I have a nice old Sako L579 .308 that has been delivering poor accuracy lately. Groups just aren't what I am accustomed to. I'm not an accomplished precision target shooter, but it was normally capable of 1-1.5" 5 shot groups at 100 yards with most any ammo. Now about 4"-6" groups and no pattern to what it "likes". The rifle is properly pillar & epoxy bedded. Crown looks good. Checked the action screws and scope mount screws for proper torque. Cleaned the barrel properly, no better.

I replaced the Zeiss Conquest HD5 2-10x42 scope in the original Sako windage adjustable QD ringmounts with a S&B straight 6x in Optilock mounts, and group size seems back to acceptable size right away with a couple loads that I tried.
So it seems like a scope problem. But the internal workings of a scope are a mystery to me, and where I live, sending a scope back to the distributor for service is neither quick or easy, and is seldom inexpensive.
Any suggestions how I go about proving conclusively that something in the scope has rattled loose internally? Or do I have all the proof I'm likely to get? Your thoughts?

When I help others are the range with bad groups or troubleshoot my zillions of beater rifles... 50% of the time the scope bases are lose with respect to the receiver. It is caused by oil getting between the scope base and the receiver. The most likely symptom is 2 shots grouping over here, followed by 2 shots grouping way over there.

The next biggest problem is dirty bore. Copper fouling.

In hard kicking rifles, it can be the stock is loose.

I have never seen a broken scope make bad groups. If a scope is broken, all the ones I saw could not get on the paper.

Sounds like the scope is bad, but if you want to confirm you could throw it onto a rifle of known good performance and see if it also opens up to ~5” groups.

Clarkm, - Sako rifles do not have separate bases. They are built in to the receiver. I checked all the screws. Ring screws and action screws. Cleaned the bore. It's a .308 Not hard kicking. Your comments were ... unhelpful.
Jordan, thanks for the suggestion. Putting that scope on another rifle had not occurred to me, so obvious but I missed your idea. I will try that.

Try JB's mirror test where you align the reticle with the reticle reflection in the mirror. Then vigorously slap the scope in the palm of your hand a few times and see if the reticle still is aligned with its reflection. I don't know whether a scope failure causing five inches of dispersion will be detectable using this method, but I've seen scopes where failure was very easy to prove using it.

Also in addition to R Riders suggestion, if you have a halfway decent borescope, attach it to the rifle and scope assembly, carefully note the point of beginning and go ahead and click a 10 or 15 "box" and see if you end up at the point of beginning. But it don't work so hot with the old Leupold friction adjustments.

When my Burris started making 1 1/2" groups I switched to another scope. Immediately it was back to sub 1/2" groups. Over the years you may have had a bad scope and didn't even know it.
When I read about folks who have a rifle with a scope from the 1970's and never had a problem with it I usually figure they don't know their scope is bad.

Take the suspect scope and center the reticle using a mirror. Once centered hold the ocular end of the scope and hit the palm of your other hand with side of the objective end. Check the reticle with the mirror to see if the reticle moved. If it has, repeat the test to verify and if the reticle has shifted again the scope is bad.

Thanks for the mirror test suggestion. I didn't know about that test and I learned something very useful! The scope checked out OK when doing the mirror test. It gave me confidence to shoot some more and pay attention to every detail again.
I was a little embarrassed that I didn't notice at the beginning, but not all the ammunition I tried shot the same, and some was simply horribly inaccurate. Mystery solved. I had not come across a load that was inaccurate in that rifle before, but I certainly found one now.
Both my Sako L579 .308 and my Model 85 Heavy barrel .308 gave good accuracy with a load of 45 gr. W748 and Lapua 170 gr. D46 target bullets, and very poor accuracy with the same powder charge and Remington 165 core lokt. So now shooting is fun again. I'll burn up the Remington bullets doing close range offhand practise.

Wrong... They can and should use separate bases. Redfield, Buris, Leupold, Control all make, or use to make bases for Sako actions with the machined tapered dovetail in the top of the receiver. The Sako Optic lock system use separate bases. There are ring mounts that clamp directly on the machined dovetail including the older Sako Factory ring mounts but these are not as stout. I would not trust them. Here are photos of Control mounts installed on a couple of my Custom Sako Predator guns (220 Swift top, 22-250 below) built on a Sako AII actions. These are expensive and a bitch to install but once the scope is mounted using Control rings and bases the scope is welded to the rifle and their slick as he11.

HEY now...that is a great idea!

Thanks. I had read about the mirror trick and tried the method I posted about whacking the scope into your hand. I verified a scope was not holding zero with the test. I sent it to have it fixed with the info of how I verified the scope not holding zero and got it back PDQ.

I'm yankin' yer chain, Dave.

Someone already posted it.

Sometimes I think it's a waste of time to read a thread before commenting, too.

I missed your earlier post. I wasn't aware he had the same idea or maybe I forgot I got it from him.