Triggers should always be fitted or checked by a gunsmith when fitted by amateurs. 
Trigger Tuning Kit, for the SMK QB/XS 78/79 models, eliminates excessive movement you get with the standard trigger. Gives finer control settings. Makes a huge difference for very little money, we have kept the price down so you can have a good trigger.
Includes Shims, Allen Screws, Moly lube, Wet and Dry, Spring guide made of Brass or Stainless Steel, all the parts needed and instructions are shown a Video on this site for free. 

 The trigger is one of the most important parts of a gun and it is surprising how many shooters don't realise this.

You can watch the video for free on this site under SMK Stripdown Video on the left side panel store links, at the moment it is the fourth one down. This video is on the XS78 but the XS 79 with the large 88 gram bottle has the same parts inside.

Option of a lighter spring is available under the option tab for £3.75.

Don't set your trigger too light as it could be dangerous.