257 Bob, Dogzapper, all,
My Webster's New Collegiate dictionary defines Custom Built as "built to individual specifications." Nowhere does it mention synthetic, or wood, or metal for that matter. If built to individual specifications, it is custom by definition.
Having said that, I think it is safe to say that most gun admirers usually think first of fine walnut stocked as examples of custom guns but that doesn't disqualify synthetics by any means.
There are two rules when dealing with fine wood stocks. Rule 1 is that wood will move; Rule 2 is that you can't do anything about Rule 1. However, synthetic stocks are not immune from movement - they just move from different causes. Wood movement is caused by moisture. Synthetic movement is caused by temperature variations.
It is also true, as someone already reported, that some custom makers shun fiberglass resins like the plague. They avoid it both in inletting and in stock finishes. That group usually will also have nothing to do with machine turning a stock from the blank. That's fine with me if that's what they want to do.
However, there is a much larger group of makers that do use resin sealers for the wood and use some glass bedding compound. They also use resin based stock finishes, usually mixed with some form of oil. Often they will use hand rubbed oil as a top coat(s) but only after the stock is really finished with resin.
I presently have a custom 458 Lott DGR being built. I can assure you that this rifle will have enough glass in the bedding to build a fair sized yacht. I do that not because the maker is incapable of tight inletting. Rather, I do it for strength and anyone that has shot one knows the Lott is a kicker.
If it is built to individual specifications, it is custom - regardless of the components used.
Tom
