I have a load that I have worked up for a rifle, when I go to make more reloads for it, its always kinda a PIA to make sure and adjust the seater plug that last couple thousandths. so my question is if the seating depth is off 3 or 4 thousandths will this have a major effect on accuracy??

It's going to depend on how close you have it to the lands in the first place. If it's not close at all then you probably won't see any difference at all. If you are close to the lands you might be pushing them into the rifling. Just depends.

its not close to the lands, its got a pretty big jump as its a ruger, I think they would not fit the mag box before they hit the lands. I was just wondering if there would be much difference in velocity or pressures, this load is on the ragged edge pressure wise, but it shoots oh so good.

If it's close on the pressure scale to max, and you have some room to play with, you might want to move it out as much as you can afford without being into the lands to cut down on the higher pressures...that is if you are worried about it.

If I have a load that shoots well, I try and save one of the bullets, and adjust my seating die to it. Generally, bullets seated with this method are very close to the original and can be adjusted to be "right on."

I do have dies that will just not seat closer that + or - one or two thousands. For hunting purposes, this is generally not an issue. If it is, then you fork out for a micrometer adj. die. Given what you said about the free bore in your rifle, I would not get to hyper about it. Kudude

thanks for the idea kudude, I will do that next time just run the press all the way down on the die with a round in it and adjust the seater plug to it, and then monkey with it the last little bit