Primers... Fact or Myth

Although most of the article is true, the part about different pistol primers having the same sensitivity is very false. I would agree about rifle primers. They are nearly identical from brand to brand.

There is a vast difference in pistol primers when it comes to sensitivity and "power". The cup is made of different material and has different priming compound. Yes, seating is essential so assuming all primers are indeed seated properly, here's the scoop:

Federal standard pistol primers have the softest cup and detonate with less impact energy than any other brand. The new Winchester standard pistol primers marked "New Surface Finish for Improved Sensitivity" are a close second followed by Remington standard pistol primers. Next comes the old style Winchester primers and finally the hardest ones are standard CCIs. The harder the cup, the more impact energy it takes for detonation.

There is a considerable difference in cup hardness between standard and magnum primers. Again, CCI are the hardest and Federals are the softest. All magnum primer cups are harder than their standard pistol counterparts in the same brand.

A S&W DA revolver is an excellent platform for a proof gun. By turning out the strain screw, you can control how hard the hammer strikes the primer. I can crank my S&W Mod 686 strain screw out where only standard Federals will detonate. As I tighten the strain screw, the other brands will start detonating in the above order. When you do the same test with magnum primers, the strain screw has to be fairly tight before even the Federal mag primers will light up. By the time you get to the CCI mag primers, the strain screw has to be completely tight. Yes sir buddy, there is quite a difference.

A couple years ago out of boredom, I decided to run a little non-scientific test with a bunch of different primers. I have several boxes of the CCI plastic bullets and plastic cases. These are designed to be fired with no powder ... only the power of the primer propels the plastic bullet. I used a S&W Mod 686 for all tests.

In the first series of tests, I used CCI, Federal, Winchester (old style), and Remington standard large pistol primers (the plastic cases use large primers) and ran a cylinder of each over the chronograph. Winchester produced the highest velocity, CCI was second, Federal was third, and Remington was the slowest. I repeated the same test but used regular brass cases and small standard pistol primers. All velocities were considerably lower but the order was the same as with the large primers.

When I switched to mag primers, I noticed there was a very wide spread in velocity within the same brand. My 686 had a reduced power mainspring so I replaced it with a factory full power spring. Walla, the velocities now had a higher and very narrow velocity spread (all brands). Armed with this information, I ran another series of tests. This time I backed out the strain screw (different for each brand) until the primers barely detonated and again with the strain screw fully tight. The difference was amazing. With light hits, just barely enough to make the primers go bang, the velocity was way lower and very erratic. When the strain screw was tightened, the velocities increased considerably and the max spread was very tight.

I went back to standard primers and tried the same test. Federal primers didn't show much difference in velocity with a light or heavy hit. Winchesters and Remingtons made a slight difference. CCI standard pistol primers made a considerable difference just like the magnum primers.

Here's my conclusions: The priming mix is definitely different between brands. The sensitivity is definitely different between all brands of pistol primers. Magnum primers are less sensitive in all brands. The amount of their "power" in most standard primers and all magnum primers is related to how hard the primer is struck.

creepyrat, U.S. made primers are the most consistent component in all the reloading materials. There are significant differences in brands, but within a given brand, they are really uniform. That goes for the primer mix and the metal used in the cup. In general, any primer that is nickel plated is going to have a harder cup and requires a stronger hit to detonate.

Inside the primer is a little bridge called an anvil. The primer mix is located between the anvil and cup. When the firing pin strikes the outside cup, the primer mix wedged between the anvil and the inside of the cup detonates, just like a cap pistol. Actually, the mix has very little to do with sensitivity, it's the type and thickness of the material used in the cup.

Many shooters use the spent primer as a pressure indicator. This could be very misleading and even dangerous. Rifle primers and magnum pistol primers have a very thick cup compared to standard pistol primers. That's because they must withstand more chamber pressure. The more sensitive standard primers such as the Federals and new Winchesters have a very thin cup and no nickel plating.

Here's a couple misleading indications when using primers for a pressure indicator:

The first example uses a standard primer in a magnum pistol load. Let's use a 44 Mag as an example. A standard primer will ignite a healthy charge of 21 grains of 2400 powder with a 240 gr JHP. This load is safe but if you look at the spent primer, it will be flattened to a point where the dent is hardly visible. This would give a false indication of excessive pressure. Using a mag primer and backing the load down .5 grains will produce the exact same velocity and chamber pressure. Because the primer cup is harder, the dent will look normal and the primer won't be flat.

Here's a worst-case example: Lets say you were loading 38 Specials with intentions of shooting them in a 38 Special +P gun (not a 357 Mag). Chamber pressure must stay below 20,000 psi for a +P load or you may damage the gun. This time we load a max charge of 5.5 grains of Unique. Everything goes good and the spent primer retains the dent and doesn't flatten. So you figure you are well under pressure and add another .5 grains of powder. This time the gun demos because you were over pressure but the spent primer still looked perfect. Point is: don't use spent primers as a pressure indication.

Max chamber pressure in handgun cartridges span from 14,000 psi in a 45 Colt to as much as 65,000 psi in a 454 Casull. The 454 Cas actually uses a rifle primer so it won't rupture under pressure. Just make sure you use the right primer for your load as indicated in respectable reloading manuals.