How it works
The foregrip increases the zoomed spread and reduces the horizontal recoil of a weapon. DICE implemented different values for each weapon, but the relationship is actually rather simple. While they are given as multiplier values, these values are adjusted to have an incremental change on each weapon's recoil/spread value.
The multiplier increases spread by .1 when ADS and not moving. The same multiplier affects both base and moving accuracy so the larger the difference between the base and moving spreads, the larger the spread penalty is while moving.
The recoil modifier is the same concept. The multiplier reduces the side-to-side recoil by .1 for the larger recoil value. This means the recoil reduction is more effective for weapons with symmetrical recoil values and affects low-recoil weapons more than high-recoil weapons (on a percentage basis). Note: for LMGs it decreases the vertical recoil by .1 as well.
Of course there are exceptions to these rules. The foregrip is exceptionally good on the SCAR-H and G3A3 giving them smaller spread penalties and larger recoil reductions. The M4A1, A-91, M27 IAR and M60 have larger spread penalties than average.
So what does this all mean?
The foregrip is ideally suited for weapons with low recoil and high base spread. So if you stand still when you shoot, LMGs will benefit the most from this attachment, then carbines, then assault rifles. If you move while shooting, carbines, then LMGs and assault rifles.
But ultimately, what is important is the context of the attachment. You will probably not want to equip just a foregrip and sight on a weapon since it offers very little for the cost of increased spread.
Use it with a heavy barrel, there's no reason not to. The ridiculous improved spread of the barrel will more than enough to compensate for the spread loss of the foregrip. An added bonus for LMGs is the foregrip will reduce the vertical recoil that was added by the heavy barrel.
The suppressor actually has a flat multiplier for spread reduction on all weapons of 0.75. From this, we can extrapolate some very useful information. If you compare the suppressor + foregrip to the stock weapons, your spread will IMPROVE for weapons with a base spread higher than 0.3, meaning most carbines and LMGs. But your spread will be IMPAIRED on assault rifles (except for the G3A3, F2000 and FAMAS). HOWEVER, you could argue that all weapons would be better off without the foregrip and only the suppressor equipped (LMGs with the vertical recoil reduction may still be worth it).
I would not recommend using the foregrip with the flash suppressor or the laser sight, it will only make you less effective at range.
Overall I do not like the foregrip changes. In my opinion, if DICE wanted the foregrip to be a 'close quarters' attachment for the run and gun playstyle, they could have made it improve one of the following: hipfire spread, spread while moving (hip or aimed), time to draw weapon after sprinting, or ADS time. In close quarters, I'm almost 100% positive most people don't really have any issues with horizontal recoil.
Note: I'm not 100% sure, but I don't think there is really a downside to using it on a sniper rifle. There is an existing spread multiplier, but sniper rifles have a 0 starting spread, soooooo yeah not sure what it does. I'd say go ahead and use the foregrip on sniper rifles, maybe it will hurt your moving accuracy but it's so bad anyways you aren't losing much.
TL;DR If you use the heavy barrel, use the foregrip. If you use the suppressor, the foregrip could possibly help on carbines and LMGs (probably not most Assault Rifles) but that is up for debate, you trade spread for recoil. The suppressor alone may be better than with the foregrip. You probably shouldn't use the foregrip without one of those two attachments (sniper rifles are the exception, not sure how they work).
