Let's back up to the very beginning and realize ALL of this has been ammo driven from the very start.
1) Based on engagement distances in Afghanistan, proliferation of cheap Level IV body armor among "potential near peer adversaries" (China and Russia), and age of current systems (mostly M249), the US Army wanted new infantry weapons.
2) US Army initially thought a 6.5mm round would be the best choice (a la SOCOM's use of some weapons in 6.5CM) but when armor penetrators were tested, 6.5 was removed from consideration.
2) US Army testing indicated that a 6.8mm was the smallest projectile that could house the penetrator necessary to meet the requirements and that muzzle velocity would need to be 3,000fps.
3) US Army designed said projectile as a 135gr bullet to house the required penetrator.
4) Lake City (first under Federal and now Winchester management) has made ALL of the projectiles used in the NGSW project testing and supplied them to SIG, True Velocity, and Textron.
5) NGSW requirements were said projectile at said velocity. Competitors were left with how to achieve this as part of their design with the additional requirements that loaded rounds be a XX% lighter than 7.62x51 NATO ball rounds (I think the XX% was 30% but I am not 100%.) and that the rifle/carbine be equal to or shorter than a particular length.
6) Textron did this with composite cased, telescoped (bullet almost completely in the case) rounds and a bullpup action with long barrel.
7) True Velocity used their composite cases and a bullpup action with long barrel like Textron.
8) SIG came up with the three piece case at 80k psi in a weapon laid out like an M4.
9) As much as the US Army loves cased telescoped rounds (they have been funding development for 20+ years), Textron was the first potential weapon system eliminated. As far as I can tell, this was because of two reasons.
A) There is no way to repurpose existing 7.62 production lines at Lake City to make cased telescoped rounds.
B) There is no easy way to retrofit the M240 to fire cased telescoped rounds and the military is not looking to replace all the M240s in inventory.
10) That left SIG and True Velocity/Lone Star Weapons Systems. My take is SIG won because they were the "easy button". The new M5 (MCX Spear) will require little to no changes to weapons manipulation training vs the M4. This is especially important since M4s will remain issued to rear echelon troops for the foreseeable future. Also, the design of SIG's three part case is closer to current cases than True Velocity's composite case so retooling and repurposing Lake City for it will be less expensive. Rebuilding M240s would basically be a wash, cost-wise. That said, part of me believes the mostly brass SIG case is the better choice for the current links used for belt fed weapons.
11) As far as I can tell, there is no "still deciding". The three part case is the case. Lake City is already retooling one line to load it in the interim while they build a new, dedicated facility for it long term which is scheduled to be complete in 2025.
12) US Army has already sent out inquires to industry for M240 retrofit proposals.
FYI - Some good info here:
