UPDATE:
We just received the new ISMI 11 pound recoil springs.  These should eliminate or greatly reduce the need for cutting 13s.  I have not done extensive testing yet but they should work great in the 9mm guns and the compacts.
 

Effects of a lighter spring:

Recoil is transferred to the shooter in a shorter duration of time because the slide is moving at a higher velocity.  This is often perceived as less recoil and reduced muzzle flip.

With a lighter spring the shooter also has less force to counteract, or you don’t have to work as hard.  This usually reduces muzzle flip.

Less force to counteract reduces the odds of producing a limp wrist style jam.

A lighter spring will result is reduced muzzle dip when the slide closes keeping sights steadier and on target for a faster follow-up shot.

Light springs are particularly helpful to smaller shooters like children, women or anyone else having trouble keeping their wrists locked.
 

Effects of a Heavier spring:

Recoil is transferred to the shooter over a longer duration of time due to lower slide velocities.

Slower slides equal a longer recovery time for the shooter.

The shooter does more work, as there is more force to counteract.  This often causes and increase in muzzle flip.

The chances of a limp wrist style jam are increased, as there is more force working to unlock your wrists.

The chance of the slide short stroking and causing a feed jam is increased.

Increased muzzle dip when the slide closes for a slower follow-up shot.
 

Brass Ejection:
It does not matter how far away it lands or if it is in a neat pile.  You are there to shoot not to pick up brass.
 

Frame Battering:
A non-issue for Glock pistols.  It falls under the category of Internet Nonsense along with the idea that light springs cause kabooms and broken parts.
 

Spring Selection and Testing:
There is no magic weight that is perfect for all shooters, loads and guns.  Each shooter must evaluate and test various weights to determine what is best for their application.  I recommend testing spring weights using the Matt Burkett Timing Drills.

For rough tuning try different standard weights.  For fine-tuning, take a spring slightly heavier than you prefer and trim it until it is just right, this is a trial and error process.

I use and recommend ISMI recoil springs for Glock pistols.

I do not recommend changing the springs or guiderod in the subcompact pistols.  The doublespring rod system works fairly well and I have not found anything better on the market yet for those pistols.

For the midsize guns (19/23/32) the free length of the spring is too long and prevents the slide from fully cycling.  Start by removing 5 coils and then check for full travel.  Trim until the slide has full travel then check for proper lockup.
 

You can go too light:
The firing pin spring can overpower an old or too light recoil spring causing the slide to pull slightly out of battery as you pull the trigger resulting in a light primer strike.  If you have off center light primer strike this is probably the cause.

Feeding jams.  The slide can be so fast that the mag spring cannot keep up.
 

Testing Springs:
To test a new spring and get a realistic idea of what it will do we reccomend do a set of Matt Burkett Timing drills for each spring.  This will help you determine the best one for you and your load.  See the drills here

Recommend starting springs weights:

G17  13lb
G19  13lb
G20  15lb
G21  13lb
G22  15lb
G23  13-15lb
G24  13lb
G31  15lb
G32  13-15lb
G34  13lb
G35  15lb

See update above on new 11 lb springs.

Matt’s spring setups:

G34 Production 13lb minus 4 coils
G35 Limited  15lb minus 3 coils
G17 Open  13lb minus 5 coils
G19C Carry  13lb minus 6 coils

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