[DTogo]

Beyond 350m, it would seem prudent to switch to a caliber larger than .223 or 7.62x39 anyway.

[Durus]

One of the requirements of the rifle is an effective range of 400meters. That would require something better than a .223 and a 6.5 Grendel would be my choice. It requires a barrel/bolt change, but otherise uses all the same parts. Mags can be adapted.

It's the cheapest choice by far. Unless a gun company unveils some new technology, which is possible, there really isn't a better choice than modernizing the m4. (how about starting with 18" barrel along with the caliber upgrade.)