I joined the Corps in 1968 and trained with the M-14.
We had a week of snapping in where we would set our sight alignment and sight picture. Once you had that figured out where you didn't have to contort your body to find the target, the rest was relatively easy.
With solid alignment and picture the rifle came to rest where it was originally set.
We trained in the use of the sling for offhand, sitting, kneeling and prone positions. Endless cocking, aiming, squeezing and firing an empty rifle over and over again at a 6 o'clock bullseye.
Qualification day was a biggy. The DI (Sgt. Napier) said anyone who fired Expert, he would personally buy a Whopper, fries and coke from Burger King. I was one of six out of 75 or so that hit Expert and I had a 226.
True to his word, Sgt. Napier gathered the entire Platoon around in a circle, called out our names and we sat in the center of the Platoon with our Whoppers, fries and coke and enjoyed every bite. The aroma of fast food wafting through the night air.
Got off track there.
In answer to OP's question, out of 75 or so recruits, only 6 of us shot to the Expert level. This was after a week of rifle training by the Edson Range rifle coach and snapping in, a really dull, dry experience. Then 4 days of live firing prior to qualification day.
I maintained that rifle Expert level through the transition to the M-16. Then later I shot Expert with .38 and .45ACP as a First Mech on KC-130F's out of Futema and DaNang.
I cannot speak Corps wide but the weapons I shot were all off the rack from the local armory. The Marine Armorer's did a fine job of keeping those rack weapons fit for use at anytime. It seemed every weapon I used acted or reacted the same to all my inputs for sighting and shooting. I shot what they handed me.
In Egypt we had to form and man up our own perimeter gun emplacements. We had ejection seat mechs, admin types, hydraulic and powerplant mechs posted with M-16's and M-60's.
This was just after we bombed Gadaffi and we were hoping he'd come out to play.
Staff NCO's all carried 1911's with full mags in place, just needed to chamber a round to attack.
Wake Island
http://www.cv6.org/1941/wake/wake.htm is an example of every Marine being a rifleman first.
This got long.
Vick