Just to make it clear, the auxillary spring also has to be overcome to "open" the toggle,
this is probably why the carbine ammo was loaded "stronger".
More spring pressure from the Aux. spring was needed to reliably close the toggle, which required
"stronger" ammo to open the toggle and achieve full recoil(and thus allow the hold open function). JMHO.
The Aux. spring works in conjunction with the mainspring to cycle the action, a much stronger mainspring would have sufficed, but engineering limitations of space and flat springs struck the engineers!
The carbine with coil spring can be made to work without the aux. spring, even with a forearm.