This thread really interests me.
One of my goals is to see what I can accomplish with cast in .223.
I have yet to watch the videos in this thread. I will, but later, when Mrs. Brass isn't sleeping. She can be grumpy when awakened by loud videos!
Your scientific approach is neat.
However, even though I have not fired cast in .223, I firmly believe some lessons from firing jacketed bullets are also applicable to firing cast boolits. They may need to be applied differently to achieve the same results.
Failure to properly prepare the brass for loading can destroy accuracy!
Properly preparing brass to load cast boolits may not be the same as properly preparing brass to load jacketed bullets. Different techniques may achieve the same final goal.
Books have been written on what it takes to load accurate .223 loads in AR style rifles. To sum up the main issues it comes down to consistent dimensions, consistent case capacity, consistent neck tension on the bullet, and not damaging the bullet's base during loading.
These same factors certainly apply to cast, but implementing them must differ!
My personal belief is the brass preparation is more important than some of the other loading stages.
For jacketed loads, I full length size, trim, chamfer, and then VLD chamfer before loading. Any burr that makes the tiniest of scratches on a bullet's base or boat tail is a flyer. Add more range and add more error which increases to complete misses off the target frame at longer ranges.
I compete at High power so my loads are fired for score at 200, 300, and 600 yards.
Where I believe brass preparation for cast boolits must differ is in neck tension and in protecting the base and sides of the boolit. A standard expander ball will be too small for cast and the mouth of the neck will also be too small for cast boolits. Without flaring the case mouth, the case mouth will likely shave off lube and metal from the boolit. The case neck might re-size the castboolit.
I suspect the Lee Universal Neck Expander die will flare the neck, but I don't know what it will do for neck tension.
I suspect a Lyman "M" die will prevent re-sizing the castboolit and flare the neck, too!
For my cast boolit .223 loading experiment, I might begin by full length sizing and trimming with a Dillon Rapid Trim and then use a Lyman "M" die before chamfering and loading. I will likely try lubed, powder coated, and Hi Tech coated to see if I can measure performance difference.
Now for some quick notes!
Most of my barrels are 1 in 7 twist. I have a 1 in 8 twist barrel. I don't believe I have any slower than 1 in 8. That could throw a wrench in my plans. I will likely try boolits from 55 to 75 grain. Some are RN and some are Nato pattern from Mihec.
Since Varget was suggested for cast and I already have a little, I will likely try it first. Reloader 15 and IMR 3031 are others I would consider!
You won't hurt my feelings by critiquing my comments and plans.