Short-stroke piston instead of DI, I think.

Nopes, long stroke gas piston, like the Ak.

1. XM106(aka Colt Automatic Rifle) epik failed with competition with M249 because of its lack of firepower. Army didn't like Rifle-based light machine gun from the beginning (it goes back to M14E2, which was light automatic rifle version of M14, which also failed from the competition with M60)

2. Closed bolt position for automatic rifle will cause overheating with 2 or 3 C-Mag. TBH, automatic weapons that designed to have heavy firepower, they need to have open bolt buy any means. Closed bolt is ideal for accuracy with rifles, but not with machine guns

3. M27 is going to fail. That's for the sure. Although people still complains about M249, it is still the best choice that army can make(i know Mk.48 and this is about special forces, but still everything goes back to M249 design)

The difference in weight between the M249 and the Mk46 Mod 0 is 5 pounds. I have had both in my hands and I like the Mk46 for several reasons compared to the M249 one is the weight reduction and the other is getting rid of that stupid magazine housing.

One of the reasons the M27 IAR was pick is a Marine Corp doctrine that states you can not use an open bolt weapon to engage the enemy from an ambush position.

Eliminating the magazine housing allows for a simpler design but now you can't use a STANAG mag in a pinch. An Amry buddy of mine shot an M249 with STANAGs on several occasions though and the gun proved to be unreliable with them anyway, so overall this is a benefit.

I know the M249 is 17 pounds and I was under the impression that the Mk. 46 was 15 pounds, but I found some weights that are closer to 13 pounds so I was a bit off there, and that is singificantly lighter, but the IAR is significantly lighter than even that.

Speaking from personal experience, I have never seen, or done so myself, a full magazine fired from a C9 without stoppages. It's a reliable weapon using a belt, but yeah, the magazine loading sucks.