[ILLusion]

1. Yes, you could put a PSG-1 inner barrel in an M4. Although it's not safe to say that ALL inner barrels are the same, it's safe to say that all AEG inner barrels will have the same mounts.

2. You could go all the way up to a 650mm inner barrel if you wanted, but you'd have to use a bore up cylinder kit with that, which also requires you to use an M120 or stronger spring. Using a standard bore full cylinder, you can go up to 590mm without any ill effects.

3. Yes, again, you could go up to 590mm with a standard bore cylinder. Standard m16 length is 509mm. And again, if you wanted to go larger than 590mm, you'd have to get a bore up kit.

4. No harm


As for the internals you've selected:
a. the Deep Fire bore up cylinder isn't required if you're already getting a bore up kit since the bore up kit already comes with a bore up cylinder.
b. Both the cylinder you've selected as well as the bore up kit you've selected is for a PSG-1, which will not fit in to your v2 gearbox. Get a proper M16 bore up kit. You don't pick the kit designation based on the inner barrel length you're selecting. You pick them based on the gun platform they're going in to. The positioning of the cylinder port (or lack thereof) and the bore of the cylinder is based on the length of the inner barrel you're working with.