Machine pistol = a full auto pistol, typically set on a standard pistol frame, can be fired in the typical two-hand fashion of a normal semi-auto pistol or revolver. Chambered only in pistol rounds due to recoil. Machine pistols are a tad hard to nail down by definition because of things that can be added on like a light shoulder stock, or come from a derivative design of an actual SMG. Typically the delineation is the length of the barrel and lack of a fore end. Examples include: MAC 10, TEC 9, Glock variations, Micro Uzi, etc.
SMG = Submachine gun. Typically chambered in pistol rounds, longer barrel than standard pistols, and have either a fore end or fore grip, to be held like a rifle, but shouldered closer to the body). Example: German (H&K) MP5.
Assault Rifle = a fully automatic, selective-fire rifle chambered in an "intermediate" cartridge, and has a detachable magazine. Examples: RUS AK series, US AR series, and a plethora of others.
LMG = Light machine gun. Chambered in smaller caliber rounds. It is lightweight and compact to be employed by a single soldier, with or without an assistant. Example: US M249 Squad Automatic Weapon (SAW).
MMG = Medium machine gun. Belt fed machine gun, chambered in typical calibers (example .308/7.62 x 51 or 7.62 x 54) of the battle rifles, and considered crew served. Example: RUS PKM, US M-60 / M240 series.
Take aways:
The FO 10mm SMG (FO3 and FONV) is not employed like an actual SMG but a pistol. I'm actually OK with Bethesda's vision of the 10mm pistol set up as a machine pistol...although chambered in a .40 equivalent, it would have fairly significant recoil held in the standard two hand pistol fashion.
With the exception of the machine pistol, I think all others that we talk about from the above list should have a stand alone model. Instead, the FO4 mod system forces us to re-chamber and modify a couple of set models to conform to the different functions of the above models. I think this is what most folks are unhappy about regarding ballistic weapons....in addition to others like damage, range and chambering.
Design issues make little sense, and to me, Bethesda seems to have taken artistic license way too far. If I didn't know better, I'd swear they have no clue what they're doing with ballistic weapons...but caveat that with the reality of their mod system being implemented (regardless of what I think about that personally). NV had it all worked out with appropriate DAM / DPS, and range stats, with infinitely more choice, variety and specialization. All Beth had to do was implement a fun and well thought out ballistic weapons system, and add to it. Instead, they decided to contract, streamline, and turn weapons modding into a blah exercise in Minecraft-like tedium.
Builds, specializations, choice and ballistics discussions were far more deep and enjoyable for New Vegas. It's a fairly flat subject, at least in my view, in FO4. After all, play long enough and you can do everything...even if with bizarre and unmemorable weapons.
Some of the real ballistic weapon issues include:
How can a combat rifle be chambered in a pistol round?
Why is the .50 so underpowered, and basically only seems to be added as an afterthought, thrown onto the equivalent of a structurally unmodified Remington 700 bolt-action.
Why the hideous and ultra-heavy design for the assault rifle? I mean, it comes at us weighing in at a "lean" 27 pounds properly modded; probably instead, should have been a medium machine gun , since it almost matches the exact weight of a US M240B, as opposed the much lighter M4 at 7.5 lbs fully loaded...the intended design of a weapon designated as such...then give us a proper "assault" rifle.
Why the loss or streamlining of weapon designs and ammunition? It is such a loss, that we now have to stoop down to the level of interjecting magic into the franchise through a ridiculous Legendary drop system? Whereas before, these "legendary" effects were far better handled through specialization perks (Shotgun Surgeon); a well thought out mod system (match barrels, etc); being able to switch between ammo types (armor piercing or explosive); or creating and using handloads (match or semi-wad cutters).
Having more base weapons would be a good start, sure. They may throw a bone or two at us to get us to shut up or merely build on their vision of ballistic mediocrity, but it won't be enough to get this "shooting" game up to speed...there are many other things that need to be addressed or redressed. Now having seen Bethesda so arbitrarily and haphazardly reject the inclusion of so much ballistic goodness that came before, and their willingness to streamline and walk backwards to force feed us a not so much fun, mod crafting system, I'm really not going to hold my breath.
Hey, at least they give us a toolkit, and after enough time we'll get much of this addressed. Too bad it couldn't have started out stellar, and gotten better, as opposed to being a repair effort.