If you want portable bullet count, you use an automatic M4 (750+ rounds per minute).
If you want less portable bullet count, you use a M134 Minigun (2,000+ rounds per minute).
If you want single-shot spread, you use a M500 shotgun.
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Compare Metal Storm's prototypes, which used stacked charges to increase bullet count (but decades later have yet to see practical implementation).
Another use is as a replacement for the Claymore mine. It's a minor upgrade. The advantage is increased safety for friendly personnel in the non-target directions. Likely disadvantages are cost, weight, and bulk.
It feels like we've mostly converged on the ideal form of a general purpose battle rifle over the past fifty years.
Based on this, it seems like multiple barrels probably wouldn't affect each other in different ways, leading to shot consistency as long as there aren't significant differences in ignition time or velocity.