Breechblock pin

[DonVoigt]

From the looks of that pin, it was worn out; and likely placed into your replacement toggle which I would bet is mismatched or worn also. It may not even be an original pin, but one made from a drill bit or rod; if an "inch" dimensioned piece were used, it may also have been undersized as the original is nominally 5mm. 3/16" is close, but about 9/1000" undersize to start.

The drawings show the hole in the breech block to be spec'd at 5.02 + 0.05mm.
The hole in the toggle at 5.05mm +0.1mm
The pin at 5.0 nominal is spec'd at 5.00mm -0.05mm

Hence designed with about 0.1mm "clearance" or about 4/1000 of an inch to the toggle, and about twice that to the breech block.

These short, "fat" pins are quite strong- they don't just "break".
The shape of the ends of the pins-flat or slightly concave - would be irrelevant.

You guys are way overthinking this, JMHO.