In recent weeks I have assembled and tested several upper and barrel combinations.... I revisited the issue of short recoil on several occasions... most could be traced to dirty chambers on used barrels (great info from this board!), but, sure as hell, on the last two rifles I assembled, all brand new parts, one worked find, and one short recoiled... I switched out the bolt carrier, also new & same manufacture, and the troublesome unit worked perfectly... and as expected, both rifles then ran flawlessly through about a hundred rounds each...  ones and two, fives and ten shot strings, fed, functioned, locked back every time!! No matter how light you held the unit....  Now, here is what I think is happening, either the uppers are experiencing some distorsion during the manufacturing process, or during the assembly process. Because the relief slot for the carrier key seems to be of a real close tolerence to the key, and also the 1" dia. bore in the upper, shows to be somewhat less in the middle of the receiver then at the ends! Now, I know what you're thinking... he's squishing the hell out of them in the holding fixture in the vice...  and normally I would see it that way also.. But I've been checking them as I get them with a straight piece of alum. tube that measures .9975" and almost everyone of them from several manufactures, is tight in the center (some as much as .010" overall)... also, I have taken great care to fixture them properly in the vice during assembly, with the proper tools, fixtures, etc, etc,...  Also, the receivers are tempered, and I tried to spring one to spec using a piece of plastic bar, it would distort a bit, then spring back to it's original spec. I figure they most likely deformed before the heat treat & anodize stage of their manufacture..  I noticed that in the Les Baer add for their uppers, they noted that their uppers are honed to the correct size... I am beginning to understand the importance of extra care like this...  although, I must admit, with a little fuss & muss, these rifles turned out fine...  so, who knows! BTW, does anyone know the correct dimension of the slot in the top of the receiver for the bolt carrier key relief??? Thanks, till...later...GT