Looking at the M4 kit from M&A parts. It says the barrel is chrome-moly. whats the difference between the to. Also has anyone build one of these with a eagle lower?

Chrome-moly just means that the barrel (like all non-stainless barrels) is made from standard 4140 ordnance steel.  It is [b]not[/b] hard chrome-lined, and I think vendors like M&A may be trying to confuse new buyers into thinking they're not buying a crappy plain steel barrel when they actually are.

Avoid these jokers and buy a quality, chrome-lined barrel.

Does that mean that RRA, DPMS, Bushmaster, Eagle, Wilson Barrels, and just about everyone else out there is selling crappy barrels? I don't think so!

Chrome-moly (Chrome and molybdenum) are 2 of the ingredients added to most steels, including stainless steel, to improve machinability, hardness. Percentages differ between different steel alloys depending on which property is most important. Chrome adds strength while molybdenum aids in spreading the strength uniformly throughout the alloy.

Listing a barrel as chrome-moly is used to distinguish it from a chrome lined barrel. Even chrome lined barrels are made from chrome-moly steel, since just about all steels have some level of it present in its makeup.