So,
@Dfou , it's great you came in here and asked these questions. If nobody has yet offered you a welcome to the forum, then please consider this a warm and hearty welcome.
Having been on here quite a bit myself the last couple of years, I recognize a lot of the people in this thread and remember a lot about what they've shared regarding their shooting habits, capabilities, fails, successes, etc, over that period. These same guys who might seem like a jumbled mashup of faceless dudes on the internet, to me are, "Oh, the guy who hammered a buck at 630 yds, uphill, seated, resting his gun on a backpack", or "the guy who goes through 1000 rounds a month of centerfire rifle ammo across different cartridges", etc.
Here's a couple of things I can share that might help set the context for the responses you're getting:
1) Every single one of these guys commenting here, is giving you the most sincere, experienced answers we can give, genuinely trying to help, not looking down on you or your son in any way.
2) There's at least 4 in here I can safely say shoot over 3000-5000 centerfire rifle rounds each, annually. Some do more. I personally do about 5k. Just about anyone giving you a detailed answer is going through at least 1000 annually - all of whom share a very strong focus on field-realistic training and riflery competence for hunting realities.
3) Virtually none of us shoot magnums...anymore. Almost all of us
used to. Those few commenting in your thread here who do, usually only do so with it as a specialized tool for very specific, uncommon purposes. This is a pattern of success that leaves clues based on high volumes of range and field experience.
4) Because the guys are so focused on field-realities and field competence, we may have a different understanding of what "proficient" would be, so please take that into account. For most of us, minimum proficiency is going to be something approximating being able to hit 8" vitals out to 300yds, in any field-expedient position, in any environmental conditions - rain, wind, snow, sundown, sunup, high-angle, shooting prone off a backpack, to something weird like being crouched low with a broken branch acting like a monopod as the only support.
This is a far more difficult standard of proficiency than one would suspect, and it's made a full order of magnitude more difficult with a magnum.
5) Across time and people and experience, almost all of us shoot high volumes of .223 as a trainer, and only escalate cartridge size to the point we can keep that same level of proficiency. It's like starting with training wheels, and just removing them, and then maybe bumping up to a BMX bike...before hopping on a Kawasaki/Magnum. Consequently, few of us shoot anything bigger than 6CM, 6.5CM, or 6.5PRC on any North American big game - and yet we also all know that .223 with a good tipped match bullet will kill anything on the continent.
There is over 600 pages of thousands of posts of proof, showing the documented photos of this. It's definitely worthwhile taking your time to read through this.
Please don't take any of this as an attack on you or your son - it's simply the best information we can provide, and it's being given sincerely, trying to help. And every single one of us here has had our worldviews challenged with all of this, especially when we first jump into this community. Much of it is very much against common, accepted wisdom.
So in my first response here in your post, please pardon the efficiency with which I shared the info I did - it wasn't meant to come across as jerkish or as parenting advice. It's just stuff I - and we - have seen our entire lives, and often went through ourselves as kids or parents, myself included.
One piece of advice I will give: If your son is really intent on a .300 Win Mag, bite the bullet and buy him 2 guns. A .223 Tikka, and a .300 Win Mag of his choice. Then set the standards of performance, with plenty of ammo for each to do with as he so chooses. It will be fairly evident for him, personally, where the path to success will be, fairly quickly.
