300 Win Mag reloading question

[Ckleeves]

Mag, I would need to run new brass through a FL sizer at least one time. Sometimes they come with obvious problems like out of round case mouths and who knows what else. After firing I can see your point. I thank you guys for posting those videos on how to set up your FL die to bump back the shoulder.
That is my next step once my cases are fired to fit my chamber. I do have some once fired brass that I have to FL size which I'll probably anneal before I do. Then go through the bumping process..
What I'm likely going to do is just use the purchased once fired brass for trigger time and then use all new virgin brass sized to my chamber and bumped for serious LR shooting.
Its been about 45 years since I had a belted cased cartridge so I FL sized everything. After getting responses from this post I'm only going to bum my shoulders back on my target rifles, this 300WM, my 6.5CM and my 308. All my other rifles which are for close range hunting (<200 yds) such as my 45-70, 30-30, 30-06 7-08 etc. get FL sized.

Case head separation doesn't care if it's a close range rifle or a target rifle.

One of my buddies was grossly oversizing brass by following the instructions from his dies and had case head separations on the 3rd firing with Lapua brass.

 

[cajun]

Just hit them with a mandrel. No need to run a sizing button down into the mouth to expand it, then size them down again just to have the button come back through again. I guess you could not go all the way down with the stoke and just have the button go in and out but a mandrel keeps things more concentric.

Yes that's all your doing. Just like a mandrel basically.

 

[MagnumManiac]

Mag, I would need to run new brass through a FL sizer at least one time. Sometimes they come with obvious problems like out of round case mouths and who knows what else. After firing I can see your point. I thank you guys for posting those videos on how to set up your FL die to bump back the shoulder.

I understand, BUT, an unfired BELTED case is completely unsupported in a FL die, so, ANY eccentricity in the body, WILL transmit into the neck. This is another reason to own Neck only dies.
Even if you don't go to the expense and level that I do with precisely honed necks and custom expanders, at least you can size the neck (Redding Deluxe Die Sets are the best) and keep everything concentric.
I have been shooting belted cases in competition for 30 years, I understand their inherent drawbacks, figured out what works and what doesn't, the BIGGEST killer is excessive brass movement ANYWHERE on the case.
Many believe that the belt gets sized, it doesn't, many believe the belt stops a case being sized because it doesn't enter the die fully, it's not meant to, never was. Many believe if the expansion line is tight in the chamber they need to size further down the case, it's doesn't, all this does is increase headspace and causes case head separations.
You need to understand HOW the case works when headspace off the belt, the belt holds ALL forward AND rearward movement, the shoulder EXPANDS to fill the void and the belt keep the case head against the bolt face. After which the case is PARTIALLY formed to the chamber, often .002"-.003" SHORT of chamber dimension. If you size at this point, you WILL BE inducing .004"-.005" headspace on the shoulder to base datum, THIS is what causes case head separations in belted cases. Period.
My custom 264WM, both chamber and throat, are based on the same principles of the A191 chamber dimension. This is one of the most accurate rounds I have ever developed personally, it is in the .2's and I have been using it for F-class this season with excellent results.
Brass movement, or lack thereof, is what is required for precision.

As @Mikecr says, more clearance is NEVER the solution.
Lond winded, I know, but this needs understanding.

Cheers.