Gonna go out on a limb and say striker barrels under 15 inch vs a 20 inch tube are comparing apples to oranges.
Velocity loss is not linear, years ago a friend cut down a 26 inch barrel 375 h&h several times each time trying to make it more and more handy for dense brush, 26 down to 23 was a drop but only 30 fps or so, 23 to 20 averaged closer to 45fps, when he took it to 18.5 it dropped to the point where rebarreling became and attractive option to him. I'd hazard that a 20 inch 300 winnie would loose no more than 40 fps per inch. Velocity loss would be from my experience will be more pronounced as you go under or over the mid weight bullets.
I have a 270 wsm chopped down to 17.5 inches, it slowed down between 250 fps and 360 fps depending on bullet weight. the 130s are a modest weight for the 270 and posted an approximate average loss of 250 fps, the 110's lost a hair over 310, the 85 e tips weren't tested in the 26 but chronied 320 fps below the barnes data for 85 grain bullets. The few 160 partitions lost the big 360. Now the 270 wsm striker pitols with the 13.5 inch barrels ususally seem to post larger losses per inch.
I also have a 375 ruger that's only 20 inches, on the 235s I'm off 150 fps from the book speeds with similar load data, on 260s with rl 17 I'm posting a negligible difference from published velocity On the 300 weight bullets I'm off 50-80 fps depending on bullet type, and on the few 350 weight bullets I was off 200 fps.
The 300 winnie is between the two on bore efficiency so that is why took a stab at 40 fps worst case scenario.
Have now had the chance to shoot 375 ruger's with 20,23, and 26 inch barrels. They all had muzzle blast, obviously mine was the most extreme. But I'd like to point out that none were pleasant muzzle blasts. Same with the 270 wsm, yeah the 17.5 makes a nice breeze in my hair when I shoot it, but at 26 it wasn't exactly something I'd shoot without ear protection either.
On the topic of powders, from reading the sp posts dating back as far as I can, it appears that the same powders that go faster go faster in short tubes as well. My best loads in the 17.5 are eerily similar in the good loads in its 26inch twin. Some powders like rl 17, magpro, and retumbo really are quite remarkable. Initially with the 375 I tried to mess with powders seeing if faster powders worked better in the shorter tube. In that limited experience, nothing was gained, actually the mid speed 4350 and rl 17 were a fair bit better than the varget and h414 recommendations.
For me it boiled down to portability, the loooong barreled 7 rum and 300 wsm I had only made it 50 yards from the truck in Kodiak before I'd had enough of carrying them through alders. But in eastern OR, I shoot for miles with nothing but sagebrush and they work great. If portability isn't a huge concern I'd go as long as the blank would allow, the added 200 fps would be well worth it to me when going past 500 yards.
As always that's just my .02$
I wouldn't even bother having a 300 win mag in less than a 24 inch barrel..... Far to many negatives having short barreled magnums
