I think the salesman was doing you a favor. His choice of words was poor, but he did have your best interest at heart.
I need to practice and might as well have the best tool for the job.
I think you are mistaken. It isn't that you need to practice. You need to learn. Practice comes AFTER learning. The rifle and caliber in question are not going to be good tools for learning what you want to learn given your apparent lack of experience.
So if you are going the "best tool for the job" route, then obviously you are going to be using nothing but match ammo. So you are looking at roughly $1.75-3.00 a round after shipping and/or taxes.
If you are self teaching, I would suggest that for what it is going to cost you in ammo to get up to speed on long range shooting with that gun and caliber, you could buy a smaller caliber accurate bolt gun in .223 and a LOT of ammo and likely come out financially ahead by the time you decide to step up to the rifle and caliber mentioned in the OP. In the end, you would end up having 2 rifles and be proficient with both for the same amount or less than what it is going to cost you by starting off with the desired 300.
It has been noted that one sure way to develop bad habits is by starting out at the top. That is probably a fairly accurate statement unless you plan to hire a shooting instructor for the first 10-20 shooting sessions.
Going with the larger caliber and gun is going to make it a lot more difficult for you to comprehend and correct whatever shooting issues you may encounter, not to mention being much more expensive.
Part of what is going to likely give you fits is figuring out what ammo/loads work best for the gun in terms of accuracy. You are going to have a hard time determining if your shooting issues issues are caused by you, the rifle, or the ammo.
You said it best. Setting it up will cost a little moola, getting a good piece of glass atop it, bipod, reloading supplies... that sorta thing. It's only money.
As far as the question of ammunition expense. There is a whole new hobby for you to get into and enjoy year round >(reloading)
You reloading for this rifle would be a mistake. You are already going to have some issues with finding commerical ammo that works well or best in your gun given your inexperience. Trying to learn to shoot at longer distance while also attempting to develop your own loads for the gun will definitely cause you a lot of frustration. As with commercial loads, how are you going to know if accuracy issues are you or the ammo? How about for consistency - you or the reloads?
Buy what you want, but keep in mind that by going the route you want to go will likely be a much more frustrating and expensive way to become the long range shooter you want to be. Plus, going your route may turn out to be a costly mistake if you learn that you really don't enjoy long range shooting or don't have the knack for it.
Very likely he was trying to goad you into buying that rifle. Most men, when told something is too much for'm, will push out their chest and grab that item just to go out and prove they are man enough. It's a common sales technique but one that prevents future sales to that customer.
That certainly does not sound like what the OP described. The is no macho-appeal in buying expensive ammunition. More over, the guy at the gun store was trying to talk the OP out of a gun that the OP wants to buy. If anything, it sounds like he is trying to help the OP make a better purchase decision and one that would not result in having a dissatisfied customer. The gun store guy would not be noting how expensive the ammo will be if he was trying to goad the OP into a purchase based on machoism. He would have completely left out that negative factor.
