It's not exactly as you suspect...
Bolt guns CAN be more accurate than Semi-Autos,
Not that they are inherently more accurate right out of the box...
Something like an AR style rifle can be EXTREMELY ACCURATE out of the box since it's all synthetic, and all the variables can be addressed in terms of machining and assembly.
Properly machined part 'A' simply fits better into properly machined part 'B', and so on...
You wind up with a very accurate rifle with little to no variance from rifle to rifle,
And since there aren't any 'Organic' components in the rifle, there aren't any 'Issues' with stuff like humidity changes, warpage, ect.
In the case of the AR, the machines have put all the time in fitting the parts for proper mount an alignment, and you don't have to 'Dink' with it...
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Now, if you flip that over to a 'Hand Made' rifle that had machined parts, but they all had to be 'Hand Fitted' to make them work together,
Then there is a lot of vairance in the assembly, and every rifle will shoot differently depending on who put it together and how it was assembled...
Also, the 'Organic' parts of the rifle change shape, warp, twist, swell...
Stocks do some pretty strange stuff, and dissimilar metals will do some pretty strange stuff...
One steel will expand/contract faster than another,
One will use steel receiver, and then plop an aluminum mount on top not realizing that the aluminum will expand TWICE as much as the steel receiver and it will expand THREE TIMES FASTER than the steel receiver....
With Wood stocks,
You either have to machine out the wood large enough to accept EVERY variation of the action/barrel assembly,
or you have to HAND INLET every single stock which takes a TON of time,
And to do it correctly, you need a dedicated, trained stock maker... And those don't grow on trees anymore!
You CAN make a bolt gun more accurate than most Auto Loaders,
But it will take time, effort, skill and a lot of supplies.
Then there is the WORST of two worlds!
An Auto Loader in a conventional wooden stock!...
This combo will drive you crazy since the auto loading mechanism doesn't always set the next round the same way it did the LAST round... Plus all the problems with organics and machining that most combinations have!
Bolt guns are VERY GOOD about NOT scratching up or deforming the bullet/cartridge as it's loaded into the chamber....
Most Auto Loaders bang the cartridge around pretty good and the bullet/cartridge case gets marked up, deformed, ect.
Then there is an issue with stuff like cartridge head spacing and bolt tension...
Is the bolt/chamber holding the cartridge EXACTLY like the one it did before?
Autoloaders have a tendency to have large variances in this department...
Autoloading bolts don't always lock exactly like they did before, ect.
Where bolt guns have very good 'Repeatability' in respect to how they hold the cartridge and head space the cartridge...
All in all, the VAST MAJORITY OF FIREARMS ARE ALREADY MORE ACCURATE THAN THE PERSON THAT IS SHOOTING THEM...
Some guys want a little bit more, so they do this or that,
Then there are the fraction of 1% of gun owners than have OCD (Obsessive/Compulsive Disorder) when it comes to hitting the target,
So they will spend HUNDREDS of hours taking the variables out of the rifle,
And thousands of hours taking the variables out of the ammo and themselves!
(No, I'm NOT EXCLUDING myself! :brickwall )