I used a mini tubing cutter & it worked ok. It was tight getting the inserts in tho because the tubing cutter squeezes the shaft.
I work in a sheetmetal shop & we have a chop saw we use to cut gutter, mostly copper but aluminum also.
One day a lightbulb lit up in my head & I tried cutting one on the saw. Works great nice clean cut & the inserts drop right in. Anybody who has a power mitre saw can do this. The blade we use is basically just an 80 tooth carbide plywood type blade. Just get one with alot of small teeth.