate ("wobble") as it spins and leaves the muzzle. This oscillation normalizes, and then worsens (bullet becomes less stable) as range increases and spin rate decrease. Hence why this is seen more at distance than up close. This oscillation also inhibits the time it takes to stabilize/normalize just after it leaves the muzzle, so you're "optimum" BC is effective for a shorter part of the bullets Time of Flight (and is generally lower than if the projectile was not deformed), since at best, we often just average a BC for ballistic calculations (even multiple BC calculations methods are still an average, just at more points along the arc).
While both are mutually exclusive options, it only makes sense to do both these days, since the mark up for the "odd" rifling or GT will be there, regardless of whether one or the other, or both, are requested. Might as well just order a bbl blank with both, unless you're in the southern hemisphere. In which case, a RH GT barrel may make more sense (from a spin drift cancellation perspective).
