This is just speculation, based on an example of one. But a while back I cut a new crown and oversized the scope base holes for a guy on the receiver of a 640. Judging from the swarf, the difficulty of the drilling and tapping operation...I got the definite impression that the 640 is made with something notably tougher than the 4140 of the Winchesters, and I have worked on a lot of them over the years. I am used to using sulfur and lard oil on my HSS tools, nope, went to cobalt and Rapid Tap after the first bite. The swarf from the cut-off and crown was like a coil spring steel, a continuous chip. Unscientific, but gut feeling is the 640 uses steel tougher than I am used to.
That or their heat treating process was different or inconsistent. Neither great, as that actually causes embrittlement. This was common in some early manufactured 1903 springfields and some eddystone manufactured m1917's. I always thought I'd like the 640, until I handled them and really looked them over. They seem to be great rifles that are comparable to the pre 64, but less refined like most mauser 98 derivatives. Basically an FN 98, which is a great platform for a build. The 640's I've handled were smooth, but a little clunky compared to the pre 64 model 70. Generally the 640 can be found for a great deal, if you can find one. They seem to be getting harder and harder to find. I wish I would have purchased the ones I have seen in the used rifle racks over the years. The most recent one I handled was only $400.00. A bargain, as compared to most pre 64 model 70's I've seen...
