Why is it that just about every non 3 lever trigger mechanism suffers from variable trigger pulls?

If you measure the pull on stock factory trigger, timney, recknagel, sako, rifle basix etc etc you will find that the first time you pull the trigger it will be heavier than subsequent pulls. In good triggers the difference might be as little as 2oz in bad ones more.

Gunsmiths' cure is to increase the pull weight. I've always found this to make the difference a smaller percentage of the release weight but not to remove it.

What is the reason for this difference? Does the temper of the spring change, does the metal of the sear change temperature or does some tiny gremlin creep in.

Are you comparing pulls one after another or over a period of time? Any mechanism will acquire a set or what I call "sticktion" over time. The longer the time, the more the "sticktion" depending on the usual variable, temp, climate, lubricant. Condition is affected by the type and viscosity of the lube. For my really particular (the ones who can actually tell the difference) customers, I use dry-types like Dri-Slide, or graphite based lock lube on trigger contacts. The Dri-Slide is liquid and just squirted on sear contacts, moly pastes are rubbed in with finger or leather pad if you have direct access to surfaces.

One after the other

sticktion, ...good one!...
with the timneys and similar systems, i think it has to do with the springs settling against the walls of the bores they sit in and the trigger and sear settling against the wall of the trigger housing itself after a while of inactivity. production conditions and affordability mandate that a certain amount of "extra" play in the pivot fits has to be there and that allows the parts to take a set. after the first pull of a stored trigger, the parts re-align and subsequent pulls are slightly easier. i wonder if a test has been done concerning lube squeeze-out. run a series of releases with absolutely no lubricant against a series with lubricant and see if the dry series is more consistant than the lubed. seeing that the parts in a trigger are constantly under some load, either from mainspring pressure or the trigger's springs them selves when not under mainspring pressure, some sqeeze-out and settle is always going to happen.

quote:

Originally posted by 1894mk2:
What is the reason for this difference? Does the temper of the spring change, does the metal of the sear change temperature or does some tiny gremlin creep in.

Let's not overlook everything above the sear. The pressure on the sear needs to be uniform each time the bolt is cocked, so while you are cautiously applying a good dry lube to the internal trigger components, don't forget to apply a little quality Moly grease to everything from the face of the cocking piece, where it contacts the sear, to the firing pin and spring and all parts in between where there is metal to metal contact.

quote:

Originally posted by Westpac:

quote:

Originally posted by 1894mk2:
What is the reason for this difference? Does the temper of the spring change, does the metal of the sear change temperature or does some tiny gremlin creep in.

Let's not overlook everything above the sear. The pressure on the sear needs to be uniform each time the bolt is cocked, so while you are cautiously applying a good dry lube to the internal trigger components, don't forget to apply a little quality Moly grease to everything from the face of the cocking piece, where it contacts the sear, to the firing pin and spring and all parts in between where there is metal to metal contact.

I think there's a lot in this in that most of my mauser shot triggers vary according to whether the 2 position safety has been applied and taken off before hand.

Quite a bit of the problem can be attributed to the shape of the sear, whether the safety physically breaks the connection between the sear and cocking piece, or how much lateral slop there is at the rear of the bolt. If the cocking piece doesn't engage the sear exactly the same, everytime, and the parts are dry and sticky, the pull weight will be erratic.

HINT: If you really want to "feel" your trigger, put some good hearing protection on when you test it.