BenchMark is what I’m using in my bolt rifles 223 & 223 AI.
I’m at 1/2 a grain under Hodgdon’s max for very good accuracy for the 223 and a 50 grain ballistic tip.
Yes, I do when trying new loads with different powders seeking the best accuracy.
Different powders will leave different fouling, so will different manufacturer's bullets for that matter.
Yes, the skulls are repos from a taxidermy supply off the web. I think they cost me about $50 each. I prefer them to the natural ones that are messy and sometimes fall apart.
Correct
In my 22-250s over the years, I've had good success with 55 grain varmint bullets pushed by VarGet, WW760, IMR 4064, IMR 4895 and a few others. Sometimes it depends on what's available to you.
If you have any VarGet, try it with 50-55 grain bullets.
What twist is your barrel? That will determine what weight bullets that will shoot well in your rifle.