I have never heard of anyone neck-sizing Jet brass, but it might be possible.

As to cleaning the chambers, I have heard of Acetone being suggested but have never used it, or alcohol, or lighter fluid, for that matter. All I have had to do to make the gun function is keep the chambers clean and dry. Nothing special, in my case.

I have had a lot more trouble making my Jets shoot as accurately as they do with factory ammunition than I have with assembling loads that function without binding up the gun. Start with a clean gun and keep is clean as you are shooting.