How Primers Work

by JimT, Texas, Monday, August 08, 2022, 08:15 (619 days ago)

On my single actions I used for hunting I shimmed the mainspring so it had more tension. My old .45 Ruger Blackhawk will light off Large Rifle primers just fine. Not only does the harder smack light the primers better, I did not get unburned powder when using my favorite powder - 2400 - like a lot of people complain about.
Here is an explanation why .......

(The following is a note from Mic McPherson on primer strikes.)

In the day, when ballistics labs used manually fired universal receivers, every ballistician eventually got bored and, instead of gently pulling the lanyard until the hoop slipped off the spur on the rotating hammer (very similar to a Colt SAA hammer design, when it rotated far enough, the spur would not hold the lanyard loop and the hammer fell), they yanked the lanyard. Every time, when they did that, they noticed two things:

They recorded higher velocity
They recorded lower extreme spread and standard deviation in any long string of shots

Note that the Universal receiver has a light hammer with a 90-grain firing pin and a hammer spring similar to a typical Ruger SA, give or take. It just does not have much striker energy. The Striker tip is, however, smaller than a typical gun -- about 0.625", as opposed to about 0.70" for a typical modern gun, so it always set off the primer, despite marginal striker energy.

Note also the testing that our late friend Terry did with revolvers where he tested progressively lighter striker springs and found progressively lower muzzle velocity.

These two are related.

When you yank the lanyard on a Universal receiver, it rotates the hammer fast enough so that as the lanyard comes free, the energy of rotation in the hammer causes it to continue to rotate against the spring and therefore compress the spring significantly more than when the hammer is rotated slowly until the lanyard slips off. And, the design allows the hammer to rotate much farther than the slip-off point.

So, we have proof that more striker energy results in more velocity and more ballistic uniformity. This is because of how primers work. Compression of the pellet compresses gas contained in microscopic pockets found throughout the pellet. Compressing a gas adiabatically heats it. If compressed fast enough, that heat cannot conduct and radiate into the surrounding material fast enough to keep the temperature of the gas below the ignition temperature of the surrounding material -- identical to how a diesel engine works. So, if you compress the pellet more violently, you get more ignition centers within the compressed portion of the pellet.

More ignition centers means that the pellet burns faster and that means that it burns in place more completely (less unburned material blown more-or-less uselessly through the flash hole). That means more hot gas is generated to do a better job of penetrating into the charge and directly igniting more of the granules.

Terry Murbach showed years ago that it is the hard whack that works the best. Primers that are hit harder light the powder better. He used a S&W revolver that had a leaf mainspring. He ran some loads through the chronograph with the spring set at full strength. Then he began lightening the spring tension. The lower the tension, the lower the velocities. After a certain point of lowering the spring tension you start getting misfires.