Some additional on the 6.8 SPC. There are still some special ops units over in the sand box using this caliber. In addition the military did some testing on it in the past couple of years and had it in actual combat. Most preferred the round well over the 5.56. However, looking at the overall picture logistically and with the money available to the military due to the economy it was a dead issue actually before it started.
Can you imagine all the extra M-4 and M-16 parts the military has in stores as well as 5.56 ammunition. Not to mention, in a foreign war theater a lot of our allied nations have the M-4 and M-16. The new 6.8 would require a massive ammunition change over as well as magazines for all the 6.8s which would be an astronomical expense. They could get by with changing the Bolts on the existing rifles but then due to some being worn from use other issues would have to be addressed like putting new bolts in and possible head space problems and other maintenance issues. The military just does not do things that way. SO it would be all new rifles to trade out the M-4s.
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-- John F. Kennedy