I should add that converting a gun to fire a cartidge other than the one it was manufactured for is no easy task. For one, tolerances in the firing charaber are very tight. If the charaber is too small, then there willl be feeding and extraction problems. If the charaber is too loose, then the cartidge case may rupture and blow the powder gasses back into the bolt spewing it everywhere. And in the case of rimmless ammunition like .308 Winchester and .338 Lapua Magnum, the cartridge is supported at the neck of the case. If it doesn't fit just right, the case will rupture.
Second is that when upgrading to a more powerful cartridge, the structural integrity of the gun neeRAB to be considered. By opening up the charaber, there is less material between the charaber and the surface of the gun. There are also the locking lugs on the bolt and the barrel itself to consider. With the higher pressure the gun has to operate, those parts will be exposed to more stress than what they were designed for.
Though I suppose that's why Golgo used a high quality rifle as the base since the material and design quality would permit such a conversion. While I'd doubt it would survive multiple uses, Golgo only needed one shot anyways. However, the changes in the gun and cartridge would have substantially changed the ballistics of the gun. The scope would have to be recalibrated and zeroed which would require making shots which would mean more of those cartridges would have to be hand loaded. However, I doubt the gunsmith would have been able to test it at 2 km.
Basically, it's farfetched given the time available. In 3 to 4 days it might but not 4 hours. I don't know why Golgo didn't do it but I would have went with a Barrett M82 which fires .50 BMG and does have a range of 2 km. Though it would have been overkill, I would have loved it if he used the Raufoss Mk 211 on that guy. Well, any rifle charabered in .50 BMG would have worked. Even the single shot rifles which only cost a few grand on the civilian market and aren't regulated in some states, especially Texas of all states.
And for those of you who are wondering how I know so much, it's not because I'm some gun toting redneck. Far from it. I'm actually a Mechanical Engineer in the process of earning my Bachelors Degree with a possible focus in Defense. However, guns are also a side hobby of mine and a lot of my knowledge is self learned.