Most machine shops will have multiple lathes with a large enough through bore on the spindle. Typically the smallest lathe at a machine shop will be a 14x40" with around a 2.5" through bore, or very close to that. Then go up from there. Out of the dozen shops I know, some don't even have a lathe that small. Some have lathes with a 30 foot bed, with massive chucks, and bores. A machine shop is alot different than a typical gunsmith shop, and you do not need a spider. There are different ways to do it than a spider. You could even have a another 4 jaw on the other side of the spindle.
It's even possible to do on a longer lathe with a small spindle bore, if it's got a long enough bed. You just gotta think outside the box.
I am by no means knocking gunsmiths, I would love to be a gunsmith, and respect the hell out of them. I am merely stating there is a difference between a machine shop, and a typical gunsmith. They deal with many different types of steels, all sorts of shapes and sizes, and sometimes have to hold to tolerances that are pretty extreme. Having to hold things, or chuck stuff up that may seem impossible to do.
And guys who dial in things for a living on a four jaw can dial them in extremely fast. Way faster than 10 minutes to dial in something concentric to the bore on both sides of the spindle.
