This drawing is inspired by the anthro MLP setting where Skyfall OC operates as part of Princess Luna's own security detail.

Princess Luna inserted a reloaded magazine into the grip of the massive semi-automatic pistol. Cycling the action she raised the pistol and started to take aim, but then paused and lowered the weapon. Remembering the first magazine she'd fired from the behemoth pistol her horn flared and small force wall appeared in front of her face. Once again she raised the weapon and pressed trigger.

The pistol roared and rocked in her hand as it sent a 11.45mm projectile rocketing downrange. The alicorn was very happy she'd remembered to erect the force wall as she watched the spent brass casing bounce of it instead of her face. The first time she'd fired it several of the spent casings had struck her.
The giant pistol thundered 7 times and then went silent. Calm returned to firing range. Removing the now spent magazine and confirming the chamber was empty Luna set the still smoking pistol down on a bench and looked over at the pony who was creator of this monstrous mechanical creation. The stallion anxiously smiled as he addressed the princess.
"So your highness! What is your opinion? Would you consider the possibility of your security forces adopting it? Yes it is a rather large pistol, but with a shoulder stock attached the pistol can also operate as a small semi automatic carbine. So it is multi-purpose system, and 11.45mm cartridge is without equal at this time!"
The Prussian Blue alicorn looked back at the pistol on the bench and then over to it's creator. She arched an eyebrow as she spoke.
"The problem I see sir. Is that I haven't test-fired a very large pistol today, but instead a very small field gun. The weapon is large to the point of encumbrance. It fires an excessively powerful cartridge for the job. It has more parts than my pocket watch. Even worse it has an alarming tendency of hurling spent cartridge casing into face of the operator with great gusto."
At this moment Luna glanced down and noticed there was a spent cartridge casing lodged down in the cleavage of her breasts. Reestablishing eye contact with the stallion, the mare casually reached in with one hand and extracted the wayward piece of brass. She then flicked it away.
The stallion swallowed and looked the princess eyes. "So, um. That's a no."
Note. The pistol in this drawing is a Gabbett-Fairfax Mars pistol. They were a series of early 20th century pistols. The Gabbett-Fairfax Mars, not to be mistaken with several other pistols from other period manufacturers that were also called Mars, never went into proper manufacturing. Instead the Mars were a series of unique, handmade, prototypes. Each one slightly different from its brethren with a wide range of features, calibers, sights and barrel lengths.

Ian McCollum at Forgotten Weapons has in the past has done a couple of video about it. Link. Link.