If your using Nosler data they used 24" barrel and it's pretty old data been used since manual #5. I know 20 let you seat out to 3.000", I have model 24. You might want to look at data that seating bullet out vs Nosler @ 2.800"
This is something that may help
http://www.6mmbr.com/gunweek095.html
Hodgdon has data for 284, 139gr Hornady @ 2.915" start 52gr/H-4350 @ 2840fps and max of 57gr/H-4350 @ 3079fps. It's not problem but 22" barrel.
My 284, I'm using 52.5gr/R-17 @ 2983 fps with 160gr AB and it long throated, 27" Lilja barrel. I have other hunting rifles so 284 is just one of my hunting rifles and haven't shot anything lighter than 160gr.
I think been few article on that model 20 and if you could get hold of Mel he may have some loads.
Well good luck