The Kidd barrels are interference fit. (slightly oversize to give a tight fit and help eliminate droop .001~) Depending on how lose your receiver is, sometimes it requires putting the barrel in a freezer for a while, warming up the receiver using anti-seize, and a little work with a rubber mallet. Then, you have to be careful to align the extractor channel. If you get the barrel all the way in and find the extractor channel is misaligned in a very tight fit, it might require putting the barrel in a vise and using some sort of cheater or rubber mallet to align. That is what happened when I installed a new Kidd barrel into a new Kidd receiver. It was extremely tight and I did have to adjust the alignment a bit which required vising the barrel.
It's not rocket surgery, but with the tight fit barrels, it's more than just pushing it in by hand like OEM barrels.
The barrel wasn't going into the upper at all with how they stood from factory. I massaged the upper with some 2,000 grit paper and gave the barrel a little treatment as well. A hammer and some time was required to get it in but it's a good tight fit now with full radial contact around the barrel extension.