Re: TRG-42 300WM to 338LM ?
A Bolt-body is NOT a complete bolt...
In addition to the 13.5mm bolt body, you'll need your firing pin assy (or buy another one), bolt handle, ejector assy, and a .338 Lapua specific extractor. Will also need a new Lapua lipped magazine at $250 per...
Probably headspace will be fine w/nothing more than a go.no-go gauge proofing. Unless you anticipate sharing ammunition or shooting factory production ammo, Go-Gauge plus .0005" or even under will produce best accuracy.
Before you spend $400 to $600 for a new bolt assy, might give some thought to other ctgs that will function at Lapua performance levels, but not require a new bolt. .338/300 ultra, .338 ultra, .340wby, .330 Dakota. Also, don't laugh; for great precision to 1200yds the .338 winchester...
Even with the 3.75" magazine length any case longer than the .300win does not leave you a lot of bullet seating room. If mainly shooting targets, the .338win gives you more bullet seating room and no need for a new bolt or magazine. I like the .338/300win for a bit more capacity and still mucho more seating flexibility.
Of course, probably no way to use the factory Lapua barrel though... I wrestled with this when I also owned a TRG-S in .30-378 and decided to opt for better barrel life and load flexibility. Think the .338 Norma might be a great choice, but really the ultra/edge and .340 weatherby will deliver all the velocity you might want. Set the belted magnums up to minimum oal headspace on the shoulder and maybe buy RWS .300win or Norma brass if you must; but Winchester is plenty good on a minimum oal chambered rifle; even Remington is fine.
I think the real ticket for the TRG42 is going to the .473" bolt body and 6.5x55AI or .284. A .338/284 would really be the ticket for long barrel life and flexibility with 300gr bullets.
Just some ideas... Have fun!