I finally got out to the rifle range today. It was crowded today. Probably due to people trying out their new gun show toys. There was a short waiting list.
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Anyway, as a part of this project, I wanted to see if subsonic pistol ammo stayed subsonic out of a long bbl gun.
In the past, I saw dramatic increases in velocity from pistol barrels. What if I went from a stock glock 4.4 inch to a 16 inch carbine?

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The test gun was my Marlin Camp-9 carbine with a 16 inch bbl.
I fired the same lots of ammo that I tested 2 months ago.
There were some surprises!
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SURPRISE#1- The 1st shot of each test string was consistently lower in velocity than the rest.
SURPRISE#2- My reloaded ammo sped up much more than the factory rounds
SURPRISE#3- The factory rounds did not speed up very much at all. One load hardly changed at all.
SURPRISE#4- Velocities in all tests showed a much higher amount of deviation than pistol tests.
UNSURPRISE- the carbine hates hollow point ammo. But I had fun shooting some ball that I loaded this morning.
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Analyzing the results, I think some of the inconsistent results might be due to loose chamber dimensions.
Beyond that, I don't really know what's going on. None of the test loads came close to going supersonic.
It would be interesting to see if the same results happen from some other carbines.
See the chart: