does anybody have a 416 rem mag that they cut down the barrel. if so how short and what velocity are you getting. i am thinking of cutting one down to 18 for the brush

Can't really help you with the Remington but my 416 Rigby is 21". Can't be of much help with velocity either as I don't have a chronograph. I don't even shoot max loads according to the book I load from. All I can say is it groups into an inch and the Elephant I took with it seemed to be as dead as the one I took with my 470 with factory loads. I don't notice the muzzleblast as being any different as it was before. It is definitely quick and handy for me.
I'm sure velocity is reduced but I don't believe it is as important as some believe it is. That of course is just my opinion and you will get MANY more that don't agree.

My first .416 Rem. was a custom Sako. I had the barrel cut from 24 1/2' to 22". The load i was shooting was the 300 Barns X at 2950. After the chop I got 2900 fps with the same load. Sorry its not a traditional load. My current one has the factory Win. barrel at 24"