I would choose 3, but only IF it was a true GPR and not the new signature series - Lyman let Pedersoli bully them into only offering a 1:48 twist barrel in the .50 caliber rifles versus using a true slow twist barrel / PRB barrel.
Also, I'd ask what is your intended purpose? For example, I have the .50 GPR and use it for matches and general range plinking but am looking forward to using it when I move back east for deer hunting - hunting in Texas is a joke with all the 'game ranches' that want to charge you the price of a new truck to shoot animals in a barrel ... If I ever get the chance to hunt for hogs, elk, or moose (bucket list items) .... I am actually saving my duckets for a custom .58 caliber build.
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1) .50 Traditions Deluxe Kentucky. Do not like the two-piece stock. If I went the Traditions Route I'd opt for the Pennsylvania rifle instead.
2) .54 Cabelas Pedersoli Blue Ridge Kentucky. Though the .54 caliber has a PRB twist barrel, their .50 calibers do not, so NO on principal. Pedersoli just twists my moccasins with all the 1:48 barrels on their .50 caliber and smaller rifles.
3) .50 GPR. Yes, but only the older rifles with PRB twist barrels. No to the current Signature Series abortion currently offered. These will start drying up soon, so going straight to the source (Investarms) and picking up their Gemmer rifle (GPR) is always an option.
4) .54 GPR kit. Yes, if you have the skills or at least a mentor that can help you out to avoid any pitfalls. If I tried to build a kit on my own it would probably end up looking like a coat rack, or worse ....