OK,
I have (and have shot 4 whitetails with) 300 Whisper,aka,300-221, 300 Fireball,
ranging in CRITICAL-yes I am shouting- with a subsonic 240 Sierra HPBT going 1040 fps it drops like a rock. A quiet painful rock but a rock nonetheless.
You need to either get the right bullet or place your shot in the brain or spinal cord to get a fast kill otherwise there is a strong likelihood the bullet will "pencil" through the chest and the deer will run off to die.
I have used Sierra 220 gr. that I have filed off the tip to make a flat point and skived the tip to help it open and yes it did work on a broadside shot but it also went through the upper heart and tore out the aorta and I never found the bullet so I have no idea if it expanded.
This year I an trying a specialty bullet from Lehigh Bullets that is a solid copper bullet that is "ventilated" for lack of a better term and you drop in a 20 cal bullet in the open cavity in the nose to aid expansion.
Very accurate round and it is hell on water jugs but I will have to see what it does on game this season-reports are favorable.
As to your ? about 300 yards-my personal belief is that it is way too long a shot to be made ethically-take another gun-around here, a shot at 300 yards would not spook the rest of the deer in the fields-I have dropped deer out at that range and had the rest of the herd just stand there and continue grazing-yes, we have that many deer - the limit is 6 and you must take at least 3 does to reduce the population.
The reason I go suppressed is to keep the noise down for the neighbors-not any other reason except that the couple of them do not need to be awakened at the break of dawn.
Just my opinion, you mileage will vary.
Gary