Well, this is my first post even though I have been lurking here for a long
time.hinkingfrom the same production line (factory) than 6920 ? The quality can be
expected to be the same ?And do the mt6400c and 6920 use same barrels?
Is it a hard operation to remove a welded on muzzle break and how is it done ?

Welcome.
My 6400c shot well from the start. It came with a pinned on comp. This is easilly removed by driving two pins out and sliding the comp off. Very simple. The comp was not a bad feature, although early examples used a roll pin which proved unsatasfactory. Later guns, mine one, use solid pins.
The 6400c also is M-4 chrome bore, seven twist with M-4 feedramps. The rollmark on the rifle states M-4, the upper is stamped M4 it lacks the bayonet lug and uses no threading on the barrel.

 Mine has been modified at GG&G with their front sight, super nice folder, and threaded muzzle which was done to accept the mount for my AAC M4-2000.  
You may, like many here, like the rifle as delivered, brake and all. It is a top quality rifle. I have heard of no dissatasfied owners.  My reccommendation is to buy it.
FWIW, I have seen 6721 guns with chrome chamber only. 6400c are tops.

Quoted: Well, this is my first post even though I have been lurking here for a long time. I’am thinking about buying a mt6400c. I was wondering if it comes from the same production line (factory) than 6920 ? Yes, it's from the same factory. The quality can be expected to be the same ? Should be the same quality, a lot of guys in this site use the parts from it for their custom build projects. And do the mt6400c and 6920 use same barrels? It's the same but I have heard that the newest models are not chrome lined, just chromed chamber. Verify first before buying. Is it a hard operation to remove a welded on muzzle break and how is it done ? Look at the bottom pictures and instructions of this thread. Assemble your own.....Step by Step Instructions! tacked thread link

Welcome to the site.