I think that the 6x45 is maxed out at 90grains
I agree, 87 to 90 grains is pushing the 6X45 to its usable limits. The additional 5 grains of bullet weight are ( in my opinion ) not worth the loss of velocity. but with the nice hard brass, nice powders, VLD mags and the 95 VLD is not that much more and with the low bearing surface gives a nice edge to the round. The bullet manufacturers will tell you they work.
There is no reason to believe it should not work as the amount of angular momentum seems appropriate up to 105grains in VLDs.
This is very hard to actually measure so the best way is to shoot a long distance and looks for any signs of yaw.
In fact it has always worked at those twists (at least with other rifling) and as I work on the powders and spreads I wanted to know if there is anything special about the 3P that would potentially make the bullet spin slower as advertised. I mean if you know about it of course.
There is nothing advertised about bullet RPM being slower. Spin is a result of rifling twist, it won't matter what kind of rifling it is that imparts it on the bullet. A 1 in 9 will impart the same spin regardless of the type of rifling. Polygonal rifling will in most instances give more speed due to less drag or friction as the bullet rides up the barrel.I measure the twist because I always do. Also head-space for the bolt I am going to use. Never had an issue in 25+ years shooting ARs but you know what they say... Morphy's law.
Is there anything else that I need to consider like type of copper or any other characteristic in the bullets?
BTW do you know if they also have them in 1:8?
Black Hole Weaponry does not make a 1 in 8 at this time. Maybe in the future if there is enough demand for it. does That might be my next one! ...because there is always a new project.
Many thanks.