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Great stuff Carl, clears the air a bit (once you get by all the conplicated explanationisms.)
Here's a bit of my experience which proves Carl's theory: I have a TM MP5A5/SD "Frankengun" (Primus knows what it looks like....... M4 slither stock and SD front end) that has the basic upgrade of PDI 150% spring and metal bushings. Also have the Hurricane reinforced mechbox (but isn't a performance part, just figured I'd mention). It would shoot pretty damn consistantly (within +/- 2fps for test shots) at 352fps using .2g BBs without hop up. When I was about to install a bearing spring guide, I had Apoc's M733 sitting open on my workbench, and it had a full cylinder in it, compared to my ported MP5 cylinder. I swapped it over to see what would happen. Since my gun was shooting about 350fps, I figured the bearing spring guide should give me around 15fps higher. Once I had the gun back together, I tested it. All shots were sitting around 330fps!?!?
So a 229mm barrel with 150% spring and ported cylinder shot 350fps, but a 229mm barrel with 150% spring, full cylinder and new bearing spring guide shot 330fps. I put the ported cylinder back in, chronied my MP5 again and was shooting about 368fps, and with a bit of hop up, 364fps. So I was right about the spring guide adding 15fps, and was neat to see what the full cylinder did in my gun. Now that I've read Carl's awesome explanation, I can fully see why.
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