There was a post here where someone said they had a 458WinMag with an extra 1/4" chamber. That begs the question, how deeply is a 500gr bullet seated in the case when using a standard chamber?

Or, rather than just reaming an extra 1/4" how long would the chamber have to be if you wanted to seat a 500gr bullet exactly .458" into the case?

Assuming you can then seat an extra 1/4" out, how much is gained in powder space? What's the increase in fps? What kind of performance would you get with the proposed max chamber?

I guess what I want to know: Is any of this worth it on a performance level?

The throat length is what I believe your after. That is the distance from the end of the neck of a cartridge to the beginning of the actual full diameter of the lands. This is what controls maximum COL. The rest of the actual chamber is std .458. In fact, you can produce a throat so long that a 500 gr bullet will never ever even touch the lands. This is all done with a std throater.
In the 458 win mag a .250 inch throat in a Single shot rifle will result in a COL of slightly greater than. 3.480 inches. This increases the powder capacity significantly enough( 5-10 grs-compressed) that QUICK LOAD predicts a velocity of approx. 2300 fps in a 23 inch barrel with WW748. I've seen 2250 fps in my rifle with 82 grs of ww 748 and a 500 gr Hornady soft point. Thats 458 Lott territory my friend! Given that factory .458 win mags are hard pressed to even hit 2050 fps, thats a 200 fps increase!
In a typical bolt gun you may not have enough magazine length in 458 win mag to ever achieve this COL( despite the throat length), but in a Ruger #1 it's no problem at all. Pressures appear to be in the in the 65KPSI range( although Quick load predicts higher) based on belt expansion measurments(<.oo1) with no stickly extraction and no flattened primers. I put a second cannelure on the bullet and use a lee factory crimp die. Freebore is a wonderfull thing in minimizing peak chamber pressures.
Did I mention < 1 MOA accuracy?-Rob

[ 10-10-2003, 08:16: Message edited by: Robgunbuilder ]

Thats all fine and good but this sort of thing does not work well with a DGR that needs to be crimped in the Cannalure, and thus the 500 gr. will seat with a big crunch of powder and compaction will then become the culprit of this poorly designed round....Your just solving one problem to create another..

Better to punch it out to the Lott, Ackley or 3" African round then seat the bullet to the cannalure and crimp it...

Ray- I totally disagree! I put a new cannelure on the bullet approx .250 below the factory oneusing a corbin cannalure tool and crimp the case solidly into it. There is actually very little powder compression this way and the round shoots perfectly out of a Ruger #1. I would not reccomend it for a box mag gun, but for a Ruger#1 no problems. I'm not arguing that a Lott or Ackley is a better DGR round, the issue is what is the max you can actually get out of a 458 win mag!-Rob

[ 10-11-2003, 00:49: Message edited by: Robgunbuilder ]

I saw something about bullets made in South Africa for .458s. They had a cannular much closer to the heel of the bullet. The idea was that they could be seated further out for more power capacity. Has anyone tried them?