I used to use copper for a couple reasons,The problem with copper (other than weight) is that it's only available in one useful size so your chance of getting a good fit are slim and it's impossible to do nesting with it.
1. I used to be a plumber so was familiar with it.
2. Solder joints are air tight and straight, but not everyone has the stuff for that.
3. It does actually come in functional sizes, repair coupling stock was my favorite, but not as commonly available at lowe's etc.
4. Cheap.
I found that 5/8 ID stock would shoot any stock nerf ammo effectively, arrows over the top, mega's poked in till the tip hit the barrel (least effective) and micros were the best.
...my two pennies...may change my mind later, you know how it goes...