NN8734, AAC has three versions, the flash hider (does have the ping sound), the muzzle break/flash hider or the double chamber. I would concur with buying one other than the flash hider. I have actually had pretty good consistency with both mounts.
It looks like there is a pile of form 4s from August 11-20 of 2018 that have been lost. Looking at this tracker
https://www.nfatracker.com/nfa-transfer-time-tracking/
It is the largest gap of form approvals on the chart. With people getting approval with less than 90 days, something seems awry.