In spite of weighing each pistol load carefully on a balance beam scale, I notice the occasional 'light' load, (about 1-2 out of 50 rounds). The sound and recoil are noticeably less and frequently the spent cartridge fails to eject cleanly. The causes that come to mind include the following:
1. human error
2. the scale (RCBS 5-0-5 balance beam) does not measure consistently
3. the powder (N310) does not always burn evenly
4. primers do not ignite consistently (Winchester small pistol)
I would appreciate hearing about any experience you might have in this area.
Relying on our senses to determine if loads are consistent is flawed. In a semi-auto, fired from a static position, where the brass lands is a better indicator and provides a reasonable comparison of round to round port pressure, although its good to use some factory ammo so you have a baseline comparison. The best tool available though for measuring the consistency of your loads is a chronograph.
During the loading procedure, human error is always present, and it is incumbent to do what you can to minimize that error.. For example, is you scale placed on a solid stable platform free from the vibration associated with working the press, or dumping powder from a powder measure? Is the scale at eye level or do you look up or down at it. Is there sufficient light to get a clear reading from the pointer on the beam. A digital scale has its problems too, but its easier to read than the pointer of a balance beam scale. When you use a trickler to sneak up on the correct weight, do you bounce the pan so that the scale gives you the correct reading twice? I've found that the scale does not always react to very small increments of powder falling onto the pan, but if I tap the pan, the scale then reads the adjusted weight correctly.
I have a 505 scale, and it has provided many years of reliable, trouble free service. You might want to clean the knives and the recess in the base from time to time with alcohol to keep it free of dust and gunk. A good investment is a scale check weight set. Regardless of whether I'm using my digital or my balance beam scale, the check weight allows me to determine that the scale is weighing correctly and consistently, which aside from the obvious merits of weight accuracy, provides the handloader with a greater sense of confidence in his scale and weighed charges.
If the powder does not burn completely, you will see evidence of it when shooting. Your pistol will have particles of unburned powder around the chamber and ejection port, in the bore, and in your fired brass. Inconsistent or incomplete burning generally relates to insufficient priming, but fast powders are the easiest to ignite, so it could result from too much airspace in the cartridge. In this condition, the flash of the primer rolls across the top of the powder surface which lies along the side of the brass case when held horizontally. Insufficient crimping exasperates the problem, as the force of the powder causes the bullet to jump forward before the powder ignites, further increasing the powder capacity which is already too large for the light charge. Ensure your loads have a good crimp, and choose bullets which have a cannelure or a crimping groove. If you are using light loads where this condition is present, prior to shooting, hold the gun vertically, which places the powder column against the flash hole, then slowly bring it to horizontal to fire, you might find that produces more consistent results.
If you don't believe that your loads suffer from insufficient priming, switch to a hotter primer, which generally means a magnum primer or to a mil spec primer, which is a magnum primer with a slightly thicker cup.
