This is best suited for the Hek due to its high Puncture damage.
When calculating total raw damage, a weapon will require a Puncture damage percentage equal to 75% of its total base damage to equal the raw damage increase provided by a standard elemental mod (which increases corresponding elemental damages by 90%). The higher the Puncture damage percentage over 75%, the more viable this mod will be as opposed to an elemental mod, and vice versa for lower values. These calculations only pertain to raw damage, and do not take into consideration enemy armor/flesh/shield damage multipliers.
As of Update 20.4, there are no Shotguns that meet this 75% Puncture damage threshold. Even the two shotguns with the highest Puncture damage percentage (the Hek and the Vaykor Hek) have Puncture only making up 65% of their total base damage.
One of the few mods that depicts a Warframe with non-standard color schemes, in this case a Nyx with her Immortal skin.
Breechloader is the correct spelling. A breech is the part of a cannon behind the bore. A breach is an act of breaking or failing to observe a law, agreement, or code of conduct; or a gap in a wall, barrier, or defense, especially one made by an attacking army.
This likely an intended deviation, as the mod does not affect reload. The name simply being a wordplay that combines gun terminology with a word related to puncture.