Tried asiing this before but I am looking for extensions that don't or minimally twist over longer lengths
Extension is essentially a solid shaft made of steel. If you want to calculate the "twist" also referred to as "angular deflection" - here is the formula for solid shafts.
https://www.engineeringtoolbox.com/torsion-shafts-d_947.html
The angular deflection θ of the shaft can be calculated as
θ = L T / (J G)
= L T / ((π D4 / 32) G)
Let's look at this formula and what it means.
L= length of the shaft - longer shaft, more twist.
T= torque - more torque, more twist.
J= is a geometrical factor calculated for shafts - bigger J, less twist
G= modulus of rigidity - bigger G, less twist
G is called -
modulus of rigidity.
Here are all the examples of "steel" from the table in the link above;
Carbon Steel 77
Nickel Steel 76
Structural Steel 79.3
Stainless Steel 77.2
Steel, Cast 78
Steel, Cold-rolled 75
Notice how similar the number is for different types of steel. In all likelihood the variation you find in steel used in extensions is so small it's impossible to detect or feel. Even difficult to measure with advanced equipment.
So basically, take a look at any extension, the only thing you need to know about it regarding twist is the diameter and length. And notice according to the formula above the twist or angular deflection is inversely proportional to the diameter in forth power. Meaning - small increase in diameter makes BIG difference in twist (less twist actually).
Ps
This formula is only valid in the elastic zone and tells you nothing about when your extension fails. Then different material properties come to play.