alright well there's a couple issues with an M4 in 308.
1. the M4 is an AR15 type rifle while you need an AR10 type platform to chamber 308.
2. the M4 has a 14.5 inch barrel making it an SBR so none of those factory rifles come with the proper barrel length and you need to file some spendy paperwork with the ATF to make it such.
3. M4 is a select fire assault rifle, all AR15s and AR10s for civilian use are semi auto only, sporting rifles unless properly licensed and registered with the ATF.
now that the technicalities are out of the way. DPMS, remington and Bushmaster rifles are all the exact same so if I were to pick one of those three I would get the DPMS and save myself $500 on the name stamped on the side. rock river arms is a good company and I would pick one up in a flash if I could get a good deal on them but being Obama Scare 2012, prices on everything is outrageous.
availability on all of those are iffy most of the time at best, my bother has been looking for a DMPS ar10 for about 16 months and none of the LGS here can seem to get them in. DPMS stopped taking orders for a while on certain models (probably while the same factory was cranking them out with bushmaster and remington stamps instead) so I would say go ahead and get on a waiting list for a RRA, you might get it before they are outlawed.
all guns lost in a tragic smelting accident.
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