Very interesting thread for this "city boy" who barely got out of High School. "Higher" education came from the School of Hard Knocks and extensive reading and practice.
I don't pretend to understand the velocity "WHY", but I can report the increased velocity WHAT when I used ChamberMate (not Gauge Mate) brand chamber inserts.
I took five of my personal 12GA shotguns, all with 30" barrels, and identified them by their bore diameter - 741" Gun, .731" Gun, .729" Gun, .727" Gun, and .725" Gun. Newly bought Winchester brand ammunition was used in 12GA-20-28-410. Five 12GA rounds were fired in each 12GA gun to establish my benchmark velocity average in 12GA in each gun. Then I repeated the tests with the ChamberMate inserts in 20ga, 28Ga and 410 in each host gun. All rounds fired over my chronograph and recorded.
Rocketman mentioned he did not think resistance (barrel roughness?)would have any affect....firing results from my .727" Gun would seem to challenge that idea, as it showed a consistently lower average velocity in all gauges fired therein.
I just could not understand how I ended up with a "velocity increase" for each sub-gauge. A Dixie gent whom I greatly respect and had the benefit of a Clemson education, said I should file the results under "gas expansion dynamics".
I wrote up my results in a "rough&ready" report for my files and reference. Anyone interested in this "report" e-mail me and I'll send it along for your perusal.
