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here's a question for those tuba physics pros...does it matter how far a mouthpiece fits into a reciever if there is no gap? I'm talking about Alexander tubas and french horn style lead pipes.

No physics pro here, but if you make the tube longer, it's going to be lower, (flatter.) No magic there. If you're asking 'does that extra 1/2" matter'?, I can tell you that on my old Besson 2-20 it surely did. I couldn't get it to play anything but flat until I had the receiver changed out.