I have good feedback on M4C, which you can check out in addition to my feedback here.
This backup upper has ~120 slow-fired rounds through it for function testing and load selection. It's in very good condition -- might as well be brand new for all intents and purposes. Loves 77gr SMK's unsurprisingly, and factory MK262 offerings from Black Hills shot very well. After the initial ~120 rounds it was coated with a heavy coat of CLP and then left untouched and safely stored. Shoots very accurately as all BCM SS-410 uppers do (less than an MOA). If you have any other questions please feel free to send me an email and I'll answer them.
In the pictures you'll see the upper with various attachments, but what I'm selling is just the complete upper (with BCG, charging handle, etc.). Doesn't come with the BUIS or sling accessories. Here's the link to the pictures:
Price from BCM is $1229. I'm selling it for $950 $925 plus shipping.
Please do not PM me on this website in order to contact me as I may not see it. Instead, email me at firstname.lastname@example.org
The 0 is a zero.