Jack, two other thoughts come to mind, but they are just speculation. First, if the first shot produced seriously abnormally-high recoil, that acceleration alone could possibly cause the second sear to disengage. Second, if the crack was opened by the first shot (abnormally heavy or not), and altered the sear-to-bent geometry, that could cause disengagement.
I agree that KNOWING what's in ALL the remaining shells is critically important. If I were doing it, I'd send three to Tom Armbrust.
Regarding the post-color casing condition of the receiver, either insufficient annealing before color-casing, and/or quench from too high a temperature could seriously pre-dispose a frame to cracking. Here are two posts by Oscar on the subject:
Re: Re-Case Coloring and Steel warpage? (J. Stephens)
Date: Tue, 14 Oct 1997 00:28:18 GMT
From: Oscar Gaddy <o-gaddy@ux1.cso.uiuc.edu>
Yes indeed, warpage and shrinkage can occur with color casehardening, but with proper precautions and pretreatment, it can be minimized to practically no problem at all. I believe that Peter Johnson's book refers to the hard fitting that Roy described in more detail. I have been told that Frank Lefever, at one time, had the jigs and fixtures used by Parker Bros. for this purpose. The new owners of Lefever Arms may or may not still have them--I know that they sold the Parker barrel rib milling machine about two years ago.
The most significant cause of warpage is the internal stresses that pre-exist in the part that is hardened. The quenching from a high temperature can exacerbate these stresses and causes warpage. To eliminate this, I always anneal by heating parts to be re-case hardened in a controlled atmosphere to 1500 degrees F for one hour and then slowly cooling for several hours to room temperature. This treatment relieves all internal stresses and diffuses the carbon in the hardened surface layer throughout the material leaving the parts as dead soft as virgin mild steel. Even with this precaution, thin parts such as tangs will tend to warp and, to prevent this, I use spacer fixtures around the tang screws to hold the top tang and trigger plate tang in the same relative position with respect to each other during the quench. Another cause of warpage and shrinkage is quenching from too high a temperature. I use the lowest quench temperature that is possible to obtain the proper hardness and the correct colors. Using these precautions, I very seldom have any problems with warpage and shrinkage and proper fitting after color casehardening. The only time I usually have any problems are with frames that have previously been casehardened several times. In these cases, minor shrinkage can occur which sometimes requires carefully removing some metal from the barrel breech faces for proper fitting or stoning close tolerance areas such as the bolt slot on Parkers for proper clearance.
I suspect, but cannot prove, that if gunmakers such as Parker had annealed their frames prior to final finishing, engraving and case hardening, they would have had much less warpage problems and would have required much less hard fitting. These frames were drop forged, subjected to machining and filing and probably bending all of which would tend to create internal stresses in the steel which can amplify warpage during color casehardening.
More On Color Casehardening
Date: Aug 02, 20:57
From: Oscar Gaddy <o-gaddy@uiuc.edu>
I agree with many of your comments but disagree with several. Your description of the casehardening process is correct for what is called martensitic hardening that is used in industry for hardening parts that are subject to extreme wear. This type of casehardening occurs when the carburized part is quenched from a temperature above the critical temperature of approximately 1340 degrees F. This produces maximally hard surfaces, but unfortunately also usually produces severe deformation and warpage of irregularly shaped parts such as gun frames. It also tends to produce microscopic cracks that can be stress concentration points and result in more cracking and frame breakage in severe cases. This warpage and cracking is due to an approximate 2 % expansion of a martensitically hardened case because of the change in crystal structure of martensite. This type of hardening can be and is used in many industrial processes with appropriate steps taken to account for the warpage and cracking, but cannot be and should not be used on firearms. Consequently, gun parts are usually casehardened by carburizing at above the critical temperature and then the temperature is lowered to slightly below the critical temperature before quenching. If the carburizing is done at a temperature of about 1400 F., very high carbon percentages are produced in the case and upon quenching from below the critical temperature, martensite is not formed, a large percentage of cementite (iron carbide) is frozen into the case and the case is very (file ) hard with very little or no warpage. While the case produced in this manner is not as hard as martensite case, it is more than ample for firearms. In industrial casehardening, wood charcoal is used with an energizer such as barium carbonate. In bone charcoal casehardening for firearms, the tri-calcium phosphate that constitutes about 80 % of bone charcoal acts as the energizer producing the carbon monoxide needed for efficient carburization.
