After bad first-hand experience: I recommend PVC pipe with threaded end caps. That's how I ship my K80 bbls after I bubble wrap them and pad the tube ends with dish towels. Triangle tubes can be bent-so can barrels. But it's a rare occurence. Like traffic tickets.
I use PVC pipe and caps, one end glued and other closed with duct tape. Guess which end they opened with a saw last time out. Put a open here label on the taped end. No names will be released. D Winter
Like others I use ABS drain pipe with the rubber cap screwed and glued on one end and the stainles steel hose clamp which I cover with tape to prevent snaging when securing the cap on the other. I wrap the barrels with the round foam pipe insulation taping it in a few spots to keep it secure when pulling the barrels from the pipe. I use block foam cut round on both ends. I've made eight or nine shipments using the same peice of pipe, works great. I perfer FEDEX as a shipper.
The post office has two container that will work. One is the triangular box and the other is a cardboard tube. Both are 36" long, but they aren't free. They cost $4 plus the priority postage. Fine for the 1100 barrel I sent, but for a
$2,000 or $3,000 barrel I probably would go with something sturdier.
PVC pipe with one end glued, other end taped & labeled. Wrap barrel in bubble wrap, pad ends & INSURE! Probably made four or five trips already. My pipe looks as if it has been thrown a couple of times, scraped up a bit, but as yet, no damage. Far more secure then the flimsey cardboard boxes. Tried one once, never again!
I always ship mine wrapped in bubble wrap, inside a PVC pipe with the ends padded and capped. I put the tube in a FedEx triangular package and ship 2nd day air. Shipped many times and never has a problem.