That's what I was thinking. A few buddies and I had this talk yesterday and in my mind they should be he same since there is the same amount of moving parts in the "gas off" mode. Thanks.
If the suppressed sound is 100db, the difference would need to be at least 10db - known as "Just noticeable difference". You should get a meter to determine if there's an actual vs. perceived difference (is your mind playing tricks?), and you should hand-load to ensure, as closely as possible, the "same" round used in each test. Since there isn't an industry-standard sound measurement test, you shouldn't worry too much about meter placement, air temp, etc. Just keep the variables as small as possible, and repeat the test a good 20 times or so for each to get a good average sound level.
If there is a 10dB or greater difference (and I'm sure the industry would kill to find a way to cut an additional 10dB from the report), I'd put it down to the ammo or barrel differences. You said the "same" barrel, but I suspect they are not identical, so there are barrel harmonics, cold-bore vs. hot, etc. to consider.
OOOOH....close, but no cigar.....yet.
The part you have 100% absolutely right is the barrel harmonics, or more precisely, the pressure waves (known as sound) caused by the barrels and suppressors whipping back and forth after the shot (think of a ringing bell). Differences in barrel profile, attachment, and support will affect the perceived audible phenomenon.
Regarding that, when measuring SPL using either the "A" or "C" weighted scale, the minimum detectable difference, by the average person is 3dB. Some trained ears can recognize a 2dB change, but only relative to a previously established level (as in: tell me when the music is louder or softer as I change the volume, not: listen to these two sounds and tell me the SPL difference between them.)