Muzzle Boost

This attachment may seem not so great at first, as the damage reduction seems to outweigh the fire rate increase, lowering the overall damage output over time.
However, situationally, it can increase the fire rate without affecting the overall shots required to kill your target. The issue is that manipulating this factor requires you to know the armor your enemy will be wearing.
For example, on an AK firing against metal armor, it increses the shots to kill by one for both headshots and bodyshots. If firing against road sign armor though, it does not affect the shots to kill the target, and is therefore a straight benefit.
Overall, if you know your target, it can be useful, but if not, it is better left at base.

This mod is highly situational. It operates in percentages, and so works best on SMGs, which already have low damage, low bullet velocity, and high rate of fire. It works well on Thompson, giving you the option of putting it on for closer range engagements to make it behave similarly to Custom SMG, or taking it off for longer range engagements. It works even better on custom SMG and MP5A4, which have relatively low damage values (30 and 35 at the time of writing this) and the highest rate of fire in the entire game - 600, so the 10 percent increase in it gives them a massive boost of rate of fire and will allow you to melt an opponent in a close range engagement by pointing in their general direction and holding down LMB. Be warned though, that, particularly with MP5A4, increased rate of fire means increased recoil, so you`ll have to compensate more for the recoil. Put on the simple handmade sight to counter this.

This mod will give your gun a similar or lesser DPS. Not worth researching it IMO, only useful if you really need another bullet on target ASAP.