4LBS is as low as I would go too, it is low enough for any practical purpose in my opinion. You can go lower if you really want to, but bear in mind that you will have maintenance issues. It will need periodic tweaking to keep it functioning at lower pull. The battering it undergoes will cause it to follow after some shooting. 2.5# is as low as anyone can get one to function, and it will only last a couple thousand rounds, maybe 10K if it is a perfect job, before it needs tuned again. Send it out and have it worked, or order another hammer and sear to replace the ones you "test" on. Beware too that full auto is possible if it is not set just right, load one at a time in the mag, then two then three.