Bullet seating starts with determining the jam.  This is somewhat subjective
and isn’t transferable.  Neck tension, ogive, and case orientation all must be
considered so jam is gun specific.  But for simplicity sake view it as the
starting point for seating relative to the lands.  My process for using jam to
seat on, or just off the rifling, is as follows:
    1) Remove the firing pin from the bolt assembly.

    2) Full length size a case that's been fired in your chamber. If you're
    using a bushing die install a ring 0.003" - 0.004" under the loaded neck.
    Grab is a must here....you want firm neck tension for this procedure.

    3) Seat the bullet way long.

    4) Polish the bullet with 0000 steel wool and then blacken it with a lighter.

    5) Clean the chamber and lube the locking lugs.  Jamming a bullet
    imparts rub of the rear of the bolt; we don't want to gall the lugs.

    6) Chamber the round.  Don't worry, it's supposed to close hard as the
    long seated bullet meets the rifling.

    7) Remove the round.  Now sometimes the bullet imprints so much it
    sticks in the bore.  If it does tap it out with a cleaning rod.

    8) Inspect the bullet for rifling marks.  Because we're seated way out
    you'll see land marks all the way around.  If present seat the bullet
    0.010" farther down.

    9) Repeat steps 6 - 8 until the marks start to fade.  At this point decrease
    the bump to 0.005".

    10) When the marks just disappear you're at the touch point on the
    lands; meaning the ogive is beginning to engage the rifling.  I like to call
    this point my "seating zero".

    11) Measure the base-to-rifling touch with a ogive stem and caliper.

    12) If you want to jump the bullet seat to the desired gap and re-
    measure.
Clark used the slowest propellants offered in the 1960’s and a lot of them
were military surplus.  Since then some of those machine gun powders have
been marketed which means improved availability and consistency.  I was
interested in trying the bottom 5% of the burn chart, specifically US 869 and
Retumbo.   8700, RL 25, and H 50BMG are also good choices.
When you’re this over-bore put some extra thought into the propellant.  
These speedsters have narrow tune windows so don’t limit yourself to just
burn rate.  Granule size, shape, load density, and heat potential may be just
as important.  The other elephant in the room is barrel erosion and here’s
where myth enters the discussion.  Three common misconceptions:
    1) Ball powders erode barrels faster than extruded.  This myth is bi-
    directional.
    2) Higher velocity means higher friction.  This accelerates barrel erosion.
    3) The mechanical influence of bullet jackets/coatings is equal to the
    effects of burning powder.
I’ll attempt to debunk each in a minute but will start with why barrels
wear.  Thermal, chemical, and mechanical events cause metal erosion.  The
three act interdependently on the lands and grooves, most notably in the
chamber’s throat.
    Thermal – bore surface phase changes (i.e. transitioning between solid,
    liquid, gas phases), softening and melting, as well as cracking due to
    expansion and contraction from the barrel heating and cooling.

    Chemical – carburizing or oxidizing reactions which are chemical
    processes that occur at the bore surface under extreme heat.  These
    cause the barrel to change at a molecular level.

    Mechanical – erosion caused by direct impingement of gas and solid
    particles traveling across the bore surface.
Thermal and chemical degrade metal to where bullet friction causes
molecular change (mechanical).  We’ve certainly become better at combating
these three over the past 100 years.  Unbeknownst to many, there was a
time when primers ravaged barrels as much as lit powder.  Early cups used
potassium chlorate as the oxidizer in the priming compound.  The by-product
of igniting potassium chlorate is potassium chloride which is similar to
common table salt.  And like any salt KCl attracts and holds moisture causing
rust.  Mercury fulminate introduced another problem.  When smokeless
powder came on the scene operating pressures rose and jacketed bullets took
off.  The performance gains were major but handloaders soon noticed their
brass becoming brittle.  Cracks, splits, and even head separations followed
causing some to blame high pressure.  The culprit wasn’t the powder or the
shell, it was the primer.  Upon firing the mercury penetrated the brass and
formed zinc and copper amalgams.  This reaction hardened the case and
reduced its elasticity.  So while not as corrosive as potassium chlorate it was
still detrimental.  Today nearly all U.S. ammunition and component primers
are non-mercuric and non-corrosive.
Burning powder in a confined hole is still your barrel’s greatest enemy.  When
sparked it explodes, hot gas is expended, flame temperature rises, and a
tremendous pressure wave follows.  That blast reaches 3,000 to 5,000
degrees Fahrenheit so the bore experiences a torch effect.  Burned powder
and copper residue are then deposited on the surface and both are mildly
corrosive.  Now imagine repeating that dynamic round after round.  It’s no
surprise the barrel melts under these conditions.  Combine that with the
pressurized imprint of a bullet and erosion is exacerbated.  Shot spacing and
dedicated cleaning help prolong your barrel but ultimately these three take
their toll.

Back to the aforesaid myths, we’ll start with number 1.  Extruded powders
are usually cylindrical in shape and single-based, meaning one source of
nitrocellulose.  They’re formed from virgin stock and their burn rate is largely
controlled by geometry.  Production quantities are smaller and lot-to-lot
variation tends to be higher. Ball powder is double based so it is comprised of
nitrocellulose and nitroglycerin.  It got its start during WWII from recycling
old extruded which eases production efforts.  Lot sizes are typically larger
and this can lower variation.  Burn rate is controlled by the detergent coating
and reduced surface area makes it harder to set off.  That same coating,
mostly nitroglycerin, adds to potential energy.   But contrary to popular belief
one isn’t more erosive than the other.  Years ago Lake City Arsenal did a test
of ball versus extruded in the 7.62 Nato.  After thousands of rounds, various
burn rates, and staggered load levels they found no difference in erosive
quality between the two.  Remember, barrels burn up they don’t shoot out.  
Flame, pressure, and gas fatigue the bore.  A powder’s heat potential factor
gives us a standard measurement for this phenomenon.  Unit measurement
is kJ/KG.  Admittedly, heat potential doesn’t capture all ignition traits but it
can provide a relative scale for comparing powders.
Powder
Heat Potential (kJ/KG)
Granule Shape
Burn Rate Chart
Number
IMR 4227
4,040
Flake / Hybrid
64
IMR 3031
3,880
Extruded
78
Accurate 2460
3,690
Spherical
84
Winchester 760
3,795
Spherical
109
Accurate 2700
3,545
Spherical
112
Norma MPR
3,774
Extruded
128
IMR 7828
3,850
Extruded
132
Hodgdon Retumbo
3,715
Extruded
138
Accurate 8700
3,460
Spherical
140
Hodgdon US 869
3,700
Spherical
144
What this shows us is heat potential, or the tendency to erode, isn't tied to
granule shape.  Nor is there a correlation between energy release and
powder geometry.  So erosion isn't a function of the powder type or burn
rate.  Instead it is tied to the amount of propellant ignited across a given
area.  Viewed another way, 70 grains of any powder with a heat potential of
3,800 will erode a 22 or 6mm much faster than a 30 or 35 caliber.  The added
bore area dampens the torch effect; granule shape and burn rate don't.  For
this reason I should be indifferent to Retumbo or US 869 when it comes to my
.224 Clark's barrel life.  That's because the two use charges within 5% of one
another. Burn rate does influence the pressure per unit of bore diameter but
that's a different discussion.
Bullet friction causes land and groove wear.  Sounds plausible but in reality
jacket drag doesn’t change the rifling much.  Burning powder and pressurized
gas is what pits the metal in front of the chamber.  When bullets impart
sliding force to those regions friction occurs and over time you get barrel
wash.  But that happens because the molecular structure is already
compromised by heat.  Copper friction in the absence of gnarled alloy doesn’t
erode rifling.  Townsend Whelen once fired eighty thousand 22’s through a
bore measuring the lands and grooves before and after.  He was wise to
choose the LR because it minimized propellant induced wear.  To Whelen’s
surprise he observed only 0.0004” in lost tolerance across the entire length.