...some folks in my last Patrol Rifle Instructors school gave me the impression that getting reliable functioning in suppressed mode with sub sonic ammo is somewhat of an art form.
There is some often-overlooked advice about loading 300 Blackout. I has to do with the shoulder rib in the mag. Most semi-auto mags (and some others) have a similar rib, designed to engage the shoulder of the loaded rounds and prevent them from slamming against the forward wall of the mag during recoil.
When loading AR mags with 5.56 ammo, the rib engages the rounds at the case neck, where the diameter is about 6.4mm. The problem is that when loading Blackout rounds, the rib engages the bullet, where the max diameter is (duh) 7.62mm. Therefore, for reliable feeding AAC recommends that bullets be seated deep enough that the rib engages the ogive at a point well forward of the bore diameter.
This isn't a big deal when loading supersonic, but the (usually) longer, heavier bullets used for subsonic loads present a problem. Unless they're loaded fairly deep in the case, the shoulder rib tends to cant the rounds inward toward the center of the mag, causing potential feed issues.
Here's what AAC has to say about the issue (particularly the note in red at the bottom):