Requires main game only.
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You have two versions of the perk:
1. The "purist" version - as the creator intended - checks correctly if the target has detected the player and uses the animation type of the 1H pistols (for missing pistols or 1H SMGs) and the original list for 2H SMGs.
Only Sneak Critical Attacks will be boosted by 20%.
2. The boosted version - All Critical Attacks will be boosted by 20% if you use any 1H pistols/SMGs or 2H SMGs included by the creator in the original list.
3. Due to the improved conditions of the perk, both versions will work with any 1H pistol/SMG (even a custom-created one) that "walks" across the Nevada desert.

Both versions of the perk will work extremely well in conjunction with Light Touch perk - the SMGs (silenced or not) will be particularly boosted (higher chance per shot to score critical hits).

New in version 1.2 - Boosted version
Increased a bit Guns and Energy Weapons skills requirements (35 -> 45). They were a bit low in version 1.1, if you have the trait Skilled.
No real need for 70 Sneak since all Critical Attacks will be boosted.

Have fun.
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The new weapons from DLCs are added to the original TheProfessionalWeaponsList via the next scripts:
NVDLC02MQ00Script
NVDLC03MQ00SCRIPT
NVDLC04MQ00SCRIPT
NVDLC05MQ00SCRIPT
...so any mod that manually modifies the original list with DLCs weapons will glitch the vanilla scripts if you uninstall that mod (see Posts section).

Credits:
Bethesda/Obsidian.
Celestial mechanics.
Me.
Image from: http://fallout.wikia.com/wiki/The_Professional
Perk Hitman - from Challenges Perks Traits NVStyle - a FO3 personal mod.

Many thanks to:
Nexus - for allowing us posting all kind of terrible ideas converted into mods.