Technically speaking, there is no gun that cannot make a SS attack, although it stands to reason that some machine guns should maybe be truly "FA only".
SA means that you will double tap the trigger to quickly fire two bullets at the same target. One bullet each time you pulled the trigger. Even if this was never mentioned in the rules I agree that it still stand to reason that you can also choose to just pull the trigger once, to fire a single bullet (treating it as if you were using a SS firing mode with listed AR and DV).
BF means that your weapon will automatically fire 4 bullets each time you pull the trigger. Either against one target as a narrow burst or by spraying two targets next to each other as a slightly wider burst. It is not possible to just fire one single bullet in this firing mode (except perhaps if you only have one single bullet left in the magazin). If you wish to just fire a single bullet you typically switch to SA mode and just tap the trigger once. Even if you know something I don't, I would be very careful about telling people that it is allowed to fire BF mode weapons using SS rules.
FA means that your weapon will automatically fire bullets whenever the trigger is pressed. You typically use it to attack a slew of targets or even a single target with a series of small dice pools.
Depending on the specific weapon's rate of fire I guess it is debatable that you could also quickly tap the trigger in FA mode, hoping that only a single bullet will be fired but this is typically not something that is taught or practiced in reality. Instead, if you wish to just attack a single target with a single bullet you would typically switch to SA mode and just tape the trigger once. Weapons that doesn't even support SA or BF mode (like LMGs) are actually never supposed to be used for single shots to begin with (they are supposed to be used for suppressive fire together with a mix of tracer rounds).
Game mechanic wise (as written) you can still attack one target once with FA without splitting your pool (but it will still cost you 10 bullets and 6 AR). Unless you know something I don't, I would be careful to also telling people that it is allowed to fire FA mode weapons using SS rules.
Note that FA allows you to attempt multiple attacks on the same target, which is an ability that the multiple attack action does not also allow.
If you for example are wielding two weapons at the same time then you can also attack the same target twice. Once with each weapon (this after all the primary reason why you would want to dual wield to begin with).
SR6 p. 110 Off-hand Attacks
If you’re attacking with two hands, the attack from your primary hand can still gain and spend Edge; for other rules, see Multiple Attacks below.SR6 p. 111 Multiple Attacks
This can be against multiple targets, or you could attempt two attacks against the same target.