feed ramps as Ron stated.
It appears to me that the 6.8 has become one of the most popular variants over the past year. I base this on the ammo sales, the additional new bullet selections coming to the market, and the over all discussion level across the net. Then, I look at the Xtreme which received 300 pre-orders in one month with no advertising...just word of mouth on this Board with many more clamoring for information as to when they can get on the list....and I think that it is the tip of the iceberg being serviced at this time.
Much of this growth in popularity has come from all the testing and development that has been carried out in the past year which has resulted in much better barrels and chambers as compared to the original design. This has allowed for more velocity, and the resulting increase in energy, over a longer range so that the round is very competitive with anything else out there in a similar caliber. Kinda like "This ain't your Father's 6.8".
One of the nice things about the round is that it works extremely well in a short barreled configuration with good velocity and it does not need an extremely long barrel to approach the high velocity limits.
One of the things that make it extremely attractive to me is the community of people involved with its development. I don't see a clique of supporters that appear to spend their every moment running down competing rounds. That has turned me off of some calibers that are quite good in their own right.
I think it is the modern day equivalent of the 30-30 on steroids...it does everything well, and, somethings superlatively.