If you have Tier 3 of Rapid Reloading, it makes particle weapons reload 30% faster. As the Big Bang is a particle shotgun, it benefits greatly from accelerated reload considering how close-quarters shootouts that it's best for can get hectic.
However, there appears to be a bug in its Effect Sequence form:
WeapFX_AmmoCount_BigBang_Seq01 [EFSQ:000CC4D4]
...where the display will not reappear after the reload is complete. It will reappear if the weapon is un/re-equipped. Normally without the Rapid Reloading perk, the display will go blank, then reappear once the reload is complete.
Effect Sequences are a bit of a black box; I was unable to make sense of it and find a setting that may make it amenable to accelerated animation speed, but I did discover that removal of a component will keep the display constantly visible even through a reload, with the ammo counter resetting after it is complete, and with any potential appearance of the "Last Shot" alert exclamation triangle also resetting.
The errant visibility control in the Effect Sequence is fxReload >>Display_Main_A >> Visibility.
On one side, the devs did intend to have it disappear during reload. On the other, it is obvious that it was not intended to
remain invisible after a reload is complete, since this change in behavior only starts after getting Rapid Reloading.
Should it get fudged a bit with removal of the errant visibility control, so that the on-gun ammo readout will be visible after a Rapid Reload?