@Pheroze,
If you reload I think the advantage of the 450 Rigby is the shoulder.
To me the biggest advantage as far as reloading the 450 Rigby versus a belted cartridge such as the 458 Lott and for that matter, the 460 Weatherby, is avoiding the belt. I always FL resize my dangerous game rounds and as far as the belt goes, you inevitably will get the slight expansion in the case body ahead of the belt because the FL sizing die can't go past it, and after a few firings (as little as 2-3 in my experience with some of the belted magnums) you can verify this by "mic-ing" the case body ahead of the belt with some Vernier calipers. There is a a Belted Magnum Collet Resizing Die that will help get rid of that "pre-belt" expansion made by Larry Willis (Innovative Technologies). I have one, and it works well. I have a few belted magnums (257 WM, 338 WM, 375 H&H Mag) that I use it on, but it's another step in my already tedious and laborious reloading regimen. It's nice to not have to fool with it. It's true that FL resizing every time will work-harden the brass and lead to premature failure of the case, or cracking of the case neck/body, but I minimize my case wear for my 450 Rigby by annealing the case every time I de-prime and ultrasonically clean, and before I FL resize the case. Another inherent problem with straight-walled cases is the fact that one has to get the flare, or bell, of the case mouth just right in order to seat the bullet correctly in the case without getting an accordion, or wrinkling, effect of the first few mm of the case mouth/body. I learned this the hard way when I was learning to reload for my B. Searcy Double Rifle in 450 NE 3 1/4 inch. It's a long, straight case, and I thought I had flared the case mouth enough, but not so much. Case mouth flaring/belling is just another step in the reloading process when reloading for straight-walled cartridges. No big deal , but just another step to learn.
You can FL resize to within 0.02 and minimize case expansion. That also helps with accuracy.
There’s different ways but with any DG cartridge I’d recommend not minimizing the resizing operation. Accuracy is a non-issue and you want to be damn sure everything enters the chamber, fully, without a hitch. A little sand or dirt is all it takes.
And while it is true you can partial FL resize bottleneck cartridges like the 450 Rigby, I would be hesitant to do so because eventually after a few firings, the case body will expand (and not spring back enough) to the point of needing FL resizing. Without FL resizing the case, it can be difficult to chamber and eventually may stick in the chamber at the worst possible time. Even if it didn't need FL resizing, the chance of it sticking in my chamber due to grit as CTDolan pointed out, or because of increased pressure in hot temps, would give me pause in partial FL resizing any round intended on potentially saving ones life. I agree with CTDolan in that accuracy is not much of an issue for rifle of this caliber and intended purpose. I FL resize the cases for my AHR DGR in 450 Rigby, and I can still get outstanding accuracy (sub MOA) at 100 yards and in.
You can FL resize to within 0.02 and minimize case expansion. That also helps with accuracy.
Minimal case expansion and accuracy are related, but also two different things in this case. What you're describing is partial FL resizing, and effectively just bumping the shoulder back 0.002 to 0.003 while also resizing the neck. This will leave the case body alone for the most part, and after the first expansion from firing, and the subsequent retraction/spring back of the brass, you will have a brass case that will have minimal gap between the case wall and
YOUR chamber. The key is
YOUR chamber, as it may not be the case in another rifle's chamber, especially tight custom chamber reamers with different leade/throat dimensions than SAAMI spec chambers. The big advantage of partial FL resizing is that you do bump the shoulder back minimally and thus allow for a little gap between the chamber wall and case in this area. Neck sizing only doesn't do this at all, and only resizes the neck. By using either of these techniques, your technically minimizing play/slop between the brass case and chamber wall, and therefore aligning the shoulder/neck area (and thus the bullet) and theoretically better centering that bullet in the leade/throat area so that once the bullet jumps to the lands of the barrel at ignition, it engages those lands equally and at roughly the same time if you will. Assuming your bullet concentricity (as it is seated in the case neck), neck tension, neck uniformity, etc., are all consistent, this technique should lead to increased consistency and thus accuracy. I believe it probably does, and most BR, F-class, PRS shooters do this as well. For my non-DGR hunting rifles where long range shots are possible, like my sheep/goat rifle (280 Ackley Improved), I do this as well. But as stated before, for a DGR, I personally would not do this as the risk/benefit equation is much more heavily weighted towards malfunction with little gain in accuracy at DGR ranges. Of course, I know you didn't ask my opinion, and realize my opinion is valued at a penny or maybe less....
I have heard the statement before that straight-walled cartridges are inherently more accurate than bottleneck cartridges, but if that is the case, why are none of the precision shooters using these cartridges versus the bottlenecked case like 6 BRA, 6 and 6.5 Creedmore, 6 PPC, etc.? I think for blackpowder cartridges there are some inherent advantages of straight-walled cartridges, etc., however I'm not sure accuracy is one of them, but I don't know. There are indeed some very accurate rifles (and handguns) that shoot straight-walled cartridges very accurately, but I'm not sure if that's the cartridge itself or the rifle or the shooter or all of the above...I'm sure there are more experienced wildcatters, reloaders, etc., on this forum that have more knowledge and experience than myself.
Sorry to hijack the thread. I actually started typing and realized that I'd deviated way off on a tangent, but had already committed to, and spent a lot of time on, the typing of a lot of this so I sent it....so there you go....