Well Marine Rifle Squads have been bigger than Army ones for a long time. The Army goes with a 9 man squad the Marines now 13 but sometimes as large as 18 but now 12. a 12 man squad would be easier to move in ACV 8x8.
Now the Marines are saying that the move to a Infantry M27 is a step up in fire power which is partially true but only when compared to the M16A2/M16A4 and M4 as issued to the Marines but a step back compared to M16 and M16A1.
. You see back in the 1980's the Marines designed the M16A2 as a "Improvement" for the M16 series. At that point in time the M16A1 and M16 were the Standard issue rifle of the US DOD. The Marines decided that they wanted to change the Sighting system for the M16. Well they were at it they made changes to the upper and lower recevers, moved to a longer buttstock, the double round handguard shell and changed from the Full auto trigger group to the 3 round burst group. Safe, semi and 3 round. This model became the M16A2.
The Army later ( early 90's) modified this with a telescopic stock, short barrel, short gas system ( more or off the CAR 15) with added feed ramps and a changed buffer set into the basis for the XM4 carbine, Socom stepped in, asked for a flat top upper receiver with M1913 rails and a modified gas block/ front sight post. The Army agreed and The M4 Carbine was created with SOCOM changing the Trigger group back to full auto for the M4A1.
There was also a Short lived M16A3 which was a A2 with the full auto group for Navy Seabees.
Eventually someone made the M16A4 by taking the A2 putting the flat top upper on it and the M4 front sight post.
then things go shook up when the US went to a major long term war or two.
M4A1 Carbines suffered for Socom because they were wearing out faster then expected. This was mostly Socom's own Fault though as they were using the same rifles in combat as training so by the time they hit the field they were pretty worn out. It didn't help that they were doing mag dumps from barrels designed more for weight reduction then high heat. The Want of Optics spread across the board so Flat tops had to be adopted and troops now with more experience complained about the poor nature of the 3 round burst trigger group. basically you have 4 trigger pull weights in one trigger as the Trigger cycles to operate the weapon.
all of this and increased foot matching by the infantry as well as increased use in vehicles in Urban made the Army and Marines re evaluate all there thinking for rifles.
Both the Army and Marines first jumped to adopt the M16A4 then the Army moved to the M4 Carbine Flirted with the XM8 until they killed it and currently the M4A1 with the same heavy barrel as socom. currently planing a improved M4A1 followed by the Eventual Next Generation Squad Weapons.
The Marines moved to the M16A4 then the M4 but then looked at the M249 as the trouble spot. M249 has been on the trouble list for both the Army and Marines due to reliability and weight. The Marines decided that instead of a new LMG, they would adopt a Automatic rifle, The Idea being to adopt a system that offered the accuracy of a M16 with the heat and high round count tolerance of a LMG in a weapon with lower weight than the M249.
This was originally supposed to be a dedicated Automatic Rifle that would fire from a Open bolt on full auto and closed bolt on semi but that was dropped. The End result adopted in 2007 was the M27 a modified HK416 rifle with a full auto trigger group but lacking any higher capacity then a M4 magazine even with the PMAG. moving the M249 back to the Weapons support element this means that the USMC infantry formation effectively has no LMG but instead that all members of the Squad are armed with a weapon that although Automatic is still just an infantry rifle. farther more despite being adopted in 2007 and now being moved as the main rifle in 2018 every part of the rifle dates back to 2004.
The Optic that comes standard with the M27 is the Acog TA11 a 3.5x35mm fixed power with back up mini dot, This is odd as the Marines themselves standardized on the Trijicon Acog TA31 a 4x32mm fixed power as there Rifle combat optic. Farther more it comes standard with a harris bipod and fixed foregrip. Well the Foregrip is not odd, the Harris Bipod is that's a bipod normally found on Sniper rifles it's a precision tool yet being used by general infantry. The Quad rail is the same rail as the HK416 unchanged from 2002. Now on the Plus side for this the M320 grenade launcher module and the HK416 were designed side by side and the launcher can quick mount to the 416 rail, but on the down with a loaded magazine the M27 is 10 pounds the M320 is over 3 pounds meaning almost 14 pounds of rifle.
The M38 which is the "DMR" version is little more than the M27 save that the 3.5x optic has been replaced with a Leopuld Mark 4 MR/T 2.5-8x36mm variable power scope not a bad scope but it dates from 2002 when it was taken off the Mk 12 SPRs that might not seem a big deal but Optics change a lot in 10 years.
I mean the Mk 13 mod 7 is also not new having been in Socom service before the Marines but it only dates back to maybe 2014.