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.
It is possible to get the gun from the safe without picking the lock, if you have Dogmeat with you, you can order him to find you a weapon and he can take it from the safe without opening it, although this may take several tries as there are a number of weapons to be found around the area.
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 be sometimes 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. A good way to do this is to leave your follower(s) in the room with the safe, and start a fist fight in the hallway. Put your fists down and the Brotherhood won't be hostile, but chances are your follower(s) will kill the person who grabs the pistol.