The SR-635 is described in the game as:

An SMG-sized AR.

You can pick this gun up in northern Ocoro, in a huge cocaine factory. The “Cooking with Coca” mission is nearby. The weapons case is inside the armoury at the back of the factory complex.

With the number of enemies around, this looks a bit intimidating. No doubt you’ll be here for the main mission anyway, but I found if you’re patient, you can move up the main road into the factory slowly, taking out anybody in sight carefully.

I did this at dawn, but you’ll need to be sure not to leave bodies where someone might find them. You can use some cover to move up the street and straight to the armoury. It certainly isn’t necessary to kill everybody!

Unfortunately, like the SR3M assault rifle which tries to be an SMG, this gun fails at both as well. It has the lowest rate of fire and worst noise reduction of all the SMGs, but scores best in class for accuracy and damage. I just think if those are what you’re looking for, you may as well use an assault rifle.

As its name suggests, the SR-635 is the only gun in the game chambered for 6×35 rounds, and has a 20 round standard magazine with a 30 round extended mag option.