I never said it was Mickey Mouse, just that it was done on a shoestring by comparison with most projects.
And, I did know that the HARP barrel was reamed to a smoothbore. OTOH, the M1A2 (and predecessor versions) Abrams Tank has a 120 mm smoothbore main gun, and it is no slouch for accuracy. Smoothbore is not necessarily inaccurate, and always produces higher velocity than a rifled barrel under the same conditions.
I do recognize that artillery is different than an armor piercing round at much shorter ranges, but I think the real difference between Bull's results and the 120 mm smoothbores is the development funds expended.
As you can see in my photo, it was very well designed and more than stiff enough for the purposes. The description you presented was not accurate - actually misleading. The system was never designed for accuracy as such. It was designed for high launch energies and exceptional range.
Good engineering of an experimental system doesn't actually require a huge budget. Just enough to get the job done, if the audience understands what the tests represent. Note that he never had any tube failures.
BTW, I am an artillery Program Manager/Senior Engineer.