starman wrote:I don't think Rag was necessarily lifting one system over the other
If you're doing precision work, metric is (in my opinion) a better choice, because you've got no conversion factors to work around - it's all decimal.
I wouldn't be caught dead using imperial for any work where precision is key.
I do use imperial occasionally, but for rough work and estimation:
"It's about 30 feet mate" - that kind of thing.
Of course, you should never mix the two systems up, as happened with NASA's Climate Orbiter mission, where the craft was expecting it's course adjustments in metric, but got given them in imperial.
...That was a $125,000,000 crater on Mars.
But in this case, yeah, you'd do best to seek out proper metric pipe. Although I'm no expert (because I've never been to the states), I believe you can get 4.5 & 6mm ID automotive brakeline in the US, both of which are supposedly good for BBs of the right size.