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(edit Most barrels...) won't last very long as fire pits. I know this from burning trash when I was a kid. By the time the barrel was full it was falling apart. I never counted, but I would expect it to work well for about 10-20 burns and maybe 50-60 burns before its paper thin and falling apart.
Edit: The thickness of the metal is what matters. 18 gauge drums are what I find to be the industry standard; 18 gauge won't last very long in my opinion. A thicker drum would last longer. I personally own a fire pit object that is very thin, which probably would not last more than 10 burns. So it goes to show that any ol' manufacturer may not necessarily make a decent product.
I have seen barrels converted to wood stoves. I have seen some of them fall apart and I have seen some stand for decades. This is due (primarily) to the thickness of the steel.
Edit2: Product reviews are probably important when you can't get the actual specifications. Amazon has reviews for the example product that you linked... 3.5 stars is not so hot.
"what (are) the advantages of having one..." "to avoid direct heating of the surrounding rock/concrete"
Yes, the advantage is to reduce direct conductive and radiant heat transfer to the surrounding material. Heating clay bricks, rocks, stones, concrete, decorative bricks/pavers, etc. will cause them to crack and crumble. Sometimes immediately, sometimes over a period of time. Sometimes explosively and dangerously, due to moisture in the stone/clay/concrete (unless you use special refractory brick).
I agree with other comments and posts, a DIY job is fine but thicker steel/iron is the key to good performance. I would say 12 gauge would be minimum thickness.
How do you all manage the campfire and cooking simultaneously? I want to cook over the coals, but I need to burn a steady supply of logs to make those coals first. The campfire rings at most campgrounds suck – the grill grate is like a millimeter above the ring so you end up burning your food if you try to use it. I can bring a grill grate from home but I don't have a support stand to elevate it. The steel ring doesn't allow enough airflow so you constantly have a smoky smoldering fire, especially with not-quite-dry "campfire bundles" of pine wood we're forced to buy. I'd like to be able to make a "two-zone" fire like you can do in a Weber kettle grill, with high heat/flames for searing, and coals for slow-roasting.
So I guess I need two things: a hot smokeless fire, and a way to separate out the coals when they're ready. We're car-camping so I can pack some gear, but would prefer to avoid anything too bulky. I've seen the tripod setup but it seems tipsy and raising/lowering it must be a fraught process. It also assumes a smokeless fire which is rare in an air-starved steel campfire ring.
I've seen the Solo smokeless fire-pit, but not excited to pay $300 for it, and it would take up a lot of room in the trunk. I see it has a griddle top option, but that looks like it's only for high-temperature direct-flame searing, not for roasting over coals. Is there any way to sift the coals out of the bottom for a separate fire?
I'm picturing something like my charcoal chimney starter – it's just a cylinder of metal with some air intake holes around the bottom. Once it's lit from the top, the convection draws in air and it gets ripping hot without any smoke after the first five minutes. I wonder if I could fabricate a larger version that would burn log splits? Something with sheet metal sides and hinges so it packs flat. Put a grill grate in the bottom so the small coals fall through, I can push them over to a separate cooking fire.
Any ideas?