All rimless ctgs. headspace on the shoulder to the bolt face. Only rimless rounds such as the 9MM luger or .45 Auto headspace on the case mouth + perhasp other rimless handgun rounds. I know of no other rifle rounds that headspace on the rim, except the M1 Carbine. all ctgs. with shoulders, headspace on the shoulder to bolt.

The .400 Brown Whelen does not present a headspace problem due to the diameter of the shoulder, compared to the standard '06 case necked up to .375 and .400".

I will add that only people who kow what they are doing, should be handloading ctgs. with tiny shoulders, ie: .375 Whelen, .400 Whelen and the 10,75x57.

Knowing NOT to shove the shoulder when you re-size the brass is the only stickler, the only problem lack of experience creates - seems too few people know this - even some gun mag writers screw up when sizing brass.

Adjusting the sizer die so only "MOST" of the neck is sized is the best way of avoiding trouble. NEVER touch the shell holder to the bottom of the sizer die, unless that is necessary to allow chambering that round.

Keeping load pressure below the elastic limit of the case itself, will ensure that neck-sized brass fits - always, as it shrinks a minimum of .001" all around, length and diameter, after being fired. That is enough to allow easy chambering and exraction. In all my rifle ctg. loading, I only neck size and have been for over 30 years with never a failure to chamber or get sticky.

I've read as have many others, that for Africa and dangerous game, one must FL size his brass. Too many people take that to think they need to bottom out the die against the shell holder - doing that can shove the shoulder back, up to .1", which creates excessive headspace over 14 times more than what the CIP and SAAMI standards are for factory rifles and ctgs., ie: .006" and .007".

I suggest you load as normal and if that brass is a bit snug to chamber, adjust the die to barely bump the shoulder and try it again - do this in tiny steps until the ammo chambers perfectly.

A failure to chamber due to a tight ctg. could be disastrus in a dangerous situation, but then, creating excessive headspace to where the ctg. separates above the web and leaves the top part of the case in the chamber will equally be nasty.

I always cycle the ammo that I take hunting, to ensure it feeds and chambers perfectly, even though I know it will. Check it and you'll do fine & probalby with only neck sizing or partial full length sizing.

When sizing brass, always lube the inside of the case mouths. Many dies will gall and stretch the necks when the expander button is removed, thus making a perfectly sized case, overly long after sizing - the headspace has been changed just by the expander button. This happens more often or to more people than some people realize.

Overreaction to this case stretch situation can create an excessive headspace problem if the die is screwed down further when that was not the problem in the first place. Failure to inside lube, was.

Note the brass - 1st is an 8x57 case. The second is an 8x57 merely necked up to hold a .366" bullet. The next is an 8x57 necked straight, then sized to hold a .366" bullet, but more importantly, sized to create a shoulder to perfectly fit my 9.3x57 chamber with a CRUSH fit the first time it is fired. The same system should be used to create brass for a 10,75x57. Merely necking the case up may leave you with a ctg. already too sloppy in headspace and will weaken the case at the web due to stretching when it's fired.

Edited to add:

Note the different height of the shoulder in the 2nd case, compared to the far right-hand case's shoudler height. The difference is substancial - .019" in actual fact and is easily seen with the naked eye. Failure to create a new shoulder before fireforming, would result in excessive case stretch above the web and shorten case life to 1 to 3 firings, probably. Properly formed to start with, prevents any case stretch, anywhere, and thus, they will last 50 or 60 firings(maybe more), if I anneal them every 10 or so firings.

The same system of case forming was done with my .375/06IMP. Necking them up straight, then down in the neck sizer until they fit with a crush fit, then fireforming.