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Powder Position and Pressure
Powder migration within large cases by small charges of powder accounts for many unexpected problems experienced with reduced loads. Be aware that loads using small charges often exhibit different pressures created by changing powder orientation within the cartridge.
A look at powder position and its affect on pressure within cartridges
All powders exhibit some form of positional sensitivity within a cartridge. SAAMI recognizes this by publishing strict guidelines governing the testing of ammunition. These guidelines are intended to control powder position within a cartridge by orienting it to generate the highest pressures for a given load.
SAAMI guidelines for testing require that the loaded cartridge be twirled slowly end over end to loosen the powder within the case. It is then carefully oriented with the primer end down to settle the powder rearward within the case. The technician then inserts it into the test barrel, being careful not to move the powder forward and away from the primer. This powder-rear orientation produces the highest pressure and velocity for that load combination and provides the baseline for published data.
Cartridges filled to near the top of the seated bullet have very little room for powder migration within the case, making them relatively immune to powder orientation caused pressure changes. This is one of the reasons that fuller cases have lower standard deviations. If that same cartridge is loaded with a faster powder, perhaps one that only fills 70 percent of the available case volume, powder may become oriented away from the primer which will reduce pressure and velocity. The ability for powder to migrate throughout a case is a major cause of higher standard deviations between shots.
Positional sensitivity is increased by unused case volume. Older cartridges designed to work with black powder and at low maximum pressures are the ones most commonly affected by this problem. The .45 Colt and .38 Special are two that fall into this category and both can have real powder position issues with lighter loadings.
Our lab was contacted by a shooter who was looking for loads in his .38 Special that would operate in the 450-500 fps range from a Ruger Vaquero using very small (he referred to them as economical) charges of very fast powder and light bullets. His goal was to create an inexpensive competition load he could use in the local cowboy action shoot. He suggested using a load far below our starting loads and wondered if it was safe. The answer was no. It was unsafe. Here is why:
A pistol carried muzzle down in a holster orients powder forward against the bullet, a situation that creates the least pressure for a bullet/powder combination. As the pistol is raised to eye level its powder charge remains forward. If the round is already under-pressured by design, it has now been subjected to an orientation that further reduces pressure. Muzzle velocity may drop to zero, as the bullet fails to emerge from the barrel.
Published data is provided with a powder-rearward orientation and the load range is calculated by regression using the maximum SAAMI pressure guidelines. The regression is based on a safe reduction of that pressure accounting for potential shifts in pressure produced by individual firearm characteristics and by positional issues that may be created by the lowered charge weights. Going beneath them risks a stuck bullet and a trip to the gunsmith.
Always treat the lowest (and especially untested lower charges) with skepticism until they prove safe in your personal handgun. There is no corner to be cut here; light loads can lead to stuck bullets. Test them carefully.
Avoiding Powder Position Issues in Handguns
Overcoming positional issues with handgun loads is not difficult, but may require a bit of research. The best solution is look for loads that come closest to filling the case up to the base of a seated bullet. When researching load data, watch for powders that have a lower bulk density like Accurate #2. This powder has BD of .635 and fills more case volume than denser powders using the same charge weight. It is a very good choice for reduced pistol loads in the .38 Special and 45 Colt cartridges.
Fillers in Pistol Cartridges
The use of fillers like Puff-Lon or Dacron usually produces higher pressures in loads that were developed by handloaders without a buffer in place. Remember, powder orientation within the case has probably run somewhere between the minimum and maximum pressure positions. Cartridges are usually stored bullet down, loaded in a revolver with the muzzle aimed downward and then brought up to eye level to fire. Each of these scenarios indicates that the powder will be orientation forward, reducing pressure and velocity. Using a buffer will force the powder rear-ward and raise the pressure potential for that combination as well as adding additional mass to the projectile.
If it is your decision to use a filler like Dacron or Puff-Lon, it important to work up loads with those components in place. Don’t try to save a step and take a pet load and add a filler to the mix. Be smart, reduce the load and work it up like you would any other handload, watching for pressure signs. When done properly, good fillers can substantially reduce standard deviations in cartridges like the .45 Colt.
Position Sensitivity in Rifles
In rifles, the two most common areas for positional issues lie with the older blackpowder cartridges and with reduced loads for large calibers. In both instances, small charges of powder within large cases can produce several distinct and reproducible pressures curves. These pressure changes are directly linked to powder orientation within the cartridge itself.
We used the .45-70 Government loaded with Blackhorn 209 to test positional sensitivity. It was chosen for several reasons, including the fact that Dacron, Puff-Lon or over-powder wads are recommended by our guide for this load.
The baseline testing was done using SAAMI methods to produce a standard load range for the powder/bullet combination that will eventually be published in our new guide. The second test used the same charge with forward-orientation and no buffer. The final test used the same charge with forward-oriented loads and a compressed buffer of Puff-Lon. The results highlight the role powder orientation can play in pressure and velocity.
The standard test, which followed SAAMI protocols for rear-oriented powder, produced a mean pressure of 17,767 psi with 26.0 grains of Blackhorn 209. The case fill was 74%. The average velocity was 1,143 fps with a standard deviation of 20 fps.
The same load, tested with its powder oriented forward against the bullet, and then placed carefully into the universal receiver, generated considerably lower numbers. The average pressure dropped to 9,000 psi and an average velocity of 950 fps. These cartridges were loaded at the same time as the test run and shot within an hour of the previous test to limit environmental changes in the data. This difference is caused by powder orientation within the case.
For the last test, we used 26 grains of Blackhorn 209 and then filled the case up to the mouth with Puff-Lon, leveling it with a straight edge. The bullet was then inserted and crimped using the same depth and crimp settings as the other loads. These loads were also tested using a powder-forward orientation like the non-buffered loads. The loads using Puff-Lon averaged 18,040 psi and a velocity of 1,134 fps with a standard deviation of 17 fps. In this case, it is clear that using a filler limited positional sensitivity within the large cased .45-70.
Overcoming Positional Issues in Rifle Cartridges
As in pistol cartridges, the best solution for rifles is to choose bullet/powder combinations that come close to filling the cartridge to the base of a seated bullet. This will typically reduce standard deviations and accounts for one of the reasons why slower powders, and their fuller cases, tend to produce the best accuracy in rifles.
In larger cartridges, especially those that were created for black powder, there may be no powders that offer a good case fill at the desired velocity. One option is to search out data using Accurate 5744.
Accurate 5744 is an unusual double-base extruded powder which ignites easily and tends to take up quite a bit of volume within the cartridge. As a double-base, it is less position sensitive and tends to produce consistent pressure and velocity in larger cases without using a buffer. Our guide has extensive data for this powder, including reduced loads for large caliber rifles. Our technicians also have access to data for antiquated or little seen cartridges that may not yet be published. The easiest step for the handloader looking for reduced rifle loads is to consult with our customer service people if the information is not available in our guide.
If you are shooting a reduced load that has been privately developed without filler or an overwad, it is important to understand that adding a buffer will orient the powder to produce the maximum pressure for that powder/bullet combination. It will produce higher pressures than the unbuffered loads and may prove damaging to the firearm. Always develop new loads intended for use with fillers with the fillers in place.
Should I use a Filler?
When handloading for cartridges designed for smokeless powder, the general answer is no. The best solution is to use bullet/powder combinations that provide good case fills and that have been tested in a laboratory environment.
When loading older rifle cartridges, load guides will stipulate when a wad or filler is used. Following this data should produce safe loads.
Utilize tested data using Accurate 5744 if available for reduced loads or antiquated cartridges.
When using fillers, develop the load from a lowered starting point and work toward the desired velocity.
For commonly used pistol cartridges like the 45 Colt and the .38 Special, which have large capacities and low SAAMI pressure guidelines, it is best to stick with published loads using the most voluminous powders available. If denser powders are used, increasing the charge weight within published guidelines will limit the differences between maximum and minimum pressures created by powder orientation within the case. The safest route is to avoid extremely light loads.
If you choose to shoot the lightest loads possible, below the guideline minimums, be sure to account for each shot fired because the threat of a squib load is very real.
Powder migration within large cases by small amounts of powder can account for many unexpected problems experienced with reduced loads. Be aware that loads using small charges often exhibit different pressures created by changing powder orientation within the cartridge. Use of a filler will control these fluctuations, but will do so by forcing powder against the primer which increases pressures. Always develop reduced loads with the filler in place if it is to be used at all. Be careful of extremely light loads, that when paired with powder-forward orientation may result in a squib load and a stuck bullet. And finally, use published data whenever possible. Let us experiment with pressure and you can enjoy the results.
Take care and good shooting.