SDC - getting close to your target may not pose a problem for escape. It all depends on how patient your assassin can be. Using a .22LR cartridge and pressing the muzzle against the victim's spine through a heavy coat will muffle the noise of the shot. Though most .22 assassins seem to have preferred the muzzle under the ear technique. If you equip the unit with a small suppressor it'll have the sound volume of a facial slap.
Many "Mob" assassinations used a .22LR pistol, like a Beretta 72 or Browning .22LR, modified with a suppressor and a slide-lock to prevent ejecting the empty case. The victim would be confronted alone in an elevator, alleyway, parking lot, etc. and removed from the gene pool quietly. The problem, of course, was if the authorities got ahold of the pistol and could track its purchaser.
Pen guns were used for a time since they could be made by any competent machinist and disposed of in a folded newspaper or even disassembled and tossed away in a crumpled cigarette pack. But if the attempt failed, the assassin had to resort to less stealthy means to finish the job.
A pen gun is either an assassin's weapon for use up close and personal, or it's a last ditch surprise weapon, much like the
.32 caliber Palm Guns of the 1880's.
A museum once displayed a "cane gun" for espionage use that was quite ingenious. Single shot, of course, but when deployed both a front post and rear V-notch sight would pop up on the cane and be triggered by a stud on the handle. The barrel was some 20" long and the last several inches housed a suppressor that "sort of" worked. Accuracy was pretty good out to about 80 yards.