Pick a weapon type. The more of this type you have in your inventory, the easier this will be.
Have the first weapon with no parts (invisible gun). This will be your control weapon, as it has base stats.
Have the subsequent weapons with only one part at a time. For instance, if you are testing barrels, have one with just barrel1, one with just barrel2, etc.
Make sure that your weapons have the same material, and the same quality grade.
Calculate the percentage difference between the various stats to the control weapon. For instance, if your control has 100 damage, and barrel1 gun has 125 damage, then barrel1 offers a +25% damage modifier. Round to the nearest 0.5%.
Repeat until you get sick of it. Remember, every piece you test makes the task easier for everybody including you!

For ease of data entry of all part names

Barrels
Bodies
Grips
Magazines
Sights
Stocks
Prefixes
Titles