So how does one measure trigger pull force. Well, if one is a gadget junkie and has money to spend, then one could acquire a trigger pull gauge like the two examples below:
The first is a mechanical spring gauge, much like an old fashioned spring balance and the second is a digital gauge. To measure the trigger pull force, the tester cocks the weapon (making sure it is unloaded first), simply fastens the hook end to the trigger and pulls the gauge backwards until the trigger releases the firing mechanism. The reading then shows how much force was needed to release the trigger. The illustration below shows how this is done.
Then there is another simple technique as the image below shows. This one is from a Life Magazine issue from 1937, but the same technique is used to this day in many official shooting competitions.
Before the reader assumes that this is an outdated method, this technique is still used in official NRA shooting competitions to make sure that no competitor is shooting with too light a trigger.
The above image is an NRA official trigger weight system and is available from some sporting goods stores.
Of course, for the casual user who doesn't want to spend $50-$100+ for gizmos like the ones above, there is a much more lower tech way of measuring the trigger pull force, which gives fairly accurate results as well. The homebrew tester simply acquires some heavy wire, such as a wire clothes hanger, cuts it to length and bends it into a S-shaped hook, using a pair of pliers.
The tester first unloads the gun and makes sure it is empty and then cocks the trigger. Then the tester simply hooks the trigger to the large end of the S-hook and then hangs a plastic shopping bag or a tin can from the small end of the S-hook. The tester then adds weights to the plastic shopping bag until the trigger releases. Then the tester employs an ordinary weighing scale (such as the one used in kitchens) and weighs the hook, the plastic bag and its contents. Multiplying this mass with the acceleration due to gravity gives the trigger pull force.
For testers who don't have access to weighing scales, they simply load the plastic shopping bag with known weights, such as grocery items. For instance, 500 gm. bags of beans, 100 gm. boxes of cocoa powder etc. can be loaded into the plastic bag until the trigger pulls. The weight of the S-hook and the plastic shopping bag and the bags that various grocery items come in are considered to be of somewhat negligible compared to the weight of the groceries themselves, so this method can give pretty good approximate results. For example, using 500 gm. bags of beans, it may be possible to determine that the release point of a certain firearm is between 2 kg. and 2.5 kg. (i.e. between 4.4 and 5.5 pounds) because the trigger didn't release when four 500 gm. bags of beans were in the plastic bag, but adding a fifth one did. Then the tester resets the experiment, adds four 500 gm. bags of beans back into the plastic shopping bag and then starts adding 100 gm. cocoa powder boxes until the trigger releases, at let us say 2.3 kg. Now the tester knows that the trigger release point is between 2.2 and 2.3 kg. Then the tester can repeat the experiment using four bags of beans, 2 boxes of cocoa powder and something else that weighs say 20 gms. etc. to get more accurate results and so on.