[Namikaze]

Kincaid, I apologize for coming off so brusquely earlier.  Truth be told, there's a lot going on IRL that is frustrating me, and I shouldn't have taken it out on you.

I agree with the common lexicon - I tried to see if I could help create one, but it got bogged down in argument about how to define certain terms.  I think we both agree on what is actually happening, and I think we actually have a common interpretation of what is going on with the rules.

[Kincaid]

No worries, the internet is an imperfect place for communication sometimes.

[Shaidar]

Very little data digging uncovered the following (all emphasis are my own):

TASERS
Tasers are legal to carry and a great choice for non-lethally incapacitating someone. Tasers, like other electrical weapons, have a flat Armor Penetration value of –5 and deal electricity damage. Use the Pistols skill to fire a taser. Tasers can take only top-mounted accessories.

HOLD-OUTS
Hold-out pistols are very concealable and are perfect for sneaking into a meet or wearing as a back-up piece. They don’t have much else going for them, being both weak and inaccurate. Use the Pistols skill when firing a hold-out pistol. Hold-outs can’t take accessories.

LIGHT PISTOLS
Light pistols are highly accurate and fairly concealable, occupying a comfortable middle ground between Hold-outs and Heavy Pistols. They’re extremely common on the streets. Use the Pistols skill when firing a light pistol. Light pistols can take top- and barrel- mounted accessories.

HEAVY PISTOLS
Heavy pistols are powerful sidearms that are a good compromise between concealability and firepower. When you shoot a heavy pistol, you use the Pistols skill. Heavy pistols can take top- and barrel- mounted accessories.

SUBMACHINE GUNS
Use the Automatics skill when firing a submachine gun. SMGs can take top- and barrel-mounted accessories.

ASSAULT RIFLES
Use the Automatics skill when firing an assault rifle. Assault rifles can take top-, barrel-, and underbarrel- mounted accessories.

SNIPER RIFLES
Use the Longarms skill when firing sniper rifles. Firearm enthusiasts will be the first to tell you that sport and hunting rifles are technically not sniper rifles, but they are similar enough in terms of game statistics to be included here rather than in their own section. Sniper rifles can take top, barrel-, and underbarrel-mounted accessories.

SHOTGUNS
Use the Longarms skill when firing shotguns. The stats for these shotguns are for normal slug rounds. When firing shot, use the special rules for shotguns, choke
settings, and flechette ammo (Shotguns, p. 180). Shotguns can take top, barrel, and underbarrel accessories.

Also, it appears that the terms Run & Gun uses the terms accessory and modification almost interchangeably.  It seems that they use accessory when you "only" add to the weapon, and modification when you first must remove something from the weapon before you add the mod.

And if things haven't changed too much between SR4 & SR5 any accessory/modification that the manufactured into a weapon don't count against after-market(character-made) accessory/modification availability/slots.