I recently purchased an older Sako Forester in 243. The rifle is 1962 production. My go-to loads in the 243 are 95 Nosler and 85 gr. sieraa HPBT. I used wipeout on the bore and got enough copper to start a mine, but the rifle was shooting right at 100 and left at 200 with a brand new VX-II. Pulled the scope and lapped the old Sako rings since the rear mount was carrying about 90% of the load. Groups have tightend up to about 1" with no crazy dispersion at longer ranges. I'm at a loss with the trigger. Even with the adjustment screw just barely toughing, pull is about 4-5 pounds. I have an old Finnbear 270 and another forester in 308 with nice crisp triggers set at 2.5 pounds. What to do?

Change the spring.

The Sako is a poor trigger design. The sear puts a heavy load on the trigger piece. If the angle of contact isn't just right, trigger pull will be heavy even with no spring.
The best option is to hone the contact surfaces but this is a job which is easier to screw up than it is to get it right. Regards, Bill

Springs alone don't make trigger jobs. You sometimes have to coerce the individual pieces into a better working arrangement. Stoning and changing angles is a big part of it. But you have to know what you're doing with that particular trigger.

Stoneybroke, find yourself a gunsmith who is experienced with Sako triggers and you will be fine.

not familiar with Sako actions but Timney makes a trigger for the A action.
I'd do that before stoning the sear engagements

Wound up with a Timney trigger and a 3 position safety from Ed LaPour on my Forester L579 from 1960. Huge improvement.

I've come across a few Sako #4 triggers that wouldn't adjust the way I wanted, but the great majority of them will go down to a nice crisp 2-2.5 lb. Sometimes on the older triggers you'll find a screw which adjusts the engagement of the trigger and the bolt sear. This screw is on top of the trigger between the trigger and the action. Raising the screw lessens the engagement and vice-versa. If you have more engagement than necessary for safety, you may be able to markedly improve your pull by slightly lessening this engagement. It must be done by trial and error since the engagement screw cannot be accessed without dismounting the trigger.

Sako apparently got tired of making this adjustment because they dropped the screw from later #4 triggers in favor of a fixed "bump" point on top of the trigger housing.

Stoning by hand is not a good idea - don't ask how I know

Whatever work you do make sure you test for safety by applying the safety catch, pulling the trigger (rifle empty) then removing the safety and lifting the bolt handle as well as testing for slam fire.