im sorry if i misunderstood the intent of your post, but calling it a "total failure" and that it took "considerable effort" to achieve said failure sounded like blame to me. lets call it lost in t translation over the interwebs
i get that subs and accuracy arent normally considered a standard pair.. but its not impossible, nor improbable.
some guns take more work to get there than others, but its very probably with the caliber without a TON of work.
here's a few links that may help you and others working in the blackout world, especially on the subsonic side of things. while obviously this data will need to be refined for your peticular firearm - it can save you a loot of trouble by weeding out some common info on what does and does not work generally within the blackout world.
http://www.300blktalk.com/forum/viewtopic.php?f=141&t=81567
http://www.300blktalk.com/forum/viewtopic.php?f=145&t=87186
http://www.300blktalk.com/forum/viewtopic.php?f=145&t=97188
the folks who own maker bullets worked with a few of the guys over there who did some incredible work with their projetiles during saaid testing. this whole testing and dev thread included a redesign of the bullet to correct some of its initial flaws. this is worth a read re: sub hunting loads.
http://www.300blktalk.com/forum/viewtopic.php?f=141&t=97863
a few summers ago we also had a 200yd subsonic challenge...
that was a real eye opener for many of us - myself included - most of which had never considered trying to launch subs accurately at those distances. Now this really doesnt apply to hunting because of the projo choices used by most, but it shows that accuracy CAN be had readily with subs, even beyond what are considered conventional distances
i'll post the link to pg4 where the results start to come in. but feel free to start back and look at the origins of the thread.
http://www.300blktalk.com/forum/viewtopic.php?f=128&t=98191&start=30
my entry is out on page 27, and i only managed to get 3 on the 3x5 card at 200 yds... but that was also ammo that i did no special load prep with.. these were simply stainless washed, FL sized, trimmed to book length with a standard chamfer and debur and primed on press. in fact those came out of a batch of ~600 that i loaded on my LNL AP using the hornady powder thrower as the powder dispenser. i did however select 10 rounds that had matching headstamp out of that lot, but otherwise there was nothing unique done to them other than that one selection option.
208 amax
2.250" oal
LC brass
cci 400
10.6gr do.63
1:7 CMMG heavy taper stainless, form-1 can, 4-12x40 diamondback for the optic, timney trigger.
shot with my magnetospeed logging data from bags. this is a ~MOA load at 100, but as you can see - that ES of 52 really kills you at the kind of distances in the challenge and subsonic speeds.
The RSO's got a real kick out of me taking a target down to 200 yds to shoot subs at lol
i suspect i could find tune that load quite a bit just by hand throwing powder for it, but i figured i'd give it a whirl with what i had on hand just for giggles.
fun day!