Update on the Thors. I received a sample pack of the 250gr bullets. They are the same length as the 350Gr FRBs. A couple of observations between the 2 bullets:
The Thors have a thinner wall on the cup at the bottom and a deeper groove, I guess because of the all Cu construction. They are also more consistent in dia. when measured with a caliper.
The FRBs have two walls joined to form the cup sandwiching the lead core. When measuring a FRB in a caliper and rotating it, it is noticeably out of round by .003-.004".
After mounting a 10x scope on my gun, I shot 2 rounds of the reference sabot load, both shots were touching. I then shot 3 shots of the FRB, they were abut 3.5" group @ 65yards. The bullet did not group with the reference load, it was about 2" off.
I then loaded the .503 and .502" Thors, neither would chamber. I then loaded the .501" THor. It was a little tighter than the FRB, but not much. It shot to the same point as the sabot reference load. I then loaded the .500 Thor, it also grouped with the other 3 shots.
I think I am going to buy a couple of pack of the .501" Thors. They seem to be ok, but there were too few shots to come to a conclusion. The fact it shot to the same point of aim as my sabot and grouped makes me optimistic.
I did note it takes about 5-shots to plug the flash hole where day light can't be seen. I used a drill bit if I could not see light out of it. I was also very careful to apply a lot of seating pressure at the end of the load.
I don't think the issue is QLA or the powder at this point. I think it is a simple case of needing to explore a few different bullets until my gun says it like it. I think I am 90% of the way there.