Love what you did, really makes them feel less like toys and more like weapons. Also appreciate you filling in some missing bits from the era of 2004 optimization and making the models look complete (looking at you, crossbow and AR2) and bringing the guns up to date! Some things, however:
Pistol is a tad bit too closer to the screen, which makes it bigger than the .357 revolver. Reasoning for this is because power should be determined by size and strength, and the revolver should reasonably appear "bigger" than the pistol. If the pistol is really bigger than the revolver itself, then it's an issue with the model itself and I wouldn't want any inconsistencies with how the origins are.
Another issue with the pistol that bugs a gun enthusiast like me very much is the hammer being VERY useless and a part of the grip itself instead of looking like and moving like, well, a gun hammer. Don't know how much you want to put effort into something like this, but a visible hammer motion and firing pin for the pistol would help make it look less like a toy. In addition, the pistol lacks proper shell ejection due to Valve's oversight.
The .357 revolver is pretty good, though I'd love just a bit of detail that probably won't be noticed with how fast it happens, but one I appreciate greatly. The hammer on the revolver (no idea if it can be moved) should quickly rear backwards and snap forward quickly, and the cylinder should rotate counter-clockwise the same time that's happening. Pretty sure the standard trick of making a cylinder "rotate" is to instant move it clockwise in a different position, and then as the action progresses make it rotate it back to its original position.
SMG is perfect, though this is just a minor suggestion. The cartridge of the real world MP7 uses is a rather long one compared to the pistol cartridge ingame, and would be interesting to change the pistol shell ejection to the unused rifle shell model as that's what the cartridge closely resembles.
Shotgun changes doesn't really impress me as much as the rest of the arsenal, mostly because it still looks the same. If you don't mind this drastic change, I think it needs to be more centered, like so (host timescale really helps with posing):
With these changes, it makes the gun looks less like a broom stick and more like a boom stick it was meant to be.
The AR2 looks pretty good, and I love the fact that you put a hand behind the magazine instead of making it hold the magazine itself (god I really disliked that)! There's one major suggestion that would help make the AR2 more believable and ridiculous as a weapon at the same time. For one, make the reload change the entire magazine at once (as that is the model for its ammunition you find), which requires chopping the magazine portion of the gun separate from the rest of the gun to achieve. Secondly, combine both the firing and the old pellet reload animation to achieve what I found conveniently on the Facepunch forums:
Fixes the problem of a single pellet having 30 shots inside it, the gun not making any sense reload wise, and explains why it has such atrocious recoil. :V
I apologize if this was quite a lengthy suggestion, but what I really felt about the vanilla HL2 weapons was its lack of polish overall, and how weak and unsightly they looked because of oversight, optimizations, and laziness. The effort you put into making the weapons look presentable and powerful is fantastic, and I hope you can take these into consideration to further improve these old 2004 relics.
Suggestion! You might want to modify the shotgun's reload animation so that the player can't see him obviously slip the same shell in over and over. A bit of an obvious oversight on Valve's part unfortunately.