I've been loading the .300 Win Mag for a while and want to know if there is a better way. Should I be just neck sizing the cases?
While held by the belt, the cartridge should fireform to the chamber of the specific weapon I am firing. Now the cartridge should fit and rest as much on the shoulder as on the belt.
If this is correct, then I should be able to simply neck size the cartridges in the future and seat the bullet as close as possible to the end of the chamber - providing it still fits the magazine. I think it will reduce the working of the brass, eliminate thickening in the shoulder and produce a better sized cartridge, for that particular rifle, that will be more accurate. Sort of like comparing a custom pair of boots to off the shelf pairs.
Can some one with greater knowledge in this area than I, let me know if I am on track or way off in the boonies?

Your 300 Win Mag headspaces on the shoulder not the belt. Only belted mags I can think of off the top of my head that headspace off the belt are the 300 and 375 H & H magnums. You can only neck size but I would only do so after you get a new load of brass and full lenth resize. After firing, it does form to your particular chamber and then you can neck size after that. There is a school of thought that this will provide more accurate ammo and give longer brass life. I normally full length resize so I can't tell you from personal experience if this is so but you can try it and work some loads and see if if does provide you with more accurate rounds.

You are on the right track.
Cheezywan

Be careful of the OAL of the round. I recently had a "pressure excursion" in a 308 because the bullet was seated too close to the rifling. It raises the starting pressure dramatically. Most references recommend .020" to start, Barnes wants more.

While yoyu are correct in asuming that neck sizing would fireform the case to headspace on the shoulder as well as the belt, you may have to nudge the shoulder back on occasion to ensure a fit under hunting conditions. On one occasion loads that fit the rife with minimum play became very hard to chamber when the rifle got cold (0 degrees F).
I have eperienced no practical benefit to accuracy from doing neck sizing. In fact, there isa school of thought that it may actually be detremental to accuracy unless a mandrel or bushing die is used because the neck is less likely to be concentric with the case.

Cossack and M1 both have good points. My personal experience is that a mandral die such as Lee sells works well and does give me an accuracy improvement. The two biggest boons I gain from using Lee's mandral neck sizing die are extended case life, up to 20 plus firings, and not having to lube the cases on resizing.

I've got to tell the truth this not having to lube the cases turned out to be the biggest draw.

Only downsides are you have to follow the directions exactly or you will have loose necks the won't hold the bullets correctly and though you have plenty of tension they are never as tight a crimped bullet. If I'm loading hunting rounds I will follow up with Lee's factory crimp die.

If your going to use the bullets in two different guns then you will want to full length size.

When using Lee's neck size die, I've found you need to spin the case 1/3 or 1/2 way and do each case two or three times for the best results. I also have taken the rod out of the middle and chucked it into a drill to take just a hair of metal off where the case neck is going to be pressed against it.

The no lube is the big thing for me but have gotten used to partial spin and press on a couple of Lee products including the FCD.

Bill

I too use the LEE mandrel neck sizing die. The biggest mistake people make with this die is using it too aggressively. It doesn't take much pressure to reform the neck and you only need it tight enough to prevent the bullet from sliding freely in the case. ALWAYS crimp, even if it is a modest crimp. It's the only way to get uniform controlled load performance.

Can't think of a single reason to size a case more than is needed to (1) hold the bullet securely and (2) allow free entry into the chamber. Anything past taht point would be for assuring free chambering if the cases get dirty/gritty...and as we aren't slogging our way through the trenches or humping over sand dunes, that's not really and issue.

I use the Lee Collet Die as well and have noted a significant increase in accuracy as well as the more obvious advantages of longer case life and not having to use lube.

My '06s favorite whitetail load consists of a 150 grain pointed flat based bullet over 52 gr RL-15, CCI large rifle primer in Remington brass. I get 5 shot groups just under an 1.5 inches at 100 yards w/ FL sized brass, but just over .75 w/ neck-sized.

One thing that you didn't mention was whether you're shooting a bolt or falling block single shot. If you're shooting a BAR or something you'd want to keep up w/ the FL as the rounds won't chamber. If you're shooting a bolt or a falling block single you're golden.

I have run into instances where the shoulder needed to be "bumped back" and caused difficulty chambering. Because of this problem I make sure to chamber all of my hunting ammo once before taking it into the field and try to do so under field conditions, ie similar temp/relative humidity/etc.