Before the questions, '300 blackout upper'. This is a give-a-way that you are loading for an AR type. Barrel length, port size and buffer will make some difference in what will work and what will work better.
To the questions:
One - IMR and H is not the same thing in all cases. Check the listings here for the listed safe data;
http://www.300blktalk.com/forum/viewtopic.php?f=141&t=86656&sid=20caddd5067192baf9336eda8387f08c
I normally use: Lt'Gun, 2400, H110/296 and Olin 630. All with very good results. So far I only load super-sonics.
Two - OAL and your barrel cutts will determine if the 110 Round nose will feed. I would think there is a good chance but only testing will tell. I have found that the more pointy the bullet the better the feeding.
Three - Why not. I cast my own and powder coat them. They do quite well. Velocities are not extreme out of the 300 Black but I would check with the bullet plater for their recommendations for their bullets. If you are going with subs, if you use heavier bullets and they are plated, they should work, see your question number two about feeding.
Four - When I make my 300 brass, I fully prep the .2232/5.56 brass before I chop saw it. My habit is to lube on the heavy side and size, followed with trimming. Then I heat treat the necks before sizing a second time. I try to use my rejected .223/5.56 brass for re-forming. I don't like to waste anything. I have gotten on the high side of 25 reloads out of my unknown usage before conversion brass. The primer pockets start getting loose.
Hope this helped a little. I don't know you use for your 300 Black. Mine was tested on a 'Bamby' not a week ago. My little 130 grain cast and powder coated bullets took out one lung and pushed it and some of the heart into the other lung. The mush was such that no bullet was recovered. If the shoulder joint hadn't been nipped, I think there would have been full penetration. One shot and one kill.
