*Nothing* is technically 'required' in homebuilts. What is prudent, or what you can get past the inspector for your a/w cert, are different matters.
I think that you will find that 4130 tubing used in certified a/c, and in all steel tube homebuilts of which I'm aware, is seamless tubing. Will you get away with using seamed tube? Maybe; maybe even probably. But seamless 4130 is the the proven standard, and never forget that a/c manufacturers are businesses, and they cut every corner they can without costing themselves money for killing people. If they thought they could safely save $2 per a/c by using seamed tubing, they'd find a way to get it past the FAA.
Grade 5 hardware is almost identical in *design strength* to AN hardware, so if the structure is designed for AN hardware, grade 5 will meet the strength requirement. But that isn't the only metric. I probably have some grade 5 stuff somewhere in the various homebuilts I've owned, but none in structural or corrosion sensitive locations (in either the bolt, or surrounding structure).
A lot (well, some) of the FAA stuff is overboard for little airplanes, and I'm sure most of us have bent some rules/recommendations on occasion, but when you're talking about structure, most of the rules & recommendations really are 'written in blood'. There are often places where we can do better, but it rarely makes sense to do worse.
Charlie
