[HarshRhettoric]

I was looking in the main 5E book, and noticed that many of the assault rifles say no stock, and have no recoil compensation (such as the Ares Alpha) but if you have ever seen a picture of an Ares Alpha, the whole back of the gun is stock.  Likewise, every AK I have ever seen in real life had at least a folding stock, yet no mention is given in the book.  Should I change the numbers to reflect this?

[Prodigy]

Your question is valid, but I would say your discretion on this one is vital. All the rifles I own also have stocks, but some absorb recoil better than others. Almost all rifles have stocks, but only a few are modded to have recoil-specific stocks.

[firebug]

A lot of rifles in SR5 that have a stock and RC generally claim it's from a shock pad accessory.  I think the idea is that, related to what Prodigy said, the stocks don't all absorb recoil enough to count.  An AR causes more recoil than a pistol (I would assume) but the stocks on the ones in the book probably don't do much more than make it comparably manageable.

I know nothing about guns, but what I mean to say is:  They all have stocks, certainly, but they don't all give free RC, and that's likely by design.  No need to change anything.

[Xenon]

Assault rifles, Sniper Rifles and  Shotguns are designed to be used with two hands and with the stock against your shoulder. This default usage does not give any extra recoil compensation. If your weapon have a rigid stock (most assault rifles do) then you can attach the Shock Pad accessory that give your rifle 1 point of recoil compensation. Sniper rifles often come already fitted with Shock Pad accessory and thus provide 1 point of recoil compensation.

SR5 p. 432 Shock Pad
A shock-absorbing pad is situated on the rigid stock of a rifle, shotgun, or heavy weapon, and provides 1 point of recoil compensation.

Sub Machine Guns and Machine Pistols in SR5 are designed to be shot from the hip with one hand Hollywood style. Some SMGs have a folding stock that let you brace it against your shoulder for one point of recoil compensation. Same goes for the few Pistols that come with a stock. Note that you can not attach shock pad on a folding stock, only rigid stock... but if the SMG come factory made with a rigid stock then you should be allowed to attach Shock Pad for an extra point of recoil compensation besides the point that the stock already provide.

[WellsIDidIt]

but if the SMG come factory made with a rigid stock then you should be allowed to attach Shock Pad for an extra point of recoil compensation besides the point that the stock already provide.

They should, but they can't by RAW. RAW specifically mentions that shock pads go on a rifle, shotgun or heavy weapon. So even if the SMG has a rigid stock, it can't take a shock pad. It doesn't fit the rifle, shotgun, heavy weapon criteria.