My guess is that the TSA would not let you through screening with tubular metal parts 20" long. While they don't specifically call out such things on their list of carry-on prohibited items

http://www.tsa.gov/traveler-informat...ibited-items#7

I think they would err on the side of caution and note the similarity to things such as baseball bats and large tools.

My personal example is that they would not allow me to carry on a 5" long piece of 1.25" diameter steel cable (it was a test article), claiming it could be used as a weapon.