Laurie Holland examines some of the mechanical issues that can affect primer performance
Before looking at primer seating, what factors influenced the results in the first two parts of this review? The first is the application itself, so my results may only apply to .308 Win and Viht N140. Then there is the manufacturer’s priming mixture recipe. Most rifle primers use lead styphnate, a compression ignited explosive, as their primary ingredient. Fuel or oxidiser compounds producing super-hot gas increase the eject temperature and volume to promote rapid reliable ignition. (Potassium chlorate, the source of the problem in ‘corrosive’ primers, performed this role for decades.) Metallic compounds, finely granulated metal, and powdered glass are options that produce incandescent particles to penetrate deep into the powder column. The amount of lead styphnate determines a primer’s ‘brisance’, ‘hot’ examples heavily loaded, but the overall ingredient mix is varied to suit the primer’s purpose.
Let’s move onto the mechanical issues that apply irrespective of which primer model is chosen. The firing pin’s speed, energy, and travel are crucial for consistent ignition, so even minor firing mechanism faults affect cartridge performance, but that’s not a handloading issue. Properly dimensioned primer pockets and flash-holes, and consistent primer seating are. Primers are miracles of complex chemistry and development, but are simple devices structurally. Working from the firing pin to flash-hole we have: brass cup; priming compound; optional ‘foil’ paper disc; two or three prong ‘anvil’ (like a shallow mini-tripod), its tip pointing backwards at the priming mix. The ‘anvil’ sits with its tip just ahead of the explosive pellet and its ‘feet’ facing the open end, therefore the bottom of the primer pocket in the case. Ignition occurs when the firing pin indents the cup crushing the pellet against the anvil-tip. To work well, the anvil’s ‘feet’ have to be anchored on the pocket floor and its tip barely separated from the priming compound.
Starting with the flash-hole, any cartridge using large primers is specified as 2mm (0.080in) diameter. Norma, Sako, RWS, and Lapua cases have closely controlled, often drilled, holes while many American makes are punched inwards through the pocket. The latter tend to have less consistent diameters and shapes, and often leave spikes of displaced brass attached to the flash-hole exit. This burring affects the primer flame shape and angle, sometimes significantly, as it exits the flash-hole and penetrates the powder column. Variable flash-hole sizes increase pressure and velocity spreads.
Lapua and other top European makers machine primer pockets giving a flat, level floor and a sharp junction with the walls. Conversely, most US-manufactured cases are swaged giving a more rounded junction. With great force needed to punch the flash-hole through the thickest, hardest part of the component, pocket floors are often noticeably dished. In combination these factors may produce inconsistent primer position and seating tension; variable primer flame behaviour. The cure for these various deficiencies is ‘uniforming’, both flash-hole and pocket floor. The former can be attacked from either direction, but combined flash-hole uniforming or deburr tools are only used from the case-mouth direction. Flash-hole diameter uniforming sees them opened up marginally. Simple non-adjustable cutters are turned inside the pocket truing its floor and cutting it to a common depth, square to the case axis. This can be done manually with a handle attachment, but obtaining a power screwdriver adaptor is a boon when many cases have to be done.
Having uniformed pockets, seating primers correctly and consistently is the next step. Most bench presses incorporate a priming tool – and they give no ‘feel’ such are presses’ weight and power. It is very important for the primer to be seated so that the anvil ‘feet’ contact the pocket floor and are then compressed or tensioned just a tiny bit more. Stop short and the firing pin uses energy to force everything into the correct relationship, equivalent to a light strike. Push the primer in too far and its cup will be distorted, even crushed, damaging the explosive pellet and causing partial ignition or a misfire. So a good priming tool and correct user technique form a vital partnership in producing consistent ammunition.
Putting priming tools for progressive presses aside, there are some small bench-mounted types available that have operating handles a few inches long and give really good feel. Forster has a very nice model with tubular magazines for around £80 that incorporates a universal shellholder. RCBS makes them in both APS (strip fed) and tube-fed versions costing rather more. All three are really fast, easy to use and give lots of feedback on the primer bottoming in the pocket. However, more people opt for a magazine or strip fed hand-tool with Lee (two models), RCBS (two models), Lyman, and Hornady producing variations on the theme. I’ve used everything bar the RCBS products and while all work well, I find I keep returning to the Lee Auto Prime. They are best described as ‘semi-sensitive’ – you usually feel the primer bottom out, but not always. For really precise operation, look at benchrest models from Sinclair and K&M, the latter even providing a little seating punch travel dial-gauge letting you accurately seat every primer at the recommended additional 0.003-0.005in after bottoming. Both handle one primer at a time, so aren’t ideal for massive sessions.
So, how important is this? If you do everything, what are the returns for your time and money? Getting primer seating right is really important for reliability and safety as much as cartridge precision and I’d recommend a bench or hand tool. The sporting shooter who never takes a shot beyond 250 yards won’t notice any benefits from pocket and flash-hole uniforming – buy Lapua, Norma, or RWS brass and forget about these activities anyway except for specialist ultra-precision and long-range ammunition. I’d still recommend using a combined flash-hole or debur tool on Winchester and Remington cases and so on – it’s a quick one-off job, and you can be sure there are no nasties hiding inside the case.