I'm going come in late on the subject.there's a variety of cartridges that are now available in .338 with the most notable being the 8.6 Blackout, .338 Razorback, with the .338 Spectre mentioned… Paul Maker has been in his mad scientist lab creating wonderful pills for these cartridges that will expand down to 850fps….. sub-sonic shooting is for only shooter who will do the work to become competent with the cartridges…. It's like all of the fanbois that bought 6.5Cms now are all snipers….
I use Primary Arms 1X micro prisms out to 300yrds effectively and probably could stretch it to 500yrds once I DOPE with the stadia provided in the reticle… the Sub-sonic.338 cal 300gr still has the same energy as a 10mmAuto at 1K yrds and enough velocity for expansion of Paul's new projectiles….
Granted, this isn't for everyone but those who understand the concepts and will do their part to be successful..