There was a short run of early XCR's where, and I quote
We've found that some XCRs can slam-fire due to extremely sensitive primers. We have added a firing pin buffer to eradiate the problem. The following is a list of rifles which need this firing pin upgrade. DO NOT SHOOT THE RIFLE UNTIL THIS UPGRADE HAS BEEN MADE. We've also made the trigger more consistent. The attached list also shows which rifles need the parts necessary to make the trigger more consistent.
The list of affected weapons is as per the attached link on RA's web site