There are four areas of concern with a trigger job: the sear and hammer hook interface, the sear spring, the mainspring, and the trigger bow. There are risks associated with the first three.
The sear and hammer hook interface can be polished and lubed. Some people will attempt to take a lot of metal off the hooks and sear face. This can make the gun dangerous by allowing the sear to slip off of the hammer hooks or not catch during the cycle. This can cause the gun to fire unexpectedly or fire multiple rounds on a single trigger pull. A polish is good but removing metal and changing the angle of the hook-sear interface is dangerous. Modifications to the sear face and hammer hooks should only be attempted by a trained and certified gunsmith who has the proper tools and know-how to do the job.
The sear spring puts resistance on the transfer bar of the trigger and on the foot of the sear (and also the base of the disconnector). Proper tension is critically important. The sear spring can be bent to give less resistance to the trigger, but this also reduces the tension against the base of the sear. If the tension is reduced too much, the sear may not re-engage the hammer hooks during cycling. This can result again in multiple shots and even full auto fire. Adjustments to the sear spring should be attempted only by a trained professional. I have done it, but when I do, I use a stock Colt sear spring as a template and I do not vary significantly from the factory curves in the spring.
The Mainspring (the spring hidden in the mainspring housing at the lower rear of the grip) controls the force of the hammer. In doing so, it also contributes to the pressure applied to the sear face-hammer hook interface. You can replace the mainspring with a lighter mainspring and get a significant lightening in the trigger pull. It will also make it feel less gritty. Factory spec for Colt mainsprings is 23 lbs. I put a 21 lb. mainspring in my Combat Commander and it did a lot toward lightening and smoothing the trigger pull. You can actually go down to a 19 lb. spring and still have a reliable gun. Factory spec for Kimber pistols is 21 lbs. If you go down to a 19 lb. spring, do extensive reliability testing before deploying the gun for serious work. This risk here is obvious. If the mainspring is too light, it could fail to detonate the primer and result in a misfire.
The last area of concern is the trigger bow. This piece is the bow that begins behind the trigger and extends back around the magazine well to engage the grip safety. The bow should be checked to see if it is moving freely and not snagging on either the frame or the magazine. I like to stone the edges of the trigger bow to make sure there are no small burrs that may be snagging on things. There are no real risks associated with the trigger, but there are also the least benefits to be obtained. Changing or polishing the trigger bow does very little toward lightening the trigger pull, but it may remove some slop in the trigger.
Regardless of whether you do the work yourself or have a gunsmith do the work, always go to the range and test the pistol carefully after a trigger job. Even if the work is done by a trusted gunsmith, test the work yourself. Be mentally and physically prepared for the gun to go off unexpectedly or fire multiple rounds. Have a firm grip on the gun when you load it.
There are some safety tests that you can do prior to range testing. With an UNLOADED GUN, lock the slide back pull the trigger and hold it back, and then release the slide release. The hammer should remain cocked. With the slide in battery, cock the hammer and slap the side of the gun briskly. The hammer should not fall. Cock the hammer and push the slide back about a quarter of an inch and pull the trigger. The hammer should not fall.
For a complete list of safety tests for the M1911, see http://www.sightm1911.com/lib/tech/safety_test.htm
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