For generations, 3-3.5lb trigger pulls were considered ideal for a hunting rifle. Lighter pulls were ok in varmint rifles, but for game rifles 3-3.5lbs was considered the best weight, heavey enough to be mechanically safe, and light enough for best accuracy under, field conditions.
Also note that for generations autoloaders were held to a generally higher pull weight, and 4-8lbs was considered acceptable. Pulls in the 4-5lb range were thought to be the best, but you could use a heavier one well enough.
This was because of two things, first, most of the common semi auto deer rifles were really tough to get safe, reliable light weight triggers on, and second the effect of hunting conditions on a light pull trigger in a semi auto, (or any other big game rifle) and the shooter.
Half the country, or better, hunts in the cold weather. Oils get thicker. Fingers get numb(er -even with gloves). When you have been in 20 degree weather for 5 hrs, sitting in a stand, or stalking, climing up and down hills, over logs and rocks, etc...you aren't in the same shape you are at the range in May or September. '
That fine light trigger pull that allows you pinpoint accuracy because of the precise feel of the trigger in nice weather, goes off early in the snow sometimes, because your finger is just "touching" it. You're actually applying that 2.5lbs force just like you did last summer, BUT, you don't realize it due to cold stiffened fingers. And that can mean anything from a clean miss to a bad hit and wounded (maybe lost) game because it went off before you were fully ready.
The smoothness, creep, and overtravel of a trigger are more important than the actual pull weight. You can actually shoot better with a 7lb trigger witha good pull than a 3lb one that stages and is gritty.
We are taught that the actual moment of firing should be a "surprise", and that's true. But, in a good trigger pull, you should be able to feel it, a point where you know that it will fire "now". And then "surprise" it does.
Semi autos have an added complication, they slam around. This is an important factor. Unlike other actions, they get jarred by the rapid movement of the bolt and other parts. One of the most common (and incorrect) methods of getting a lighter trigger pull is to reduce the sear engagement. Bubba (the guy who thinks he's a gunsmith) takes his stones (and god forbid, sometimes files) and "fixes" the trigger. Often he fixes it so well that it is just short of getting jarred off from the action cycling.
Add in some thickened oil from cold weather and you have a potential for disaster. Things can stick, and then go off seconds later (really bad). Or not reset for the next shot (bad). Or even go runaway (really, really bad). Lots of things are possible.
A 2.5lb pull on an M1 carbine is sweet, but waaay lighter than milspec. That means that either the gun has been worked on, or something is worn to just short of the failure point. Either way, you should have the gun checked out by a competent gunsmith (not Bubba) to find out which, and if it has been worked on, was the job done well, leaving the rifle safe?
What if it is to the point that the next time you chamber a round, it jars the trigger group enough to fire? OR fire when you take the safety off?
Personally, I've never had a gun that I haven't dry fired a couple of times, at least (using snap caps if needed), to get a feel for the trigger, before I load it with live ammo. Some of my guns have very light triggers. They are safe that way, in my hands, because I know how light they are (the feel, not the specific pull weight).
All guns are different in small details. Knowing each one you use, and knowing it fairly well means you get the best performance from it, and the best safety on the range, or in the field.
