Found this on another site, hope it helps...
Many of the Glock's parts and sometimes the whole gun are dictated or limited by the BATF's system of "import points". To qualify for import a pistol must score 75 points based on the following system:
Characteristic Points
Length: for each 1/4" over 6" 1
Forged steel frame 15
Forged HTS alloy frame 20
Unloaded weight w/mag (per oz.) 1
.22 short and .25 auto 0
.22 LR and 7.65mm to .380 auto 3
9mm parabellum and over 10
Locked breech mechanism 5
Loaded chamber indicator 5
Grip safety 3
Magazine safety 5
Firing pin block or lock 10
External hammer 2
Double action 10
Drift adjustable target sight 5
Click adjustable target sight 10
Target grips 5
Target trigger 2
For example, here is how the G17 scores: Length (7.28") 5
Weight (24.79oz) 25
Caliber (9mm) 10
Locked breech 5
Firing pin block 10
Double action 10
Target sight 10
Target grips 5
Total 80
Here is how the Glock 26 scores:
Length (6.29") 1
Weight (21.87oz) 22
Caliber (9mm) 10
Locked breech 5
Firing pin block 10
Double action 10
Target sight 10
Target grips 5
Target trigger 2
Total 75
Changes and features made to Glocks to comply with the BATF rules:
- all models are imported with a cheap click-adjustable rear sight that is replaced at the factory with the stock sight
- target (grooved) trigger added to compacts & subcompacts
- target grips added to all models (required for the subcompacts)
- due to their caliber, .380 autos (G25 & G28 do not score enough points to be imported)