It’s not just velocity, there is a theory that I believe to be true about spin rate also being a crucial part of the TSX expansion equation.
It was covered on another post in this forum, but basically someone was testing TSX in ballistic gel and fired 2 rounds of the same projectile and at equal velocity into gel by 2 different rifles.
One TSX expanded and the other did not, the only difference was the twist rate of the rifle barrels.
As the author said… “that’s when the lightbulb came on!”
That was my post, and I was quoting Steve the Owner of Hammer Bullets. Although not exactly the same as a Barnes, minimum rifling twist rates still apply.
Curious about the rifling perspective (but I am always curious).
Is it twist rate, rifling profile and depth, number of grooves and lands?
It makes sense, that "harder" alloyed mono-metal bullets may require different rifling design, twists than traditional lead, or soft cupronickel cup and core bullets.
It is twist rate.
This is a quote from Steve at Hammer bullets.
"We learned this when we first started impact testing bullets in the beginning of marketing Hammers. We were low velocity testing to confirm proper performance at 1800 fps. Launched a bullet at 1800 fps and...
I have discussed this issue at length with nearly every manufacturer of monometal bullets, and they have all noticed the same phenomenon. When SPIN RATE is below the minimum, the bullet will likely fail to expand, even when velocity is above the recommended minimum. Adding velocity when the spin rate is below minimum will not solve the problem.
From the Hammer bullets website.
https://hammerbullets.com/about-our-bullets/
Recommended Minimum Twist Rates
Each of our bullets has a required minimum twist rate that is calculated at sea level using the Miller Twist Formula. We use sea level as the standard to ensure that there is enough rpm’s for proper terminal performance. While the Miller Formula calculates stability for ballistic flight,
we are most concerned with hunting performance and ensuring that there is enough stability for proper terminal performance. The required minimum twist rate is based on a gyroscopic stability factor (SG) of 1.5. An SG of 1.5 or higher is needed to achieve full bc value and expected terminal ballistics.
In our impact testing for terminal performance, we have found that marginal SG (less than 1.5sg calculated at sea level) hinders a bullets ability to stay on track and open reliably for proper terminal performance. SG factor of 1.5 or higher will keep a bullet oriented point first on impact
and promote proper deformation of the frontal area for good terminal performance. We have never seen a downside to running with a faster twist than the minimum required twist. Infact extra stability is always a good thing. When referring to a rifles twist rate, note that a 1-8″ twist is faster than a 1-10″ twist.
If hunting at low elevation and cold temperatures it is recommended to run faster than minimum required twist rates.
Hunting with less than listed minimum twist rate can result in poor terminal performance and should not be done.