I got my sample package of Thor bullets, it took about a week from request which I thought was very fast, the owner told me he would ship them last Friday, and I got them Monday.
I had my Omega all cleaned up anyway so I grabbed the bbl in the vise (with some padding to avoid damage) and pushed each of the 4 samples through the barrel from the muzzle end. The .500 simply fell through, the .501 had a very slight resistance, the .502 had more, and the .503 was a snug fit. Even the .503 pushed through easier than a Hornady bullet/sabot combo
I measured the bullets and found that the diameter change is only in the skirt area...the body of each bullet is the same dia.
I think the .502 or .503 would be my choice for my rifle.
In the very near future I am going to order some bullets to shoot for accuracy in my rifle. Their stated BC for the 250 is equal to a Barnes 250 Spitfire TMZ.
Here are some lengths for both the Thor and the Barnes TMZ's
Barnes 250 TMZ 1.110"
Barnes 290 TMZ 1.210"
Thor 250 1.161
The hollow base in the middle is .211 inches deep, the wall thickness at the very end of the bullet is .018(per side) tapering up to a .055(per side) wall thickness near the bottom of the pocket.
The barnes 290 TMZ is an iffy situation in some 1/28 twist rifles from what you read, mine seems to shoot them ok.
I'm not sure with the greenhill formula if there is any factoring of a hollow base or not, bullet stability in a given rifling twist is often said to be a matter of the length of the bullet.....but a hollow base changes things enough that a forster type shotgun slug is stable with NO rifling twist....so a hollow base is sure to have some impact on stability. With the length being between the 250 and the 290 barnes I have tested already in my rifle I would expect the Thor's to be stable in a 1/28 twist.