There is no spotting scope you can afford that will see 6mm or 277 cal holes at 300 unless you use one of those "marker" targets that splotch out in a bright color where the bullet hits.
3" to 3.5" high @ 100 will put you on the paper at 300
I use the Redfield targets (red on white) as they have a neat grid system
What do you plan to hunt a 300 ? That's a far poke given wind, mirage, out of breath.
The bullet weight depends on the game and velocity. Killed my last 2 speed goats at 230 with an 80 TTSX starting out at 4000 fps from a long bbled 25-06, #2 at 232 yds with a 270 M 70 BOSS shooting 140 accubonds. at 2900.
Either one would kill an Elk at the same distance with a selected shot (double lunger).
Hope this helps.
