I assume that your short barrel will be legal, because it is a "pistol" right? Also, since it is a "pistol", which was previously a legal caliber are you still restricted to 10 rounds on your person? I have an 8.5" 300 blk pistol too, but my deer gun this year is going to be a 16" Noveske 300 blk rifle, which will replace my 458. I am going to look this up, but I feel like I keep reading that you don't gain much from your powder in 300 blk past 9", so I am not sure it matters that much to your velocity. I went 16", for legality obviously, because I want a butt stock on my hunting rifle if I am going to make any shots longer than 100 yards.
I guess I would think that the 300 blk may NOT be considered a "good" deer hunting cartridge, but it sure seems like a lot of hunters are having great success with it. I have a friend who hunts deer and antelope out west with 223, so I don't see how 300 blk is inappropriate as long as you have a realistic view of it's limitations. Unlike with my rifled shotgun or my 458, I feel like shot choice/placement will be more critical. What I really look forward to is carrying a nicely balanced rifle with low recoil, relatively flat trajectory, and a usable range totally up to anything I need. I have a 300 WSM as well, but I was told that I cannot use "high powered rifles" on the property which I hunt.
Anyhow, I have ladders worked up with the 110 Barnes Tac-TX and 110 Hornady Vmax. I chose w296 and Lil gun for powders. I have 1680, vv N110, 5744, but it seemed like you could get higher velocity on the 110s from w296 and Lil Gun. I am saving my 1680 for subsonic, as I only have 3 lbs.