I've always wondered if porting a barrel, for example the Marlin 1895, would effect the accuracy, energy or velocity of the bullet fired from it?

The only thing that I am aware of that porting does is allow gasses to be released behind the bullet before it exits the bore, resulting in reduced recoil and lots of noise!

The performance difference is usually less than 20fps. Accuracy is not effected. Point of impact may change but that can be adjusted.

Put a muzzle brake on it then you will not have any affects on velocity/energy, or accuracy. Good luck.

I was thinking about porting the barrel on my 444S and asked a very knowledgable instructor at school and his response was "I would not even think about it unless I had an EDM to do it with. The small burrs inside the barrel are extremely dificult to remove and you can damage the rifeling trying to remove them." He also suggested the muzzle brake for a barrel that is not already ported or if you don't want to spend the money sending it out to someone with an EDM.