pi * radius squared * length
for a .22 with a 19" barrel
3.14 * (0.11 * 0.11) * 19 = 0.72 or 12cc
so a plenum of 36cc would be good, the 50cc MAX would be optimal.
25cc would not hurt but it would lose pressure before the pellet left the barrel
since you were looking for FPS increase then you want the pellet accellerating the full length of the barrel to maximize FPS
The cylinder volume calculator helps in the calculation of the right, oblique and hollow cylinder volume.
www.omnicalculator.com
or you could get crazy and build a ~300cc plenum with a 1.1L bottle
although the parts needed for that are on AliExpress and in China so you would get the pleasure of paying an extra 125% tariff come 5-02-25
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