I don't know how big this half barrel is; if you can give actual dimensions it's more helpful. I'm growing Seminoles in a 'planter' made of slabs of marble. the whole thing is about 4 feet wide by 2.5 feet high by 2.5 feet deep. They're running up a dead tree and I have around 13 fruit set now. One thing that helps is, if you can get a longer container, to plant the seeds on one end and let them trail across the top of the soil before they actually go up the trellis. This gives the vines the chance to send roots into the soil from each root node. Normally regular pumpkins trail and root in as they go; this helps the plant. You can't really achieve than in a planter but the more roots you can get, the better. And of course you'll want to give them a really rich mix with lots of rotten composted and manure mixed in.

Here is a link that might be useful: Garden Freak in Istanbul

The pot is about 24 inches diameter and about 20 inches deep I didn't think about the longer container idea though. I'll see what I can find. I'm mostly dealing with limited space so trying to grow them up instead of out.

Don't pumpkin vines put down roots?

Yes they do put down roots if they have the chance, and the ideal way to grow them is to let them root into good soil along the vines. But if you don't have the room for that, you can still get some as long as you're not trying to grow an award-winning Atlantic Giant. :) BTW my Bungkans have now climbed up into the plum trees as well and are forming two squash up there. It's getting a bit wild around here...