| 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. |
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| 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. |
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| 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. |
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