All of the above can be and most likely are the cause of your rifles performance. You say it's scope mounted with a 3-9 Burris. Does this indicate that the stock has been cut as well, or that a replacement stock was installed? Before or after its last fireing?
Any rifle left sitting for 16 years is more likely to change its point of impact than not. To expect it to stay the same is like expecting to win the lottery. If it was stored on its butt, it will definitely change somewhere. The scope is probably OK, those older Burris models were great. Now one thing that can and does happen, is with time the tension on the mount screw threads will cause slight stretching, therefore loosening the mounts. I know, not very much, but it does happen and more than is realised.
The bedding screws may have caused the stock to collapse slightly, if not pillar bedded, etc.
Take the rifle apart and reset all of the screws on the scope mount and stock bolts. Clean out all of the oil from the bedding and clean the bore. Check the barrel channel for contact points and remove with light sanding. That should clear up most of your problems.
Oh, by the way, that dry Alberta air may have also caused some stock shrinkage. The above should also take that in stride as well.
Just an add on on the original theme. About 1978, I bought 50 lbs of #44 powder, swedish bofors surplus meant for the 6.5x55, from Century International, damn I miss that company.
I stored the powder in the freezer, useing it up as needed over the years. It closely approximated IMR 3031, so it was very useful for a wide range of cartridges. All went well until 4 years ago. All of a sudden, my Mod 70, 257 Roberts, featherweight was grouping into 30-40 inches at 100 yds. Now this rifle was noted for sub MOA groups with 87gr Remington bullets.
I hadn't cleaned it after the last shoot, so there was about a 3 month interval before it was used again. Upon close inspection, the bore looked like someone had gotten inside and and peened it with a ballpeen hammer, the bbl was trashed. Big mystery, lots of crying on my part.
Couldn't figure it out until a friend had the same problem, it turned out that the only thing the rifles had in common was the powder we used.
I pulled the bullets from the 257 Rob round that were loaded and found the powder was rusty and smelly and the brass and bullet base were pitted. Then I went back to the stored powder. Even in the freezer, the remaining powder was starting to show signs of rusty powder and had a strong odor when thawed out.
I called my bud to tell him what I had found and darned if he didn't have the same problem. We burned all of the remaining powder, not much of a loss at $2/lb. In my buds case he hadn't stored the powder in a freezer, but under dry, controled conditions. The metal shelves over the original cans, were covered in rust and the inside of the cans were also covered in rust.
Powder can and will go bad, even under loaded and ideal conditions.
We also had some problems with some Portuguese 7.62x51 Reformado (reloads) FMJ from Districorps, who to their credit, recalled the whole batch and either replaced it with other ammo or paid for it and the shipping, both ways.
