I am thinking about using fast-gun cannons for secondaries on my Battlestations build. My question is what will happen to performance if I cut the barrels down to 3 or 3.5 inches to meet scale requirekments. i have heard that shorter barrels don't perform as well, but would this length seriously affect gun performance?
Shorter barrels will reduce your muzzle velocity, how much depends on cannon design, gas pressure, etc. If it matters or not also depends on what you're trying to hit and penetrate. A nice perpendicular shot to someone's hull above the waterline and you wont have any problems, short range (~20 degree down angle) sidemounts hitting belows still probably not a problem. low angle sidemounts (~10 degrees down) going for belows or glancing shots get more problematic.
I am loking at 1/8" supply line at 150psi to MAV-2s actuated via MPA-3s linked to a SMAV-3. Down angle would be about 15 degrees. Would it help if I went with a tight-tolerance 3" barrel from BC?
Yes, it should. I plan on using tight-tolerance (thick wall) barrels for the secondaries in my boats for that very reason.
The standard barrels and the "tight-tolerence" barrels I have from BC have the same ID so they wont really get you anything performance wise. They are more resistant to damage so that is a plus.
The thick-walled tubing i got from Smallparts a few years back has a smaller ID. I'd go with something similar and make your own barrels.
