I just acquired a sporterized Swedish Mauser which has been cut down to 24 inches. Since I am going to thread the barrel for a suppressor, what barrel length would you folks recommend for a hunting rifle? I don't plan on ever using it for game bigger than deer, and likely no further than 350-400 yards. I also want to hunt with the silencer.

OK so you are adding a NFA  Class III suppressor $$$ to this rifle and knowing it will effect handling by being front heavy  -   so you are considering cutting the current 24 in barrel to a shorter length - correct?   If so  20 in would be a good choice but you must have the barrel longer than 16 in or it becomes a short barrel rifle needing a second tax stamp.  You loose about 150 ft per sec for each inch you cut off the barrel.  The reduced velocity changes your balistics but the up side is improved handling characteristic and reduced over all length. Discuss your rifle with the gunsmith who is going to do the work.  I'm guessing you already have the suppressor for something else because it would not make a lot of sense to spend that kind of money on a pooky old rifle.

Yes, I already have the suppressor. I am aware of the SBR legalities. I guess I'm just asking where is the point of diminishing returns with regards to the barrel length for this cartridge. I intend to cut it as short as I can without loosing too much velocity.

6.5x55 is a good round. There is no way it will lose 100+fps per inch you cut off, the Swede is not overbore by any means and those are usually the rounds that lose the most. I'd bet if you take it to 20 you might lose 140 - 160 fps TOTAL.

It's off being cut now. I settled on 21 inches.