The barrel, bolt, and upper receiver for my ongoing MK12 Mod 0 build came in yesterday from Les Baer. What a beauty the barrel is; it's a shame I'm going to have to have it turned down. It really isn't as heavy as I thought it would be. If you can't see what the cartridge it's chambered in is from the pictures, it's the .264 LBC AR.
The barrel extension fits very snug in the upper receiver, which I suspected would be the case considering that Les Baer rifles are reported to shoot so well. Normally when you slide the barrel into the upper, there is some notable play because the inside diameter of the upper can not be of the exact same size as the outside diameter of the barrel extension. I have an AR with a .004" difference between barrel and receiver that I seated with a bed of Locktight to help with point-of-aim shift when the barrel gets hot. To do that, you literally coat the extension with Blue Locktight and bolt it onto the receiver. Another remedy is to measure the difference betweem the two like I did, and then buy the appropriate sized shim stock to fit the pieces together snug; this I learned from reading tips and tricks from Highpower shooters. I won't have to do any of that with these parts.
Weird thing is that the Les Baer receiver needs to be modified slightly in order for it to fit onto the DPMS lower receiver. The front take-down pin goes through the upper just fine, but I have to put a good amount of downward pressure on it to get the rear take-down pin to fit. I don't necessarily have a problem with this because it makes the upper and lower fit together like a vice; I just hope that there's no issue with the bolt carrier binding.
For fun, I tried to bolt the barrel up as it is so that I could shoot a few rounds downrange and have the once fired cases. Using these cases that have been stretched to the inside dimentions of the chamber, I would then fill them with water and note the difference between the empty weight and water weight. The figure you get from that gets entered into QuickLoad, and you can predict what a bullet/powder combo will do before you even make it. Too bad though that the barrel nut from the PRI handguard will not fit over the barrel, so it's off to ADCO for some turning on their lathe. Once I get the barrel back from them it's getting a coat of parkerized-black Alumahyde from Brownells -- can't have the Tangoes (groudhogs) spotting my shiny barrel, now can we? I'll also coat the Les Baer upper with the same dark earth Alumahyde coating as the rest of the rifle.
I expect a range report in the next three weeks or so.