Seperating cases is not necessarily a sign of hot loads, but that of setting your shoulders back to far when sizing. It sounds like you've got excessive headspace, thus the seperations. What die set up are you using? Maybe we can help you adjust your headspace, stop the seperations and allow you to keep what sounds to an incredibly accurate load!
I've been reading the posts and there is some good stuff but the last one you posted may your answer. You don't have to full length size. If your getting separation, it is possible that you are setting the shoulder back to much. I would just neck size the next group. Then when the bolt starts to get too snug then bump you shoulder back a little bitty bit at a time. Start with just barley hitting it then try it. If it is still tight do it again just a eight or quarter turn down or so and try it again When the bolt closes with just a little pressure lock the die down and you should be set. The issue is this cartridge was designed to head space off of the belt you need to make it head space off of the shoulder if you want to get any longevity out of your brass as well as get more accurate. Almost all of the factory 300wm that I have worked with showed a large case length grownth from the head to the shoulder after the first shot. The die instructions say move the die to the shell holder but I just start a little high and work the die down to where it is where I want it. Another note here is that many people have trouble with the case over expanding just above the belt this is more than likely because they are over resizing there cases. In other words putting to much pressure on top in order to return the should back to where the factory makes them. Factory rifles adhere to the SAMM specifications which in belted cases entail the belt not the shoulder this is why there is a lot of extra room in front of the shoulder.
The bullet jump your talking about should be fine unless you are using VLD's which don't like to jump normally but some times they do. There was a suggestion about opening up your mag well to some degree. If that is possible give it a try just make sure your cartridge will feed properly. I wouldn't worry about the length of jump you have until you have tried everything else first. I have loaded plenty of 308 mags and usually I have better results backed off a little anyway. The powder your using is an old "good" standard for this cartridge. It was suggested earlier that you try 4350 if your barrel is short I would agree its worth a try. If it is a longer barrel say at least 24 you might try a little H-1000 but more than likely your powder is not the issue.
I don't see a problem with the bullet choice The sst is a good hunting bullet, but an alternative might be a Sierra 180 spritzer boat tail. or the Nosler Accubond both very good bullets. The Sierra is more affordable.
One thing I would defiantly try is changing your primer many may disagree because many are fond of federal but I really don't like there primers it seams that every time I work a load with them I don't get it where I want it. The bottom line is I believe they are too hot.I feel they burn a column out of the center instead of lighting the powder somewhat evenly. I almost exclusively use Remington primers. They just produce more consistent loads this is verified from a oehler chronograph. In your case the 9 1/2 mag would be my choice. The powder you are using is probably what you need but it just needs to be lit more uniformly and the 4831 isn't that hard to lite per say at least in comparatively to retumbo.
Sorry about going on and on but I think your resizing practice is your first issue then I would try the primer change with the load your shooting back it off a bit and work up I have a feeling you might get what your looking for. I load for several people and I work with what most would consider less than perfection in guns. It is more rewarding to make a mediocre rifle shoot than the custom built rifle shoot I can usually get a custom to shoot within two trips to the range but the mid-grade is a real challenge . I may bed the gun or put a little pressure on the barrel who knows what ever it take but it is fun. Speaking of this what is the stock made of??I'm not real familiar with this rifle
I realize that your rifle isn't what most like to call a great rifle but it may shoot good if you put enough work into it. but then again sometimes they just don't. This is not the case with your gun your shooting into a quarter and for a factory gun that is pretty darn good maybe you can get it to shoot better.
I hope all this rambling has helped I would be curious what you come up with so please keep us up dated.
The Lee Collet doesn't work the shoulder area, from my experiences as I use them religously. Its sole focus of operation is on the neck being squeezed around the mandrel. If you have the die adjusted too deep into the frame, you run a risk of springing the collet- no cause for alarm...just disassemble and un-spring it.
Back the die out and ..with the ram at full stroke.. screw the die down until it just engages the shellholder, then give it 1 (one) turn. Size your case, take an ID reading of the neck w/your calibers and you might have to give the die another quarter turn.
Another thought- most compound presses that cam-over-top stroke can present a problem w/overworking the collet. Be sure, when you adjust the die, that your ram is at the top of its stroke.
__________________ Go Ahead- Melt the Copper Off. . .