There are about 40/1000 of an inch difference be-
tween the 550 Express' bullet diameter and it's
base measurement just forward of the belt. (True
too for the 550 Magnum.) So that's a pretty darn
straight case. With bolt actions they extract with
no problem it seems. HUBEL has his 585 HE at about
50/1000's spread. It also has a very straight case
and sound extraction in B/A rifles. If using a
Farquharson action and a RIMMED/FLANGED cart-
ridge, at the minimum how many thousandths
of an inch do we NEED, between the bullet
diameter and the base of the brass diameter,
to insure easy extraction when seeking 2200 FPS MV
or better, and using flat tip CEB 13s or NF's, at .280
sectional density, calibers from .458 - .585 ?????
Figure a brass length of 2.8" give or take.

{NOTE: Per MICHAEL458'S research it seems this
MV and Sec. Den. is all that's needed with the new
flat tip banded solids from GSC, NF & CEB}

You need to specify the difference between the loaded brass neck diameter, not bullet diameter,
versus brass cartridge base diameter,
if you want to have a meaningful discussion about case body taper.
Only 1 or 2 thou of diameter taper per inch of brass case length is required for reliable extraction.
For ease of feeding in a bolt action, or chambering a loaded cartridge in a single shot with some stray dust and dirt in the field:
That is a different matter.
Better to have about 10 thou of diameter taper per inch of case length to ease that.
The more, the better.
Case taper does not increase back thrust.
Straight cases do not have less bolt thrust than greatly tapered cases ... but I digress ...

Brass neck wall thickness is usually about .010" per side at the neck, beside the loaded bullet.
It is often more than .010", rarely less than .010", depending on the brass make and the cartridge specified.

This is IMHO.
I would not discuss a rule of thumb for cartridge taper based on bullet diameter versus case head diameter.
You need to add the neck brass wall thickness (as loaded) to the bullet diameter and compare that to the brass base diameter.
Add about 20 thou to bullet diameter then see how much difference there is between that and the base diameter of the case, for a start at assessing hypothetical wildcat case taper.

Brass maximums and chamber minimums are to be specified.
The brass must fit into the chamber safely and functionally.
Amount of slop allowed depends on purpose intended for the cartridge.

GREAT answer RIP! I appreciate your detailed
break down very much. Based on what I've learned
from hand loaders in person, and what I have read,
my tendency has been to add .025" to the bullet
diameter when I ponder these questions that arise
in my head. And since nobody really cares what
neck diameter is of loaded ammo in CASUAL chit-
chat about this stuff, I just talk in terms of
bullet caliber and how "fat" the brass is at it's
fattest. Your words above though, are truly the
way to size things up when getting serious about
doing a wildcat, NO doubt. Thanks as always.

My 585 has a total taper of both sides of .027"
from the OD of the base ahead of the belt and the
OD of the case mouth. In 3 inches of the case length
from the front of the belt to the mouth.
I prefer no less taper.

I made one to try that I didn't go with--- where there was only
about .007" total taper both sides, and it was not enough
for best feeding and reliable extraction...

My 700HE total taper is .038" in 3.5" of case side length.
And they both extract fine with 70k loads. My 585HE in
NEF with 45k loads the cases fall out.

I could just say 45acp...

I have never had extraction problems with the express.

Jeffe,

I think your 550 Express is really fine! I'd love
to see you take Hubel's 585 down to the same
OAL. Is there a short action that could hold such
a cartridge with a mag that would have room for
FOUR rounds?
Link to 585 HE: Good Stuff

DR,
well.. it wouldn't work due to action availability .. bolts to take the gibbs case head PROPERLY arent short enough actions... no return .. and no good reason to make the ultimate big bore standard length round to have inherent weaknesses...


and ZERO reason for more taper.. a perfectly straight case of .997 will extract perfectly from a chamber from 1.00 to .998 (base to case mouth) .. unless pressures are stupid. a straight plug always extracts from a normally tapered hole.

this isn't fly specs in the pepper.. this is rice specs in the salt... while on paper it might make one feel good to discuss, the engineering term "proven by use" is perfectly just here

My case can be cut back even to 1.8" and
have same bore size, with same die with same taper..

IE-- the inside is parallel back to shorter than 1.8" and
IE the taper of the increase in brass thickness matches the
taper of the case sides.

And that's why I have a bunch of plain
headed cases for guys wanting shorter, they can cut them
off and have same bore if wanted or neck them.

The venerable 470NE, 500NE 577NE have mmuch
more taper angle and the 458 Lott and similiar
cases about same taper, and they extract/feed under extremes of
heat, dust, etc, due to the proper taper.Ed

I have cases with 0.003/Inch taper with zero extraction problems at 65kpsi. Much ado about nothing!-Rob

Link to 45 ACP: Thank You AmmoGuide
3/1000 taper in this really short round.

RobGun speaks of his experience with brass of the
same, (almost no) taper giving NO extraction
difficulty. Good news for all. HUBEL's 585 HE can
be shortened WAY down and still use the same
dies. How excellent! Now the bolt action makers
need to make a short version to handle Ed's and
Rob's rounds cut down, say to brass length of 2.60
inches or so.

Back to the FARQUHARSON, I am now understanding
the taper severity needed for a reliable hunting rifle.
And as always, thanks guys!