One o f the biggest problems with the M14/M1A as a DMR is that no matter how accurate you make it on the range, it will go inaccurate in the field. That was shown over and over again with the M21 and with the incarnations of White Horse, and EBR systems. I worked with the 82d for a day trying to get their weapons back on course after a deployment. No go. The number of problems are just insurmountable with that system for accuracy in the field.
The problem is a balance of accuracy and durability. The more accurate a weapon, the less durable. The more durable the less accurate. The M14 falls in the durable less accurate zone. To make it more accurate makes it very much less durable and it falls back to battle rifle in the field. It is the nature of the beast. That was the problem with the M21.
Everyone figured that since the M14 NM could shoot like a house afire it should make one hell of a sniper rifle. Problem was the M14NM was babied by the armorers and every night it was twicked. The M21 came out and rules forbade anyone but NM armorers form touching it and no one but Benning had those. Now add the fact that spring set would very with temp, and your cold bore would change, as the springs warmed up your zero would change as would the barrel harmonics. As a sniper you did not have sighters to get on paper. And when you crawled around some mud hole getting in position your zero was toast. Reason why when we started looking at a new sniper rifle in 85, all semis were banned from consideration. None could pass the field shooting part.
Now can you have a very accurate M14, hell yes, will it stay accurate in the field, hell no. The closest we came was with the stock insert and scope system made in MA by an old man whose son was a sniper in Viet Nam. That was Brookfield Precision and our incarnation was the M26. The Army dropped it due to Gen Guest having backed the M24 so hard, and the Navy took it over. It still went no where though.