The AEP7 laser pistol was put into service to replace the AEP5 model. With a generous battery size, tight shot grouping at range, resilience to extended use, and fairly decent damage output, it is a very good choice for a sidearm and is used extensively in this role by the Brotherhood of Steel and the Enclave in the Capital Wasteland.
As the successor to the AEP5, it boasted a redesigned focusing crystal pattern, an improved photonic focusing chamber, and selectable laser focus, wavelength, pulse energy and repetition rate. In addition, it was lighter and less bulky than its predecessor and was perceived as a step toward replacing the N99 10mm pistol with a laser weapon as the standard issue sidearm for military forces.
The AEP7 was produced in large numbers thanks to newly developed manufacturing technologies in the United States, although logistics and supply issues (caused by prolonged war with China) prevented its wide-scale deployment.
Smuggler's End shoots twice as fast outside of V.A.T.S. and does 6 more damage per shot than the AEP7 laser pistol. This weapon has a weight of 2, compared to the standard version's 3, and also has an additional 150 HP.
It is Elder Lyons' personal weapon, found in perfect condition, and located in his private safe in the B ring of the Citadel. The safe is locked with a Very Hard lock or opened via Elder Lyons' terminal, requiring Very Hard hacking, located on the desk to the left as you enter his room.
The description for the weapon in Fallout Shelter reveals that the weapon was owned by an "anti-hero" who often took the weapon with them into dirty cantinas, and even had the weapon on them at their death.
Although stored in his safe, the gun is not "owned" by Elder Lyons in game terms. Taking it is not considered stealing and incurs no Karma loss.
This weapon can sometimes be found by pickpocketing Elder Lyons.
There is also a chance if you go on a rampage in the area while the Brotherhood soldiers are sleeping one may pick it up and use it against you.