mp7- i never use it. still haven't since launch pdw- honestly this gun is pretty ridiculouse with grip/select fire vector- i like grip/stock. long barrel works well. msmc- no real way to go wrong with this weapon. might recommend fast mag or extended mag. the reload seems a little slow to me. skorpion- i like grip/long barrel. peacekeeper- with this you can get away with grip and quickdraw.
oh yeah, forgot the chicom. grip and fast mag for that little guy
I generally add stick to every weapon I have lol..lovely little attachment. Um a RF'd Skorpion is able to hold it's own against shotguns...when used correctly. Havent played alot with the other SMG's so....
Lol, seriously? The grip is virtually useless, literally ANY other attachment is a better option. It gives you a 2% increase in center speed, which is just about nothing and almost completely unnoticeable, the ACOG gives you the exact same benefit.
Of course, you can't use an ACOG on an SMG, but you CAN use an EOTech sight, which has a similar bonus, a 1% increase in center speed. Basically, don't use grip, you can achieve almost the exact same effect by using an EOTech, but you get a great optic as well. For more info on the grip check out Drift0r's video, he breaks it down pretty well.
To actually answer the OP's question though: laser sight is excellent, I run it on all my SMG classes, SMGs have great hipfire and the laser really gives you an edge in CQC. The EOTech is also excellent if you don't like the iron sights, although most SMGs have good iron sights (the MSMC and PDW especially). Fast mags are always helpful. Supressor if you want to be sneaky.
Long barrel only gives you a 15% increase in range, so it's not really all that useful, but if you do want to use it I'd suggest it on the MSMC.
One more thing, if you throw select fire on the Vector, it turns it into a 2 round burst weapon with incredible accuracy and a ROF that rivals the Skorpion. Great choice if you have a fast trigger finger.
EDIT: I highly, highly recommend NEVER using the rapid fire attachment. The range penalty is absolutely massive and I can't think of a single situation where the positives outweigh the negatives, except for MAYBE (that's a big maybe) the select fire Vector.