So long as we are having this debate about the details of which spring does what, let's get the designation right. The striker spring is the mainspring. The recoil spring is NOT the main spring. In any breechloading, caplock, flintlock or snaphaunce firearm, the mainspring is the one that makes it go bang. Hence, in the Luger the striker spring is the main spring. All of these firearms have a mainspring, the majority do not have a recoil spring. In some itty bitty pocket pistols, the mainspring and recoil spring become one and the same spring. I am referring to English terms here without regard to German ones.
It's probably better to refer to them as striker spring and recoil spring and side step the confusion. Referring to the recoil spring as the main spring probably happens because it is the biggest spring. But keep in mind that Thor, unlike us mere soft pawed mortals, is in the habit of shooting the cannon with no recoil spring present. But he is still using the striker or main spring.
The recoil spring exerts a closing force on the toggle train at any and all positions. In the last stages of closing, the striker spring presents an opposing force. It then becomes a balancing act as to which will exert the greater force. Which one dominates depends upon the individual pistol and spring set. Momentum does play a significant role in the final stages of closure, paricularly with a light recoil spring. If deprived of momentum by holding the toggles and allowing them to slowly close, some Lugers will close on spring forces alone, some will not close. Keep in mind this only counts when loading a cartridge. The principal difference is in the strength of the recoil spring.
Minus the recoil spring, the striker spring will tend to keep the action closed when it is in battery and cocked since the "knee" is past center. This is a rather small force though. If not cocked it will have no influence.
When in battery, the closing force of the recoil spring is required to insure that the action is not opened by the random forces of handling or dropping the pistol. This force is of liitle or no significance in keeping the action closed at the time of firing. The back thrust tneds to hold the action closed due to the "knee" being below the center line of the front and rear toggle pins.
In contrast, the Erma Luger look alikes are blowbacks, the knee never goes below the center line and the spring force is absolutely required to keep the action closed in handling. Strangely enough, the spring force is not very significant at the time of firing. here, the inertia of the breech block and toggle train tend to oppose opening and slow it down to the point that the bullet has left the barrel and the pressure has dropped before the action has opened far enough to expose more than the web area of the case. The spring forces are then significant for decelerating the action while momentum opens it the rest of the way.
The spring force is not significant in holding the action closed at firing in the blowback because the backthrust is on the order of several hundred pounds while the force of the spring is a few tens of pounds. The case expands and drags on the chamber walls providing more resistance to opening than the spring does.
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