Depends what you want to achieve and where you play. Some sites will only accept a DMR based on a platform that is 7.62 IRL. If that's not an issue then if you are building a DMR you'll want to think about power level. The whole point of semi locking is to alow you run higher power levels, my local site allows 1.9j for a DMR with a MED of 30M, lots of sites seem to work on 1.6j for a DMR. What power level are you getting at the moment ? a longer inner barrel will generally increase the power level of a GBBR .