Jerry,
I hate you! Your pictures are always the BEST!
Just wondering though, that is a serious grease groove. Deeper and wider than all others I have ever seen. I even had an H&G #503 for a while and I don't think the relationship between the bands and groove were quite that much different. They were all the same size, if memory serves me right.
There should be no problem with leading if you can get the alloy right! I bet lube is all over the muzzle of most guns you shoot that out of!
WOW!