If it's only with MP53 weapons it may be a bug related to the new ammo pooling feature. To test, you can open the gunspec and comment out or delete the lines near the top that look like
poolammo = 556NATO
altpoolammo = 40MM
to comment out just ad a semi-colon in front so it looks like
;poolammo = 556NATO
;altpoolammo = 40MM
If that solves the issue then it's something that will need to be fixed in FPSC for v1.19. Please note that disabling ammo pooling may require you to edit the ammo FPEs so that it gives ammo for the specific weapon used rather than the default weapon in the ammo pool.