My plan was to buy this 6cm and have the factory barrel cut to 16” and then have a shoulder added and threaded 5/8x24, are you saying the factory barrel is to thin to do this?
I'm not a gunsmith. I'm not even a carpenter, so anytime I throw out measurements, you should take them with a grain of salt.
Per my best measurements, if we assume that a 5/8x24 thread needs 5/8" of thread depth, which is what my Tikka 6.5cm has right now, then for an 18" barrel I'd want to slide back 2" from the existing barrel shoulder and measure. When I do that I get 0.69" which as I understand it is sufficient for threading 5/8x24 as most gunsmiths want .625"+0.050" for the shoulder, or 0.675" total. When I measure the same for a 16" barrel by moving back 4" from the existing shoulder, I get about .648".
Assume a straight-line taper of 0.021" per inch, you would drop below that .675" mark at right at 17.29"ish.
Put another way, the minimum length at which *my* barrel would keep a 0.050" shoulder on a 5/8x24" thread, is 17.3"ish.
Whether another gunsmith would try to thread the needle by going a bit shorter, I cannot say.
ETA: But I can absolutely say that I'd be 100% fine with a 17.3" tube. Or even 18". If I were spec'ing a custom barrel I'd do a contour that juuuuust let me keep the shoulder I needed at the length I wanted. But working with what's available.....ehhh, whatever. 17.x" is fine.