I do not agree with some of the respondents who say there is an automatic weight penalty when using square tubing over round tubing. Many years ago in the early 1970s, I designed a steel tube fuselage for a very light aircraft (it was an ultralight before that term had been invented--you can see a photo of the airplane on the Gallery page of my website www.sportaviationspecialties.com).
I was interested in minimizing the weight as much as possible, so I put a lot of effort into the design of the fuselage, carefully analyzing the stresses on each member of the fuselage truss using rigorous engineering methods. (Bear in mind, that this was before any computerization of the process--it was all done with a scientific calculator).
What I found was this:
1. In optimally-designed trusses, the individual truss members are subjected only to pure tension or pure compression loads, no matter whether the truss is bent one way, bent the other, twisted one way, or twisted the other..
2. When a truss member is under a tension load, the best strength/weight option for tensile strength considerations is sometimes round tubing, as determined by its outside diameter and wall thickness. However, because 4130 tubing is available in only discrete sizes, if a given round tube does not have an adequate cross-sectional area and the next larger size (either tube diameter or wall thickness) has excessive strength, it may be possible to select a square tube size/wall thickness that has a tensile strength in the middle.
3. However, when that same member is under a compression load (which will most always happen in one loading condition or another), the limiting condition is often buckling strength rather than tensile strength. Whether the tube buckles depends on several factors including how long is its unsupported length, the modulus of elasticity of the material, and the "moment of intertia" and "radius of gyration" (engineering terms) of the tubing cross-section. Here, the choice of square vs. round is not as clear. Assuming we stick with 4130 steel, for a given compression load and a given unsupported length, the lowest-weight situation is obtained by selected tubing that has an adequate radius of gyration for its weight-per-foot. It may turn out to be round tubing or square tubing of a given size and wall thickness.
4. There are other practical considerations in designing tube trusses, as listed below.
a. To reduce compression buckling problems, other truss members can be added to reduce the unsupported length (like a jury strut on a wing). Or, local stiffening of the cross section near the middle of the unsupported span can be done. However, the additional weight and complexity may offset their value.
b. Some truss members such as longerons span several bays and engineering analysis shows that the loads in various bays is different. That would mean that if we were being rigorous, we would select a different tubing size for each bay. However, since that is impractical and undesirable, the tubing cross-sectional shape and wall thickness are a compromise.
c. For lightly-loaded structures, it is sometimes possible to select a tube size and shape that will meet the required load requirement, but will be too fragile to withstand normal handling, moving in/out of a hangar, etc.
d. Cost is always a factor and thus a practical consideration is to minimize the number of different tubing sizes used in the design.
e. The weight of a fuselage truss is sometimes not the deteriming factor in the total weight of the airplane. Often greater weight savings can be obtained in other aspects of the design.
In the design I did back then, I wound up selecting square tube for the upper longerons (most often subjected to compression loads) and round tube for the lower longerons (most often subjected to tension loads) and for uprights and diagonal members.
And you are right: joint construction using square tube is much easier than with round tube.
Regarding the welding process you use, MIG is being used in aircraft manufacturing facilities (ex: CubCrafters and RANS) daily, so it can be successful. However, in those facilities, the welders are well trained and perform the same operations day after day; thus they have a very carefully-controlled process. Not casting any question on your MIG welding ability, but it is much easier with MIG than with oxyactylene to produce welds that look good but don't have good penetration.. I do encourage you to learn to do oxyacetylene welding. Either that or learn TIG welding, understanding that additional considerations exist in welding 4130 using TIG.
So, I encourage you not to write off square tubing without doing some good engineering analysis.
Hope this helps!
Sport Aviation Specialties
G. Michael Huffman
SportAviationSpecialties dot com