Easy opening method for combo padlocks
This method relies on the lock owner's laziness and lack of security awareness. There are lots of these people.
1. Observe the number at the index. It might be the last number. This is the easiest number to find even if the number you see is not it.
2. Slowly and softly turn the dial left. Stop when you feel an increase in resistance. If the user is as described, this is the middle number.
3. Keep turning the dial left slowly and softly, feeling for an increase in resistance. This number is one gate off from the real number.
4. Determine all gates by applying soft opening pressure on the shackle, turning until the dial stops. Let up pressure and keep turning. Master and Yale have 12 gates. A few other types have 10. Thus, on a 40 position dial you will find stops 3.5 numbers apart on 12 gate locks; 4 numbers apart on 10 gate locks.
When the shackle locks, a lever kicks the third wheel (i.e., first number) about one gate off of the opening number.
5. Try the combo with the first trial number, known middle number, and pull in each gate position for the last number.
If step 5 fails, try the gates to the left and and right of the first number.
If the user spun the dial, you can try shim bypasses, pick the back of the lock if it also opens by key, or use the brute force combination approach which is well covered by the book on opening combo locks.
In the brute force method, you determine the number of gates as in step 5 then make a table (10x10 or 12x12 cells) with the gate numbers as headings for rows and columns. Almost certainly, you can eliminate trial combinations where the first and second numbers are the same. This forms a diagonal thru the table. The numbers to the left and right of these numbers are also improbable.
So instead of having 40x40x40 or 64000 combinations, you have at most: 12x12 - 12 = 132 combinations on which you will apply step 5. On statistical average, you'll hit the working combo within 66 trials. You have to be accurate to within 0.5 number (e.g. 1.5, 3.0, 4.5, etc) when applying trial combinations.