I bought 2521 recently (16" HF CL MOE Rifle Kit), it is my first AR, but I was surprised to not see a marking (ex. 5.56 HF) anywhere? Is it under the MOE hand guard? Also what is the difference between 2521 and 2551 it says it is "4150 Chrome Lined", I assume that is just a better lining? its $10 more than the kit I bought, didn't see it though when i bought it. Oh well.