The low sprocket rating of 40-42 teeth is because of the angle the derailleur is designed to travel & be a moreorless consistent distance from the relevant cog.
Too close & the derailleur will shift by simply crashing the chain into the next cog or dragging it away.
Too far & the angle necessary to derail the chain won't be enough. Shifting will be vague & inconsistent.
There is little safety risk in installing a cassette too small, but at best it'll shift like garbage. Even if you get half the cassette to "sort of work ok" the B-screw adjustment will only be appropriate for one side of the cassette or the other. The other half stands a good chance of not working at all.
The key to smooth, reliable shifting is the derailleur "floating" a consistent & proper distance iaway the cogs. So that the chain is guided over until it self selects the cog you desire. This is the function of the B-screw adjustment. The angle of the parallelogram, spring tension, cog stack, & other factors are specially selected to this end.
Last edited by base2; 07-24-22 at 11:42 PM.