Basic stuff:
Most common custom actions- Turbo (two and three lug), 10x (two and three lug), Holeshot Arms (Jerry Stiller) (2500x (two lug), Trident (three lug), Hall. Some others you'll run into are the Lonestar (made by Stiller), Falcon (made by Leonard Baity), Swindlehurst, Time, etc. But mostly it's Turbo, 10x or the 2500x and Trident. Lots of Hall actions are out there and being used also.
Jerry Stiller of Holeshot Arms (Texas) makes the 2500x, Trident and Lonestar. Anthony DiOrio (Virginia) makes the Turbo, Lonnie (can't remember his last name) from PA makes the 10x, Alan Hall makes the Hall (easy to figure that one out eh?), Leonard Baity (NC) makes the Falcon, etc etc...
Main differences
PAS (Pin Around Spring) Ignition-Turbo, 10x
SAP (Spring around Pin) Ignition- 2500x, Trident (three lug version of 2500x), Falcon,, etc
What it means? Calfee likes the PAS ignition. Some say it doesn't matter, some say it does. I don't know. Same with the 6 vs 12 O'clock firing pin. Most popular in the customs, personally not sure it matters though.
Bix and FF triggers are the more popular triggers used on these. Jewel triggers are still used by quite a lot of shooters, but most of the big name shooters use the Bix or FF triggers anymore.
Why these customs? Closer tolerances than factory actions. Not to say factory actions can't win, they can. Remington 40x, Win 52D, Suhl, Anschutz are great actions. Advantage of the Suhl is that the factory trigger rivals any of the aftermarket custom triggers. Most use Bix or Jewel triggers on the 40x. Anschutz of course has their own trigger, as does Win 52.
But in general, custom actions should have closer tolerances than factory actions. Not to say custom actions don't need work when you get them. Turbo and 10x can be rough as a cob when new. Either requires knowledge on the owners part, or a gunsmith familiar with them to sort out. Not to say they won't shoot when new, but there's a lot that can and should be done to get them "right". Personally feel, the one action that is the most user friendly is the 2500x or Trident. More of a stick on a stock, stick on a trigger and barrel and away you go. Very well made actions.
Known gunsmiths were listed earlier. Several others do very good work with any of these actions. Go with who you're comfortable with. Gorham and Eck are two of the more commonly used. Think Eck only works on the 2500x/Tridents anymore. You'd have to ask. Penrod, Langley, Blue all work on any of these actions. I'm more familiar with Don Blue so he does mine. Good friend and close so easy choice to make. Langley does some great work as I shoot against a lot of his guns and they do shoot well.
Factory vs Custom...Personally have owned Win 52's, 40x's, Anschutz, Suhl, etc. Have shot most of these in matches and done pretty well. Even did very well with a lowly Kimber 82G in matches. As to custom actions, have owned Turbo, Falcon and Trident. Never really shot my Turbo much as it wasn't quite what I expected. Falcon was a struggle until I learned what makes these things tic. Worked great then. The Trident is the one that blew me away. From the start it has been everything I think a custom action should be. But again, this is all from a personal perspective.
In a nutshell it's ignition, barrel and ammo. You get ahead of those you're going to be ok. The other stuff has to work well together, but any of these actions can and have won. Of course, personal ability plays a part I suppose
May have missed something but in general this should help get you started. Good luck
Kenny