I will try to answer best I can.
First, like everything else a poor job, be it lapping or machining will cause many other problems that otherwise would not exist so I lap after the machining is done on the recoil surfaces to reduce the resistance to lift the bolt after firing , because as you know We tend to load very heavy and bolt lift can be difficult. Bench rest folks normally don't load hard so this is not a problem for them.Also even the machined surface is not as smooth as it can be with lapping.
And yes the trigger/cocking piece, does apply upward force to the bolt and it tends to ride on the top of the bolt while cocked. with a preferred tolerance between the bolt and the raceway of .001 to .003 this allows the bolt to rise off center by .0005 to .0015 (Not very much misalignment). but upon firing, the bolt will seek true center and this is where I want it to be and have machined and trued for. The bolt cant misalign the case head permanently but 55,000 to 65,000 psi can.
Reaching these pressures takes Milliseconds and the bolt will be forced into what ever alignment it allows. This is also the reason we machine the lugs and the abutments square to the bore center line. If you have .0000 head space this phenomena is not encountered because the bolt will be self aligned by compression. most smiths prefer head space of .0005 to .0015 for accuracy and .001 to .004 for function. in these cases the cocking piece will thrust the bolt upward and remove the clearance on top until the sear releases the cocking piece.
I was trained by a Bench rest shooter/builder and many of his philosophies carry over to our sport but not all, because of different requirements between the two sports.
I hope this explained why I do things the way I do. Others disagree with some of these philosophizes and that's fine This works best for me.
PS: Don't worry about challenging or questing something I do, Because I don;t know everything about anything, and can learn something if I am lucky. I only post what works for me.
J E CUSTOM
Too funny! I never mentioned that I also lap AFTER machining. I don't lap to improve the fit. I machine to improve the fit. But I do lap to improve the finish and I also use a spring loaded tensioning fixture that screws into the barrel tenon threads at the end of the receiver. I lap for the same reason as you do - not to square the lug fit or to improve the % coverage, but simply to improve the finish and smoothness of the bearing surfaces and reduce friction. I doubt I remove more than a few tenths but I've never measured it. For me, it's not lapping in the same sense that others use the term. I guess I prefer to say polish the lugs..... AND THEN GREASE them with a good anti-sieze grease - another thing most shooters seem to overlook.
Great post as always JE!
Your assessment of the time factors involved in any movement of the bolt is interesting. I don't recall exactly where I read it but it wasn't an authoritative source. It was just an off-hand comment someplace. I'm thinking it was someplace like froggy's facts or an article on primer brissance testing. Regardless, the author made the point that "lock time" is much more critical than most of us think because the lock-time is the longest duration event in the entire sequence of internal ballistics events. By his definition, lock-time is the time interval between trigger drop and primer ignition. According to David Tubbs, the flight time of most bullets through the barrel is 1.0-1.5 milliseconds while the lock time of most conventional bolt action rifles varies between 2.6 and 9.0 milliseconds. When one considers that the bolt body is at least an order of magnitude heavier than the firing pin, it becomes difficult to predict how much the bolt can align itself in response to the firing pins spring powered trip forward and whether it will bounce a bit after arriving. Similarly, a calculation of acceleration due to gravity shows that the bolt CAN drop to the bottom of the raceway if the lock-time is long enough. For short lock times, it cannot. In other words, the bolts ability to drop or align itself appears to be highly dependent on lock time as well as the clearance between the bolt and the raceway. The overall physics of that movement is quite complicated.
You stated a clearance of 1 to 3 thou. So I assume you are using custom fit bolts too. My measurements of factory bolt clearances are more like 10 to 20 thou. Once in a blue moon I find a good one. It's always been a parkerized bolt so I have assumed it's probably the coating that achieves the fit. That's a purely anecdotal observation. It's probably statistical garbage so I don't lose sleep over it.
Until the primer ignites, the only thing reacting the firing pin spring is the bolt body and locking lugs. So perhaps it is possible that the act of driving the pin forward pulls the bolt body back against the lugs and starts the alignment process. Regardless, it seems to me that there is a whole lot of clattering around that is possible before the rearward pressure of the igniting charge takes over.
I certainly agree that the charge pressure will align the bolt in microseconds once it gets going. The high rear ward force on the bolt face totally overules gravity, spring pressure, inertia, and everything else that is going on.
In the absence of any information to the contrary, I have to agree that machining everything to the pressure centerline is the best way to do it. That's why I also machine everything to the raceway centerline. One has to begin someplace and since the raceway centerline is hard to change, I feel that it's best to leave it alone and make everything else as concentric to that baseline as possible.
Nonetheless, I remain quite intrigued by that long period of clattering around that takes place before chamber pressure becomes dominant. Assuming that Tubbs is correct, it seems to me that due attention to minimize and/or at least control (make it repeatable) all that movement is a worthy goal.
Tubb advocates a lighter pin and stronger spring to reduce lock time. While I accept that, I have not actually seen a difference on targets. Perhaps this factor depends on bolt clearance...... I've never tried lighter pins on a high clearance bolt. Given the crude timing and dimensional analysis above I suppose it's also possible that the two approaches both accomplish the same thing in different ways. It's not like bolt drop is an order of magnitude faster or slower than the lock time. They are both quite similar. Therefore it's possible that tightening up the fit probably accomplishes the same thing as a faster lock time.
Which brings me to my latest fascination. Trigger over-travel. Many shooters assume that minimum over travel is a worthy objective. Less is always better...... NOT! When champion shooters like Boyer, Ratigan, and Tubbs say otherwise, I listen and therefore I have always adjusted triggers for MAXIMUM overtravel. Furthermore, I believe maximum overtravel becomes more and more important as trigger pull settings increase as they do on a hunting rifle. Higher forces significantly reduce the time it takes to reach the hard stop as well as the inertia that gets transferred to the rifle when that hard stop is reached. Both factors significantly increase the amount of movement the rifle will see in those critical early moments before it even fires.
Enter Trigger Tech. One cannot argue with TriggerTech's success in the market and more importantly on target at competitions. Yet their design and even their advertising claims zero overtravel! How can this be? I don't know the answer yet. I have written to them to see if they will provide a technical explanation but have heard nothing back yet. The only thing I can come up with myself is that there is no movement or trigger tension release (only increasing force) in the Triggertech design and therefore there is no inertia or momentum to transfer to a hard stop........ Only time will tell........ Too much fun!
Again, thanks for sharing!
