Make sure you matched the proper cylinder to your barrel if you change barrel length. Other than that, it doesn't really matter what the bore is of your barrel is.

Also to reap the full benefits you need a good quality barrel. I would choose a KM out of the choices you gave. I have heard of people recoating these. Other people buy brass barrels and just use Brasso or some other brass polisher to "refinish" the barrel. Just get the promy barrel. Also did you order overseas or something? Have you tried contacting your retailer about your F'ed promy barrel that lead to this thread?

Just for reference I am using a DBC barrel. Haven't used it enough to see drastic benefits. But it is better than my other setup.

you cant just slap in a bigger cylinder though

You mentioned nothing about putting in a longer barrel. You just said putting it in anything longer than an M4 would degrade performance.

That isn't true.

If you put in a tightbore that's the same length as your stock barrel you'll gain FPS.

If you go too long, you will not have enough air to continue pushing the BB, so the barrel will create drag on it.

Same goes with a loose barrel...

Yea you can.. It's called a "Bore-up"

i ment the extreme cases of psg1 length barrels in a standard v2/v3 box

thats what ive been trying to say!!!!

and about the bore up, how can it be bigger if it fits into the same slot

Sounds like someone needs to quit giving advice on things they have no idea about.

There's lots of them out there for sale. Nearly every cylinder manufacturer makes a bore up cylinder for AEGs. They really do fit.

he said that the bore up is a bigger cylander, and i said how could it fit into the same slot as a stock one if its bigger

Length is the same, but bore is bigger. Usually, they are tapered as well.

Volume = pi x r(squared) x length

And thus the name "BORE - UP" :)

how much bigger are we talking here, a few mm?

we are talking a few 10th of milimeters, multiplied by the lenght of the cylinder, it adds enought air to clear the barrel.

And, reducing bore to 6.01 would need LESS air, as there is LESS leaking around the BB. No the opposite. LESS leak does not mean you need MORE air, quite the opposite.

And the reason why the 6.01 is dropping FPS is because the BB will bounce a lot more times for the same lenght of barrel, and thus will loose a bit of speed. It will however be a lot more stable when exiting the barrel.

The point is that the difference that a 6.01 will make is stability is compromised by the reduced FPS. So the best ratio stability/speed is achieved with a 6.03.

Bore-up kits are simply cylinders with thinner walls, and the corresponding piston head. Reducing the thickness of the walls while keeping the same external diameter causes the internal diameter to rise, thus creating a larger volume.

Most kits are made "taper", meaning the the internal diameter is reduced near the piston head. It helps creating a better airseal as the air pressure in the cylinder is rising.

LESS leak = more efficient = more FPS/longer barrel

i was always under the impression that it was also due that air couldn't slip past the bb as easily, so it needed more air/higher compression.