First I would much rather sell you one of my 338 THUMPERS since it is the best cartridge that will ever be designed.
But I do understand wanting to use the action you have. You never said what you are going to be using it for which can effect if it is suited for what you are doing with it. I would shoot a hog at 500 yards with our 50 THUMPER but until I do some more testing I don't think I would shoot one with our 338 THUMPER at this point, but the 338 is great fun to shoot steel targets at 500 yards.
In researching subsonic rifle cartridges I found that they all have close to the same drop which at 500 yards is about 23 feet. The limiting factor with subsonic cartridges is the amount of drop and not the range they can shoot accurately.
You are correct in barrel length, I would suggest to not go shorter than 10" for what you are doing and 14"-16" would probably be ideal. I use a 8" twist on my 338 THUMPER and have not had any problems with stabilizing anything.
As far as subsonics being a good choice for that you want to use it for its kind of up to you. If you can deel with the drop you should be fine.
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