Buddy has an opportunity for a 300 Jarrett wind runner at a smoking deal. But neither of us know much about it. How does it compare to 300 wm

Neither, I would do 300rum first...here is a pretty good read on the Jarret......Main reason, there is quality brass available for the Rum(including bartram), the rum has more power, is an inherently accurate round, is available in many different rifle configurations and is a long range killer....

300rum...Throws a 230 berger @3200 fps at 62*.........
Attached File

Distance to Chrono(FT): 15.00
Ballistic Coefficient: 0.717
Bullet Weight(gr): 230.00
Temp: N/A °F
BP: N/A inHg
Altitude: 0.00
#

FPS  FT-LBS

PF
5    3218    5289.51  740.14  
4    3221    5299.38  740.83  
3    3214    5276.37  739.22  
2    3208    5256.69  737.84  
1    3214    5276.37  739.22  
Average: 3215.0 FPS
SD: 4.9 FPS
Min: 3208 FPS
Max: 3221 FPS
Spread: 13 FPS
Shot/sec: 0.0
True MV: 3220 FPS
https://www.longrangehunting.com/threads/300-rum-or-300-jarrett.32848/

Thank you. That helps. My friend was offered the Jarret with a Swarovski scope for a grand but he doesn’t reload. I told him I would help him reload for it and then I started looking into brass. It seems as rare as a unicorn that will fetch you a beer.

For a grand, I think I'd purchase that combo and either rebarrel the rifle to something more common or resell both scope and rifle separately. Neither Swarovski glass nor Jarrett rifles are cheap, and I would think you could resell for a profit or have a very nice rifle for a very modest investment. The .300 Jarrett is based on the 8mm Rem Mag which shared a rim diameter with the .375 H&H of .532" Surely you could find a more appropriate and common cartridge to make that rifle your own.