AR question Dissipator configuration

[Baron]

I had a question about Velocity
Would a Dissipator upper with 5" flash hider have the same velocity as a 20" barrled upper? I was thinking of getting a 16" Dissipator upper and add a long xm177 5" flash hider, then scope it.

Here's a picture of what I was thinking...


I'm currently building a 16" XM-177 "style" carbine and was thinking of getting a seperate upper to scope and stuff, but I figures I have to keep the same gas system so that both uppers would work with the carbine buffer and spring
Is that correct?

So would the dissipator with 5" flash hider have the same velocity as a 20" barrel? Or am I way off?

On a side note this is my first AR build, so I don't know much about them. I've already put together the lower and stock and just waiting for funds to buy my brothers 16" bushmaster upper

[DirtRacer151]

No. The 20incher will have more muzzle velocity

[Baron]

Would the 16" with long flash hider have more velocity then the regular 16"?

[sd1023x]

No. Flash hider does nothing to add velocity...its just a muzzle attachment. Once it leaves the muzzle, no more energy is imparted on bullet.

[DirtRacer151]

And if it does help w/ velocity at all I'm pretty sure the help is next to nothing

[Baron]

aHH OK... I just figured the longer flash hider was adding at least another 3-4 inches of length to the barrel and would hold the pressure behind the bullet till it left the flash hider seems how the slot are at the end of the flash hider.

How exactly is velocity generated? is it the pressure behind the bullet plus the twist rate?