Interesting thing I just ran into and figured I would post it. I just changed out my ICS M4A1 w/M203 to a CQB-R with no M203 and as such swapped the inner barrel. I had a 363mm 6.03mm Madbull second gen barrel in the gun before, occasional feeding issue and decent accuracy. I swapped it out for a Prometheus 247mm 6.03mm tightbore and my accuracy had a huge jump and my fps went up 15-20. Most people run into fps increases when they install a tightbore, but I didn't expect one from switching from one to another of the same tightness but almost 120mm shorter.
Yet another mark against Madbull in my books and another for Prometheus. The price difference isn't huge either.
The only thing changed was the inner barrel (and outer.) Even used the same hopup rubber that was in it before (Guarder clear type.) I did have to switch the piston body but kept the same piston head (this as my 4th Modify piston that has stripped on me, each with less than 2500rnds.)
My CQB upper mechbox went from 345-350 fps to 360-365fps
My outdoor upper mechbox went from 395-400fps to 415-420fps
Now I need to bring both of them down to be legal
Just food for thought, I am in love with my Madbull Noveske RIS though.