I thought the Corp just fielded a new lightweight titanium alloy 155mm a few years back. It is
extremely light and can be a carried in on most fixed-wing and rotary transport. With the V-22 they can bring one down with its crew and a couple of days of rounds. Perfect for rough terrain where speedy insertion is better than a self propelled solution. I think that this particular mortar are for more "mechanized units" like the LACV companies. Correct me if I am wrong but I think that they plan to use both. The point here is that they do not want to use a 155
self propelled tube,
not that they will have no 155's in the corp.
With the addition of those small light weight MLRS that the corp is getting they are really developing a very powerful "mini battery brigade? and the MEB and MEF level that can also be spun out to the MEUs. It makes them capable of taking on good sized regional armies while still maintianing the expeditionary profile.
You're close: the Marine Corps is still getting the Lightweight 155 (not exactly light - about 9,500 pounds all by itself). The V-22 can carry one of them for short distances since it has to carry that gun externally - and there isn't much load capacity left for crew or ammunition (and the truck). I don't know if you've ever had the pleasure of horsing 155mm ammunition around, but it comes in 800-pound pallets and if you don't happen to have a 4,000 pound forklift with you, it's going to take a long time to move that ammo!
The concept is to have the medium-caliber, medium range EFSS covering the direct support work for the fast-moving maneuver units, the LW 155s providing the battlefield shaping/reinforcing/counterfire missions and the HIMARS (High Mobility Artillery Rocket System) providing the long range high value target/counterfire punch.
The EFSS is aimed at supporting the deployed MEUs with something a bit more portable (the 155s aboard ship are referred to as "ballast"). Depending on the mission and the forces we're dealing with, we will bring in the heavy hitters for well-balanced fire support.
