To my way of thinking there are two elements necessary for a firearm to be considered custom. First is that the combination of elements that make up the firearm are not available in an off the rack gun, and secondly, that the piece is made to the specifications demanded by the customer. I have 3 rifles and a handgun that I would consider semi-custom, and have had another three that qualify as full custom.
The semi-custom groups includes my M-70 Express in .458, the factory barrel as cut down to 21" and the LOP 13.5" with a Decelerator recoil pad. My ZG-46 .30/06 had a small crack behind the tang, so while this as being repaired, I had the action bedded, and cross bolts installed in the factory stock. My second 602 was, like the first one, chambered in .375 Ultra, but because I retained the factory barrel, I consider it semi-custom despite the McMillan stock, and NECG sights. My primary ATC gun is a semi-custom Ruger Vaquero .44 magnum. Aside from an trigger job, it as dehorned, meaning the sharp edges were broken, and the front of the trigger guard narrowed to prevent a blackened fingernail. I had considered a lanyard ring, but decided it would be under utilized. The grip frame and stocks remain factory, and the sights weren't messed with. I put a Magpul SGA stock on my 590 Mossberg, which dramatically improved the the gun's handling and shooting, and to me qualifies as an improvement, but not as customizing.
My full customs include a couple of 700 Remingtons, the first was my short action .308 target rifle; McMillan A-5 stock with an Anschutz rail, Krieger 1:8 .308 #5 target contour barrel, trued action, Jewell trigger, Dlask bolt handle, single shot adaptor, PH bi-pod.
The other is a work in progress, a switch barrel 700 long action, with Ron Smith gain twist barrels; a 24" 1:7 .243, 1:8 a 24" .280 Ackley, and a 22" 1:10 .375 Scoville, all unfluted stainless #3 sporting contour. The .375 is the only one with irons, an NECG barrel band brass faced post that will be used with an XS Weaver ghost ring attached to the Near rail. Of the three, so far I've only shot the .243, which feeds smoothly from the magazine, and without issue through the long action, and the 90 gr Scenar/Lapua brass/N550/CCI BR-2 combination grouped 3 holes touching . . . at 200 yards, several times. Not bad for a light hunting rifle. The rifle was bedded in a Wildcat stock, but due to a minor misadventure, the stock was damaged, and an HS Precision stock will take its place, I wanted another McMillan but the price has doubled. If the HSP stock doesn't work out, I might have to bite the bullet eventually.
My Brno 602 qualifies as a full custom; 20" Ron Smith fluted barrel chambered in .375 Ultra, pillar and glass bedded into a cross bolted McMillan stock, knuckle deflector, custom quarter rib inletted for Talley QD rings for a scout scope, and a Talley base installed on the rear bridge to accommodate a Talley ghost ring. This was my go to rifle, it got carried almost daily, and shot a lot.