Google "colt 6920 problems" and you will find about a million threads about issues with what you think is God's gift to the AR world. Everything breaks, and there are lemons on the market from every manufacturer. Stick with the ones mentioned above and you will be fine. Most of these manufacturers have put out stuff that has been torture tested by some LE agency and adopted. Don't worry about reliability as much as features for the price.
BTW, "Colt 6920" is actually a word and a number.
Google "Toyota Corolla problems", "Apple computer problems", "Acme Brick problems". If it sells in significant numbers, there will be a bunch of people with problems.
I have a Wyndham Weaponry HBC model AR. The HBC is for heavy barrel, to stand up under continuous fire and not warp. Wyndham is a company started a couple years ago. It is in the Wyndham, Maine plant where Bushmasters were made until the
Greedom Group moved production to Remington in New York. The founder of Wyndham is the guy who rescued Bushmaster from bankruptcy in the 1970's. He built it into a good name, then sold to
Greedom Group with a five year lease on the plant and a five year non-compete for himself. When the lease was about up, Greedom announced they were moving and any employees who wanted to move to New York were welcome to apply for jobs at the Remington plant.
The lease ran out, the owner of the plant and former owner of Bushmaster decided to start up again. He had a trained workforce from which to choose the best new employees, and he had the desire (so they say) to reverse the corner cutting that
Greedom Group had phased in over the last five years. It's a great story. The reviews I've read about the rifles are excellent. The rifle has a lifetime warranty, although I've had no opportunity to test it. My experience with the Wyndham HBC is no problems of any kind in the last year. The price was around $300 less than the Colt 6920. I can think of nothing bad about this rifle.
That said, I suggest that you buy a Colt. Three years from now, the $300 will have been forgotten, but you'll still have a Colt. Colt is the original manufacturer of the AR15/M16/M4 rifles. The gun has 50 years of testing behind it and has been slowly upgraded by actual combat users experience (I was one of them in 1967-68) and is the culmination of that 50 years of development and evolution. Even though I have nothing bad to say about the Wyndham, I still wish I had bought a Colt, and unless Comrade Obama shuts us down, I probably will in the next couple years.