Ahh the old "Marines are riflemen first"... myth. Yes myth. A Marine who's an admin clerk is the same as Soldier who's an admin clerk. Shooting once or twice a year or even four times a year does not make someone a "rifleman". Having served along side them in two theaters of war I just smile at this. The Marine who's carrying her weapon in a chow slung position is no better prepared than the Airman or the Soldier doing the same.
However, to the argument at hand. The Marines do start out with a better focus on marksmanship. However the Army has far more courses and professional development for our shooters. This speaks to money, time, and personnel- we have it and can rotate more people to shooting courses than the Marines can. Which is why a lot of these courses are joint.
Bottom line, a company of Rangers will spend more time at a range in one month than most do all year and focus on everything from 300m+ shooting to shoot houses and other stress fires. Much can be said for a company of infantryman in either the Army or Marines, just less time at a range. Now obviously those in the SOF community spend more time at a range but will focus on different distances and techniques. Its not unheard of to watch them go through 2,000 rounds per individual per day for a week. During one course I (non-SOF) personally shot a 1,000 rounds a day for a week straight and learned more about me, my weapon, and fundamentals than anyone who spends 5 hours on a 500m or 300m range zeroing and qualifying.
You want to watch some wizards go through some rounds? Visit the Army's Marksmanship Unit at Ft. Benning, GA, that is all they do all day long. No special optics for the most part and their weapons are pristine. They are as much as a training unit as they are a demonstration and rewards program that gives outstanding shooters a way to demonstrate skills and take a break from multiple deployments. Hell one of their shooters won a gold medal these past Olympics.