I recently shot a Marlin 30/30 , with iron sights and factory ammunition and had no trouble shooting 2 inch groups at 100 yd. I was able to hit the 300 meter pigs at the silhouette range. With a 2.5 power scope and leverevolution ammunition, it is a usable 300 yd rifle but not ideal.
When I was young and could see sights, I shot a 2 1/2 inch, five-shot, group with a model 94 carbine at 200 yards. This group was fired with 170 Silvertips from the prone position. Lyman peep sight, bead front.
If one is well practiced with the rifle, used to shooting out to 300 yds, and has a scope on the rifle, a 30/30 is a viable 300 yard rifle. If one is not well practiced, no rifle is a viable 300 yd rifle.
If a person can consistently put shots into a six inch circle, centered on the point of aim, then he and his rifle are suitable for big game at that distance and from the position from which the shots are fired. So, if the guy can hit that six inch circle, at 300 yards, from a rest, he and his rifle are viable at 300 yd, from a rest. There is no doubt in my mind the 30/30, with modern ammo, is capable of 300 yard deer, from a ballistics standpoint. The ability to deliver the bullet is entirely up to the shooter.