I could be completely uninformed here, I was under the impression that the tight tolerances of a custom action was the whole point. If that is not the case then what actually makes a custom action better or more accurate than say a production rifle?
That is a large reason to buy a custom action nowadays, but not the only one.
ARC specifically has made a lot of changes to their actions over the years, and the Mausingfield in particular had a lot of revisions early on and even now. At serial number M5C1-TSx-00380 they created a fixed headspace that would not change in all future revisions and future actions (headspace is the same for Nucleus, Mausingfield, and Archimedes actions). Before that serial number, with my action being in the M5C1-TSR-0037x range, the headspace could vary from revision to revision and there were enough different specifications that nobody makes pre-fits for the older ones since it would serve very few customers.
I bought this action despite that because I like the other features the action provides. I like the combination of controlled round feed and inertial ejection. I really like the feel of the bolt handle/knob (the old knurled barbell). I also like how smooth it is, with DLC coating + tens of thousands of rounds combining to make it run fantastic either dry or lubed up. The action also has a reputation of performing well in dirty conditions compared to those with particularly tight raceways like a Defiance.
Nowadays headspace tolerance is much more of a big deal, but before the Impact Precision action came around it wasn't really something that anybody cared about. There are thousands of custom Defiance and Surgeon actions that have fantastically tight tolerances, but you still can't buy a pre-fit that will screw onto any Deviant or 591 action. From what I've seen the timing of the action threads is the main difference, but you'll also see some variance in headspace from action to action because that tolerance was never before considered as important as the fit between bolt and receiver. People sent their guns into the smith to get a new barrel, that was just how it was done, so it didn't matter if that dimension was perfect every time.
Impact was the first company I saw, excluding Accuracy International, that specifically controlled those tolerances and advertised it, even selling pre-fit barrels themselves. AI did it first since they were designed to have quick-change barrels, but Impact Precision seems to be the company that really kicked off the trend for all other custom actions. It makes sense thinking about it in hindsight, but before then it wasn't something anybody cared about because the smith was going to measure it anyways. Noawadays the complete interchangeability of parts is taken for granted, but it wasn't all that long ago that the idea of swapping bolts and barrels between rifles was considered ridiculous and useless.
As to what makes a custom action better, it could be the features or it could simply be the quality and looks or feel of the action. A Remington 700 action can be made to shoot just as well as a top of the line custom action. The difference is that it might lack the features, it will probably not feel as nice, and it will cost you a decent chunk of change to get the action trued up. By the time you buy the action, pay the gunsmith, and buy the scope rail you're in for enough money that most people would rather spend another couple hundred dollars to buy a custom action instead.
. If you don’t want it I’ll give you a little money for it. Good luck and hang in there all is not lost
