well, as usuall I cannot resist the urge to be the antagonist here.

Who defines what is custom(ized) and what is good enough to be called custom (built)?

Surely, SOMEONE 'built' Chad's SKS into what it is today. was it a licenced gunsmith? no. was it a fancy factory? no. It was chad.

Is there a licence or a certification for making 'custom-built' guns? I'd like to see what the placcard looks like. or are they each 'customized' to their owners?

Even Les Baers makes small runs of stamped-out parts for their 'customizations'....now how is this different from the SKS polymer stock chad bought at the army surplus store?

Where is the line that defines a small enough production run to be a 'custom-building-part'? Where is the line of expertise needed, or the price tag charged, for the work being done?

If we were lawyers and this were a major decision, I think the ONLY clear-cut enough definition to all agree on in any situation and any case, is ANY modification from factory-stock.... Does not matter it it was finely engraved, or gouged with a hammer and a nail in Bubba's garage table steel vice.

And along these clear-cut definitions, I'm sorry to say that a Ruger MKII with integral silencer is not a custom built gun, it is an alternative production run on a factory gun.

Get a normal MKII and fit or have a silencer fitted to it and integrated, yes. it is now a custom weapon.