All three companies competing for the NGSW contract have there own new cartridge. The Army has specified a specific 6.8mm projectile. No official specs for the projectile or ammunition performance requirements are public yet but I have see bullet weight estimates being about 8 gram (123gr) and a muzzle velocity target of 3500 fps. Like I said barrel burner.
Sigs systems (remember this is two different guns, a rifle to replace the M4 and machine-gun to replace the SAW, that share ammo) will use a hybrid ammunition similar to this:
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The steel base is there to help deal with the increase pressure.
The Textron systems is using a case telescoping round similar to the right three rounds in this picture.
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General Dynamics rifle will be using this new cartridge developed by True Velocity.
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I personally think General Dynamics and True Velocity have the better system so far but the whole program is very much in the early stages and alot can and will change and it could all evaporate just as quick.