Based on an article by John Barsness - GUNS magazine pg 26 May 2009. [JB, formerly of Handloader is one of the most qualified gunwriters when it comes to primers and reloading in general]
Information from the Speer #14, Hornady #7, Nosler#6, and Lyman #49 reloading manuals, Alliant and Accurate Arms data.
Additional Information from James Calhoon - "Primers and Pressure" Varmint Hunter Magazine, October, 1995
Hopefully this explains a bit more about, not only primers in general, but specific characteristics that can aid a reloader in choosing the optimum sparkplug. Pertinent information will be added to this section when more information becomes available.
Primers come in different strengths, technically known as “brisance,” a word defined as “the shattering effect of a high explosive.”
Primer brisance mostly depends on the length of the flame that leaps out of the flash-hole after the firing pin whacks the primer cup. This flame can also be manipulated to last a little longer, by adding tiny particles of other flammable material to the priming compound. These differences really can effect not just accuracy but pressure.
For instance, in a very small rifle cartridge such as the .22 Hornet, a “hotter” primer might start to dislodge the bullet before the powder really gets going. Instead of a relatively gentle, slowly accelerating push, the bullet gets cruelly hit hard. This is why some Hornet fans use small pistol primers, with much milder brisance than small rifle primers.
Really huge rifle cases such as the biggest Weatherbys, Remington Ultra Mags, and older British African cartridges require a lot of very slow-burning powder to operate at all. Slower-burning powders are normally more difficult to ignite, and a bigger flame of longer duration helps, especially in cooler weather. The first “magnum” primer, the Federal 215 was designed for this very purpose. Many handloaders think the 215 is still the hottest commercial rifle primer, but the CCI and Winchester magnum rifle primers are just as hot, if not a little hotter.
Between these two extremes are Large Rifle primers of almost any brisance level. Remington and CCI primers tend to be the mildest “standard” primers and Winchesters the hottest (the reason that Winchester never had a magnum rifle LR primer until recently), with Federals somewhere between. Deciding which to use depends not only on the size of the case but the powder.
How fast a powder burns depends not only on granule size (bigger granules have more relative surface area) but on exterior coatings. Extruded powders, such as relatively small-grained 4895 or large-grained H-4831 depend mostly on granule size to control burning rate. Ball powders don’t vary much in granule size, so depend mostly on relatively flame-resistant exterior coatings to control burning rate. By definition, these coatings make ball powders harder to ignite.
For example, in the 30-06, IMR 4895 is very easy to ignite, one reason it’s often suggested for reduced loads down to 2/3 of a case’s capacity. We’ll probably get the very best accuracy from a mild primer such as the CCI 200.
To make the 30-06 zip however, we might try Ramshot Big Game. The Ramshot ball powders burn cleaner than most ball powders, but they also require more flame. Winchester Large Rifle primers are the hottest “standard” rifle primer and often perform very well with Ramshot powders, but if they don’t definitely try a magnum primer. This can often result in smaller groups.
Something else to remember is that competition rifle shooters often favor mild primers i.e. primers that produce just enough heat to properly ignite the powder. They feel that as primer brisance gets higher, it also gets less repeatable from primer to primer. Another train of thought is that the powder is ignited a tad more gently. When this happens, the front slope of the pressure curve is less steep. Which means the bullet is pushed a tad more gently into the rifling which tends to deform it less. Whatever the scientific reason, competitive rifle shooters seem to feel that the milder primers give both better velocity uniformity and accuracy.
The same principles also applies to handgun cases. You might find that magnum primers aren’t good for milder loads, especially with cast bullets for some reason or another (Elmer Keith claimed that the hot flame tended to slightly melt the base of the bullet - no way of knowing if that is true.) Whatever the case, often using a standard pistol primer can reduce group size with milder or cast loads.
On the other hand, magnum primers are almost always recommended for magnum loads, especially if hard-to-ignite ball powders like W296, or its H-110 twin, are used. In fact, magnum pistol primers were developed for the large case revolver magnums like the .357, .41, and .44 Magnums. They seldom are needed for standard autoloader rounds or standards like the .38 Special.
Some powder manufacturers recommend standard pistol primers with certain of their powders even in magnum pistol loads. Alliant 2400 is one where the use of magnum primers is strongly discouraged, and another is Accurate Arms, which recommends standard pistol primers with their handgun powders, including #9, unless “they provide better accuracy in your firearm.”
There also is an unusual situation that should be considered when deciding whether to use standard or magnum primers with ball powders that is pointed out in the Speer manual: Powder manufacturers may state that their propellents do not require magnum primers. This is generally true at maximum safe pressure levels. But Speer’s ballistic testing fully explores propellent behavior over the usable range of charge weights. They often found that a particular propellent works fine with standard CCI primers at the maximum safe pressure. However it may not consistently ignite with lower charge weights. In the lower pressure regimes typical of “starting loads” they commonly saw increased extremes of pressure and velocity. Some ball powders ignited by standard CCI primers will even produce short hang-fires–called “click-bangs” for obvious reasons–at start load levels but not at maximum safe pressure. In those cases the use of magnum CCI primers to insure performance over the range of charge weights is recommended (or perhaps a switch to a hotter standard primer such as the Winchester WLR).
So as you can see, picking the right primer brisance can be very important and can give you optimum accuracy and consistent performance. Fortunately for us there are primers of every brisance level in every category of primer, whether it be standard or magnum.
Different primers have different cup thicknesses. You can see the importance of cup thickness when primers are considered for semiautomatic rifles that have free-floating firing pins. This topic is discussed in greater detail in the post "MILSPEC PRIMERS FOR SEMI-AUTOS FAQ AND INFO" that follows the primer chart.
Handgun primers have thinner cups than rifle primers, making them easier to ignite with the typically weaker firing pin fall of handguns. Small Pistol primer cups are .017" thick, while Large Pistol primer cups are .020" thick. This is the reason using handgun primers in .22 Hornet rifle loads sometimes results in pierced primers in some guns. Obviously their substitution in the high pressure .223 Remington would not be a good idea.
Even the same type of primers from different manufacturers can have different cup thickness. Federal primers tend to have thinner cups than Winchester, Remington and CCI primers. On occasion this can be handy. Some revolver trigger and action lightening jobs may result in a lighter hammer fall that results in not all the primers going off. A switch to Federal pistol primers can make the load 100% again. The same thing can happen in cold weather with some “modern” bolt actions with light, fast firing pins. These are supposed to whack primers with the same approximate energy as an old-fashioned 98 Mauser strike, but under some adverse conditions they can occasionally use a little help. Federal primers can provide that help.
With Remington small rifle primers, the 6 ½ primer has a thin cup and is not recommended for higher pressure rounds like the common .223 Remington. It was intended for the .22 Hornet. When Remington introduced their .17 Remington round in 1971 they found that the 6 ½ primer was not suitable to the high-pressure .17. The 7 ½ BR primer was developed for this reason. According to Remington, the 7 ½ has a 25% greater cup thickness and they state on their web site: "In rifle cartridges, the 6-1/2 small rifle primer should not be used in the 17 Remington, 222 Remington or the 223 Remington. The 7-1/2 BR is the proper small rifle primer for these rounds."
CCI/Speer Technical Services says: "The CCI 400 primer does have a thinner cup bottom than CCI 450, #41 or BR4 primers... [with] the CCI #41 primer... there is more 'distance' between the tip of the anvil and the bottom of the cup." so that is their AR15 recommendation, although it seems like there are no complaints with using the BR4 and 450 primers by AR15 shooters and reloaders, in general. The #41 just gives you a little more safety margin for free-floating firing pins and would be the best choice for commercial reloaders who have no control over the rifles their .223 ammo is used in.
Another factor which determines the strength of a primer cup is the work hardened state of the brass used to make the primer cup. They are made with cartridge brass (70% copper, 30% zinc), which can vary from 46,000 psi, soft, to 76,000 psi tensile strength when fully hardened. Manufacturers specify to their brass suppliers the hardness of brass desired. It is possible that a primer manufacturer could choose a harder brass in order to keep material thickness down and reduce costs. Winchester WSR primers are somewhat thin, yet seem to be resistant to slam-fires and this is likely due to this hardness factor.
Large rifle primers all appear to have the same cup thickness of .027", no matter what the type.
This also affects pressure tolerance. Cases that utilize small rifle primers and operate at moderate pressures(40,000 psi) should use CCI 400, Federal 200, Rem 6 1/2, or Win WSR. Such cases include 22 CCM, 22 Hornet and the 218 Bee. These primers can also used in handguns such as the 9mm., 357, etc. Other cases that use the small rifle primer can use the above primers only if moderate loads are used. Keep to the lower end of reloading recommendations.
Cases that utilize Small Rifle primers and operate at higher pressures (55,000 psi) should use CCI 450, CCI BR4, Fed 205 and Rem 7 1/2 etc.