I regularly use square ASTM A500 tube for all sorts of stuff. I like it a
lot, because MIG welding is much easier, it is very cheap.

It seems to resist corrosion much better. I left a rack on my pickup
unpainted all winter and there was very little rust when I painted it last
week. 4130 would have been badly corroded by now.

Square tubing also means that when I cut an angle with my cutoff saw, it
fits perfectly...no filing or joint jigger gadget needed.

My Adventurer Amphibian kit uses square tubing of some sort. I expect it's
just this same cheap structural stuff, even the engine mount and landing
gear are made of it. The Adventurer has a bad reputation (for such a cool
airplane), but not for THAT.
http://beta.communities.fr.msn.ca/Ad...entalAmphibian

I think the reason airplane types use 4130 is mostly tradition, and an
insistence that we need to have the best. But "the best" hasn't been
redefined since the 1950s.



For a typical rag and tube plane, properly choosing the tube sizes
should result in a weight gain of less than 15% for the same strength
which is, what, around 20 pounds for something like a Tailwind or
Aeronca. This to me seems like a good trade off to eliminate the
possibility of cracked welds due to poor technique. Not to mention
maybe saving a few bucks and being able to get the steel locally.