I too have a stainless barreled .300 Wby. Barrel is matte finish, no problem whatsoever.Mine is a Shilen match grade.
Interesting that the concern is the outside finish and reflectivity. Many years ago when stainless barrels were first being tried, everyone was interested in how well they would last and shoot. There are many, many (probably 100's) stainless alloys and most are great for the application they were designed for. In the marine industry lots of people thought any stainless alloy would work....wrong! I have a pair of stainless Ruger Vaqueros that I use for Cowboy Action shooting. I have shot thousands of blackpowder rounds thru them (over 10,000 per year!) in competition and practice without a single gun problem. Durable, good looking and they shoot!