He's not demanding a change. He's asking for openness and competition, which seem like the American way to me.
I might be inclined to agree if we were talking about purchasing a new weapon system; but we are talking about purchasing another batch of a weapon system we have already adopted. Colt is the sole manufacturer (currently) who has the legal rights to the M4 TDP, so they are the only people to buy the rifle from. It isn't like DoD could walk over to Bushmaster and buy them right now.
If Sen. Coburn is suggesting that we should have new competitive trials anytime we buy another batch of small arms, he is introducing even more expense and problems into the process - and one of the "problems" with the M4 is simply that some carbines have been in service for decades now and need replacement and/or repair. Adding a competitive bid before that can happen means less rifles for the same money and a longer wait for the replacement.
As to special operations guys using the HK416 - they have different requirements and some of those requirements (shorter 10" barrels and sound suppressors) are areas where a gas piston has an edge over a direct gas system.
For all the people pimping the HK416, I've yet to see any key pieces of data from H&Ks marketing department like:
1) What is the difference in the mean rounds between stoppage between a newly issued M4 and the new HK416 over a random sample of 100 weapons?
2) What is the total cost (logistics, new rifles, etc.) of replacing the M4 with the HK416?
The threshold MRBS for SCAR was 1 per 2000. Usually the threshold requirement indicates what is already being achieved, so let's assume that the M4 is making 1/2000 MRBS. The highest threshold number I've seen is for the Swedish AK-5 at 1/3,500 MRBS. How much money is it worth to have three less stoppages in 10k rounds?
If we all took an objective look at the two different systems. One blew hot gas and crud into the bolt carrier group and the other used a piston to move the bolt.
Objective eh? If we all took an objective look at the two different systems. One torqued the bolt carrier group violently with every shot and flexed the barrel and upper receiver. The other used the gas impulse of the fired round to move the bolt straight back with minimal weight or disturbance. Which would you choose? Everything in engineering is a tradeoff - you don't get something for nothing.