As an FYI, ShootingSight should be releasing a tool steel hammer for the M4 in a couple of months.  If you are interested, please email [email protected], and we will get in touch as soon as they are available.   These will be CNC machined Rolled Plate S7 Tool Steel, so the highest grade you can get.  Price TBD - under $100.  It will be 100% US made, so 922R compliant.   Art Neergaard ShootingSight llc