I was looking at buying a colt 6940. A buddy tells me he has read the military is having problems with the 6940 wearing out too soon. Since these are carbines the gas pressure in the shorten gas tubes causes a lot of wear and tear very quickly on the upper and I am likely to only get to shoot 5000 rounds before needing a new upper. I understand what he is saying, but cannot find this information on the internet. Anyone know anything or have any info? I initially was thinking about getting a standard ar15 a2 with a 20" barrel. Can I replace the carry handle piece with a flat top piece?