Re: slow and surplus powders
I just looked up several of Kirbys old posts, and he reported powder bridging issues using Rl25 and HRetumbo in his 22/6mm AI (40) with the Wildcat 100 grain ULD.
So far (400 shots) I have used AR2225 (HRetumbo in US), AR 2218 (H50BMG in US), VihtN170, and H870, in my 224 Clark (22/257 AI with 30 degree shoulder) using the same bullet, and have never experienced powder bridging problems. I am unsure whether this actually proves that the shallower 30 degree shoulder is the reason, but it certainly appears that way.
Kirby also reported having powder bridging issues using slow burning extruded powders such as H50BMG in his 257AM. So far I have not experienced powder bridging in my 257Weatherby with the 156 grain Wildcat ULD using AR2225, AR2218, and H870, but have experienced the problem using Viht 20N29.
On burning rate I have found that AR2218 (H50BMG) is about 1 grain slower burning than H870, but unfortunately I have never had access here in Oz to aa8700, WC872, US869, IMR 5010, and H5010 so I am unable to comment based on personal experience. I hope the above is of some help. Regards, Brian.