I've built a G&P WOC M4 as a DMR - it's fairly consistent and will hit a body sized target at 40-45 metres, but you need to be willing to put down a lot of money for a very nice wide bore barrel, R-hop, bolt with NPAS, decent magazines and the rest.
Also a lot of time and patience. I wouldn't recommend it.
Oh and miracle barrels are over-advertised, average barrels that have made it in the states where they tend to buy most of their kit domestically. Cast the net wider and ORGA, PDI and Laylax/First/Prometheus make far better barrels. Wide bore is your best bet with a GBBR as your power source is basically limitless. Turning off the NPAS on mine gives me 520fps on Guarder black, so making up for that extra bore diameter isn't hard.