History Primer 196: Spencer 1886 Shotgun Documentary | C&Rsenal

It appears from the animation that if the magazine is empty and the slide is fully to the rear, one could manually push the breach block all the way down as the breach block is only held up by a spring, and not any part of the action bar stud. This would expose the chamber for single loading. Once a fresh cartridge is in the chamber the breech block should rise again to full height under that spring that ejected the previous fire cartridge. The right side extractor would just skip over the cartridge rim and allow the slide to go fully forward to center the firing pin on the primer.

Given the age of the gun, I'm assuming those were 2 1/2-inch (not 2 3/4) shells loaded to the low end of the pressure scale.

Will this thing slam fire?

My best guess as to the single loading claim: you could pull the slide all the way back, "single load" by topping up the magazine tube, and slide forward. Thus the magazine always remains full in reserve and you load one each shot.