I recently ordered a set of Armored Core weapons (by Kotobukiya), and have been assembling and modding them to work with the 5mm fist holes on TFs.. In order to make them usable, I shaved down the grips on the guns down to 2mm thickness, so that I can then fit a piece of 5mm tubing over them.
I found some 5mm tubing, meant for radio-controlled car fuel lines. Think it was about £5 for a metre or so from Amazon.
Nice mod. I think Armored Core weapons are too realistic looking for my tastes though, they wouldn't look right to me on Transformers You oughta do the same mod on Kotobukiya's MSG weapons. More futuristic and Transformersy
I was intending on ordering some after Xmas, since postal services are gonna suck over the next few weeks..