Okay I'm a little confused about buffer tubes, the springs, the weight. I bought a 16 inch mid-lenth gas system upper in 5.56. Are all buffer tubes the same length? I know I need a mil spec, I plan on using the mag pull carbine fixed stock, what about the buffer weight in relation to the gas system? Post links if you want, it would help, Everytime I've googled this I can't get the right answer, I'm worried I'll get a tube that is to long for the stock? The wrong weight? I only want to buy these parts once so your feedback will help me a lot. Thanks in advance.