More on hot loads:
There will be a good deal of researching, calculation and experimentation before I come up with any final answers. I am in the process of developing a setup to measure spring strength and am planning some experiments to measure how far back the action goes with various loads. The ideal would be to have the action go back to .001 inch shy of hitting the stops. In the meantime here are some preliminary rough guides.
Usually you buy a replacement spring rated for so many pounds. This rating means the number of pounds force to compress the spring to its fully loaded length. It is an oversimplification as there are several parameters. First the diameter, if this isn't right the spring just won't fit. Second is the bound length when the spring is compressed to the point where the coils jam against each other. Again, if it isn't right it won't work. Since we are buying a spring alleged to fit a Luger we shouldn't have to worry about these two measurements. Springs are available in two types, linear and progressive or variable. For the Luger we want to stick to linear springs. That's the easy stuff.
The next three measurements are free length, preloaded force and loaded force. The last is the rated strength of the spring. In the Luger the loaded length is 1.29 inches. ( 33 mm ) Wolff Gunsprings sells springs for the Luger rated at 36 lbf, 38 lbf and 40 lbf. These are on the heavy end of the scale. The lightest spring you can get away with is 30 lbf.
The mildest commercial load I am aware of in either caliber is the Fiocchi 7.65 load at slightly less than 0.5 lbf*s. This load seems to function with a 30 lbf spring, which is the lower limit for spring strength. I had a problem with the pistol not going all the way into battery when using this spring and some loads. It closed far enough to fire but was not locked so this is a very touchy situation. If your action is a little slicker than mine you might get reliable lockup but WATCH it! If you fire the pistol with out complete lockup you may have a true collector's item. Walk around the range and collect a piece here, collect a piece there.
For the 9 mm load with 124 grain bullet at 1050 fps we have around 0.61 lbf*s worth of impulse. For this cartridge a 36 lbf spring might be about right. If we get the same weight of bullet up to 1150 fps I'd use a 38 lbf spring. At 1200 fps use the 40 lbf spring. For the time being I'll not recommend any load hotter than that. With further research will refine the recommendations.
I have not yet run any actual experiments with impulse at the higher end of the range. All in due course. (You'll notice the 9 mm cartridge seems to be sneaking into my 7.65 mm project.)
Between now and the time you decide to mess with your spring start with the lightest available load and work up gradually until you get reliable functioning. Along the way make sure your magazine is up to snuff and not causing any malfunctions. Start by loading a single cartridge at a time and look for a load just hot enough to make the hold open work reliably. The typical malfunction with a load that is too weak is for the cartridge to stop half way from magazine to chamber but otherwise in a normal position. The breech block will stop with the lower edge against the side of the case a little in front of the extractor groove. This situation results from the breech block not going back far enough and the lower edge dragging the cartridge forward.
Incidentally, 0.5 lbf*s may sound like a small number. It represents a force of one half pound force applied for one second. But in the actual case the time involved is less that one millisecond. Divide the above number by half a millisecond and the actual force applied to pushing the slide back is approaching 1000 lbf. Remember it's all over with by the time the toggles hit the ramps.
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