Since HKs run on blowback pressure & not gas bleed off, I doubt you'll ever get it to reliably cycle w/subsonics & if it does & you put a supersonic load in it I imagine it would be dangerous.

I've tried the 80g .224 bullets subsonic in my 1/9 twist ARs & it will not stabalize them.
They were almost all hitting the target sideways @ 25 yards. Glad my can wasn't on. Impact sound was impressive.
I get 69g to shoot fine but have to reload w/cocking handle.