[JohnKSa]

The only advantage to DI is a slight saving in weight and recoil, IMO.

And potentially better accuracy given that there are fewer moving parts, particularly fewer moving parts impinging on the barrel.

Don't forget that you're comparing apples and oranges, in that the M14 is cycling full-size rifle rounds with a much heavier action than the AR platform.

Direct Gas Impingement has been around a lot longer than the .223 round.

The Swedish Ljungman (1942 design) was DI and chambered in 6.5x55 Swedish.

The Egyptian Hakim was DI, a variant of the Ljungman, chambered in 8mm Mauser.

And, last but not least, the DI, AR-10 was chambered in .308 and was developed before the AR-15.

The point is that there is absolutely nothing about the DI system that keeps it from performing well with "full-size rifle rounds".

You can start with a DI and upgrade to piston with a conversion.

If you want a piston system, buy a system that comes directly from the factory as a piston system. The conversions can work very well, or they can be problematic depending on a number of variables, since the energy is transfered to the bolt in very different ways in the two systems.