I do not really have an opinion on most of the above posts, but I do have some data to add here for those who might be interested.
I use WC820 (like H110) for all of my magnum pistol loads and subsonic rifle loads. It meters very well and gives low standard deviations, usually less than 20 fps.
My first attempt at a subsonic 223 remington load was with 70 grain Speer semi-spitzers and standard primers. As I had no idea where to start, I loaded ten rounds with loads from 12 to 4 grains. Starting from the top, each load clocked a bit slower each time, and the bolt carrier moved less as well until it did not move at all. The 9th round gave me about 750 fps. The tenth round surprised me by extracting and holding the bolt open. The chrony did not show a tenth shot, and there was not a hole in my light senser (again).
The bullet stopped past the gas port, about 2 inches from the muzzle and was easily tapped out with a cleaning rod.
My next trip to the range with various loads gave me about 1050 fps with a spread of 40 fps and stdev of 19 fps using 7 grains of WC820. Eight rounds at 100 yards using a the peep sight set for 200 yards gave me a 4 inch group about 16 inches below the aiming point. My 20" Colt H-bar has a 1-9 twist.
My goal is to develop a good subsonic load for my contender in 22 k-hornet and use it with my 223 suppressor. However, the contender barrel has a 1-12 twist which is very good for 40-55 grain bullets moving 2000-3000 fps. I am thinking that I will get keyholing or large groups using the 70 grain speer bullet. I may have to try seating them in the case backwards.
Any sugggestions? Thanks.
PS, I do think that shooting subsonic rimfire ammo in a rifle and expecting anything with power is a pipe dream. I will stick to heavy bulelts at 1000 fps minimum.