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Thoughts on Rimfire ammo - Q & A part 1

 
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Old 09-17-2011, 07:46 PM
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Thoughts on Rimfire ammo - Q & A part 1

There's alot of questions on rimfire ammo, be it .17, .22, 5mm, or other. I thought I'd put something together to help pull together at least ten common questions and somewhat ambiguous answers to them, without going into a history lesson of rimfire ammo origins. Some of the information was excerpted from different corresponding articles, patent office information, technical bulletins, manufacturer information, and personal experience.

1. What ammo works the best in my "X" model rifle, or what is the best ammo?

Of all the questions out there, this seems to be the granddaddy of them, with no absolute answer. Why? It's been well documented that even though there are a few ammos that work well in many different rifles, it may not be the "best" ammo for your particular rifle. Slight variances in the rifle itself, sometimes the same manufacturer and model, produce different results even with the same ammo. Many shooter's actually try several / many different types of ammo before they choose one as their "best". Shooting all those different ammos at different distances also determines what is their "best" ammo. Some ammos that shoot outstanding at 50 yards barely group at all at 100, and ammos that don't seem to group well at 50 sometimes shine at 100 compared to others. Unless you are dedicated to one distance or another, this is also a factor when choosing an ammo - variable distance consistancy, or a dedicated distance ammo. It's not uncommon for one to give up a little accuraccy at one distance to gain it at another over another ammo.
Then there is bullet design - Hollowpoint, Round nose, other - generally a hollowpoint, ballistic tip, or segmented design combined with a higher velocity round works well on game due to it's quick expansion / fragmentation and ability to cause a large wound and quicker harvest. A round nose design is generally used for target, but isn't the rule - many hollowpoint designs work well on paper too. A round nose design on game will give you less expansion, but more penetration. What bullet design you choose depends on the size / heartiness of game you are hunting, or how well it consistantly impacts your point of aim.

All .22 rimfires (except the WRF and WMR) are black powder designs, and use tapered heel bullets. If you examine a .22 S, L, or LR cartridge, you will see that the case and bullet are the same diameter. The part of the bullet inside of the case (the heel) is reduced in diameter to allow it to fit inside of the case. Case-diameter bullets also limit bullet shape, weight, balance and bearing surface. Another significant problem is that .22 rimfire bullets have an undersized, cupped base that the propellant gases must expand reliably and evenly into the rifling grooves for proper sealing and stabilization.

2. Hyper Velocity, High Velocity, Standard Velocity, Sub Sonic?

Again, this depends highly on how each performs through your particular setup and the distance you are shooting, but generally for target shooting standard, match, or sub sonic velocity ammo will more consistant. The reason being as high velocity ammo starts to loose speed, it causes a mini sonic boom. This disturbance causes the bullet to destabilize, wobble, and "veer" or get pushed off course. This transition is known as the transonic barrier and it varies depending on the original velocity of the ammo, air tempurature, altitude, and humidity. Also high and hyper velocitied ammo tends to cause greater harmonics in the rifle and actually can cause a slight barrel whip, known also to degrade accuraccy. For a rule of thumb, hyper velocity is usually an ammo 1400 FPS and above - achieved with a lighter bullet and higher powder charge; 1100 FPS to 1400 FPS is considered high velocity (approx. 1125 FPS is the sound barrier at 68 deg F); 1100 FPS and below is considered standard velocity, and because it is below the sound barrier is technically sub sonic. For ammo's sake, ammos in the hovering in the 1050 FPS and below are more of a customary sub sonic designation, and those hovering around 1000 FPS or lower are the "truer" of being slanged sub sonics as the lower powder charge, FPS, and the amount of sound they give off when fired is greatly reduced. While a standard velocity round that is near that 1100 FPS can go supersonic depending on environmental factors and take on those destabilizing characteristics mentioned previously, what is marketed as a sub sonic round generally has such a low velocity that it will stay sub sonic in even most extreme conditions. The higher the velocity, the less arch the bullet path will be to your target, alleviating a larger amount of elevation adjustments as compared to lower velocity ammo.

Advertised and even actual FPS will vary depending on barrel length, temperature, altitude, and humidity too, and even changes with the seasons of the year due to those environmental conditions. So use the advertised FPS as a guide, not as gospel. The best way to determine the actual FPS of a chosen ammo is to actually meter the ammo through your setup using a chronograph.

3. So what differentiates Match ammo from the above?

Generally it comes down to the manufacturing process - more care or tighter tolerances are given to this type of ammo than a regular bulk batch of ammo. Consideration to better / more consistant casting of the bullets (shape and weight, design of the driving band, diameter, length, lead hardness, etc.), more uniform powder and priming charging, case dimensions, rim thickness and so on usually leads to a higher performance when it comes to groups, FPS deviation, etc. This type of attention to detail usually comes at a higher cost, but also usually comes with greater reward. Referring back to #1 (which will happen alot during this post), will it be the best ammo all the time? No, but most of the time it will.

4. What is Eley primed?

First, a "quick" lesson on rimfire priming...
Because there is no anvil (as a dedicated, removable primer has on a centerfire) to initiate ignition of the lead styphnate rimfire priming compound. The addition of ground glass is used as a frictioning agent to ignite the priming compound once the rim of the cartridge is crushed / struck, working as the anvil would. Ground glass is not "bore friendly," but without it, .22 rimfire priming will not function. 10 to 30% of the priming mixture is comprised of this glass. And, to date, no lead-free priming compounds suitable for .22 rimfire have been found, however in the 1980’s Eley had developed a more eco-friendly dry compound very different from the lead styphnate and also gives off the tell tale smell that is as unique to shooters as Chanel No.5 is perfume shops.

Centrifugal force is used to push the priming compound into the hollow rim of the cartridge case. This is accomplished by dropping a wet pellet of priming compound into the bottom of the cartridge case, inserting a closely fitting steel pin, then using the pin to spin the case at speeds of approximately 10,000 rpm for a few seconds. This is a tenuous process at best and frequently fails to completely fill the rim with priming compound.

Preparing the small, wet primer pellet and inserting it in the cartridge case is hazardous hand work—part art and part training. The moisture content of the priming compound must be carefully controlled within narrow limits, otherwise the mixture will not work at all. The ground glass in the priming compound increases wear on the steel spinner pins and excessive smearing of the priming compound up the case sidewalls (a common problem) can adversely affect ignition and interior ballistics.

Ignition begins on the side of the case where the rim is crushed by the firing pin; there is no flash hole to focus the ignition gasses into the center of the powder charge. Failure to press the priming compound reliably and evenly inside the annular rim cavity can lead to misfires and high variations in muzzle velocity.

The dry composition and application of the Eley priming compound facilitates a better uniform filling of the rim, giving a more uniform ignition, creating better consistancy.

5. Why are rimfire ammos lubricated?

Rimfire .22 ammunition makers fight a constant battle with bullet lubrication. It is ironic that such a cheap cartridge requires highly specialized, micro-crystalline, synthetic-base waxes for lubrication and costly systems for application. Many also use a combination of beeswax and tallow, along with other well guarded recipes. Often, what works today does not work tomorrow due to minor variations in temperature, humidity, bullet hardness, propellant variations, etc.

Many reasons are given for it's purpose - reduce barrel wear and lead removal on the bullet as it travels the lands, create a better gas seal, acts as a filler to smooth imperfections in the bullet, help to reduce friction between the bullet and rifling.

Because .22 S, L, or LR cartridge are heeled, which means that the bullet is the same diameter as the case, and has a narrower "heel" portion that fits in the case. Such bullets are also called "outside lubricated," because they are ordinarily waxed or copper plated. In all other modern cartridges, the bullet shank is of constant diameter and the case is slightly larger than the bullet to allow the heel of the latter to fit inside. This old fashioned term for this design is "inside lubricated," as the lubrication grooves of lead bullets are inside of the case.

All .22 rimfire bullets tread a fine line between function and accuracy. The bearing surface of .22 rimfire bullets is the same diameter as the outside surface of the cartridge case. This makes lubrication of such bullets difficult as the case-diameter bullets must be lubricated on the outside where it may be wiped off or contaminated. Lubricants for lead, center-fire bullets are unsuitable for rimfire ammunition, and, unfortunately, copper-plating serves no ballistic purpose, does not eliminate the need for lubrication, adds cost and damages the bullets. This damaging of the bullet is another reason why you'll rarely if never find Match grade ammo copper plated.

6. Why are some 22 ammos copper plated?

.22 Long Rifle bullets are generally either plain lead (for standard velocity loads) or plated with copper (more of a lighter copper wash) or gilding metal (for high velocity or hyper velocity loads). The thin copper layer on the bullet functions as a high velocity lubricant reducing friction between the bullet and the barrel, thus reducing barrel wear. It also prevents oxidation of the lead bullet. Lead tends to oxidize if stored for long periods. Oxide on the bullet's surface could increase its diameter enough to either prevent insertion of the cartridge into the chamber, or - with hyper velocity rounds - cause dangerously high pressures in the barrel, potentially rupturing the cartridge case and injuring the shooter. Standard and subsonic cartridges use a wax lubricant on lead bullets for the same purpose at lower velocities. It is also believd that the copper adds a slightly harder surface and "hits" harder.

7. What powder is used in rimfire ammo?

I've researched that Vihtavuori produces powder for Lapua rimfire ammo (which well may also be used in Wolf and SK jagd). It's also rumored to be used in Eley ammo. Winchester (St Marks powder) also produces a rimfire powder, a search of the patent office produced a hit of 2.8 gr of WC371 (Winchester ball) propellant.

It is important to remember that the .22 Short and Long Rifle cartridges were originally designed for and loaded with easily ignited black powder. However, smokeless propellants have dramatically different ignition requirements and burning characteristics. Propellant makers soon found that the .22 rimfire design was not friendly to early smokeless propellants. They struggled for decades to find suitable smokeless propellants and the search continues to this day.

Essentially, the .22 rimfire requires unique smokeless propellants with a high energy content that are easily ignited and burn progressively. Limited case capacity dictates a dense powder with a small flake or ball configuration. Propellants with large flakes, sticks or coarse grains cannot be used as they will not drop uniformly through the holes in .22 rimfire plate-loading machines.
To facilitate ignition, .22 rimfire bullets must be heavily crimped into the case mouth to increase shot-start forces. Of necessity, this deforms the bullet. However, even on a good day, only about half of the propellant in a .22 rimfire cartridge burns completely.

Ammunition makers also struggle continuously to find suitable rimfire powders. The ideal .22 rimfire propellant must be competitively priced and compatible with plate-loading systems. It must have a high energy content, ignite easily and burn progressively while leaving a minimum of unburned propellant. Very few propellant powders meet these requirements.

Due to the weak ignition, powder residue from partially burned and unburned powder are constant problems—just ask any indoor shooting-range operator. The high ballistic performance required by many modern .22 Long Rifle loads places severe interior ballistic requirements on propellants. On the other hand, low MAPs can cause uneven expansion of the lips of the cup on the bullet's base and poor accuracy. In some instances, high MAPs can blow the lips of the cup base completely out.

In order to obtain the high muzzle velocities advertised for many .22 Long Rifle loads, MAP limits must be pushed to levels that leave little margin for error given the weak case head. Also, the proper case hardness gradient must be maintained to prevent extraction and/or ejection problems in spite of variations in brass strip and tooling.

8. Why are all rimfire cases brass?

The weak case head is the Achilles heel of the .22 rimfire cartridge. For this reason, Maximum Average Pressures (MAP) of .22 rimfire ammunition must be kept below 24,000 psi. Rimfire cases must have enough spring-back to assure consistent extraction in blowback-operated semi-automatic guns, yet remain soft enough to prevent splitting. This is a narrow margin that also eliminates steel as a .22 rimfire cartridge case material. Design parameters require all rimfire cartridge cases to be rimmed. Necked rimfire cases require several additional production steps which adds considerably to their cost.

9. What is the difference between the .17 mach2, .17 HMR, all the .22 cal rimfires, and 5mm?

Besides the diameter of the bullet (caliber), each type of ammo has a different case size to allow for powder capacity which equals a higher or lower velocity for each type of ammo caliber. Each provides similiarities and advantages over the other, and each fits a certain niche depending on your needs / application.
The .17mach2, .17HMR, and 5mm are necked cases (like a bottle neck shape) and uses a relatively light weight bullet in relation to their powder charge. The .17HMR (Hornady Magnum Rimfire) has the flattest trajectory compared to the .17mach2 and the other straight walled cases (in order of case size) of the .22 Short, Long, Long Rifle, WRF (Winchester Rim Fire or .22 Rem. special) and WMR (Winchester magnum rimfire or .22 magnum). But the .17HMR also has the most concern dealing with wind due to the light bullet. Of all the rimfires, the most widely used is the .22LR, and it also offers the most various selection of velocities and bullet types. The 5mm rimfire is based on a case similar, but not identical, to a necked-down .22 Magnum case. Its case is a little bigger and it has to be stronger as it must contain 50% greater pressure, 22,000 psi Vs 33,000 psi. Bullet diameter is .2045", or .20 caliber, the same as the .204 Ruger centerfire cartridge.

The Remington factory load drove a 38 grain Power-Lokt semi-spitzer hollow point bullet at a muzzle velocity (MV) of 2100 fps. This offered an increase of about 25 yards of practical range compared to the .22 WMR., and offered the best knietic energy of all above mentioned out to 150 yards. Unfortunately, the 5mm rimfire was a commercial flop shortly after it's introduction in the early 1970's and although somewhat obscure, is very limited production today with the only ammo that I know of being produced by Centurion.

The 3 most popular rimfire cartridges of today are the .17HMR, .22LR, and .22WMR. Each of these rimfire calibers can be researched in further detail with a simple Google search. Each are a fine addition to any shooter and a choice can be narrowed down further depending on what your application is. For hunting, all three perform well while the generally more expensive .17HMR and .22WMR offers more "punch" than the .22LR, the .22LR offers a relatively more economical target / hunting caliber that offers a variety of ammos for versatility. However some Match .22LR ammos can rival and surpass the cost of .17HMR and .22WMR. The design of the .17HMR cartridge not only contributes to it's flat trajectory, but is also devastating on varmint type game. The .22WMR (although not quite as flat shooting as the .17HMR) is also quite effective on small to varmint sized game, and is also able to utilize a heavier bullet to help retain more downrange energy. Due to the availability, versatility, and rifles chambered in .22LR, it appears to be the most popular rimfire of all. The more pronounced arch of it's trajectory also aids in its duty of a training round, having to adjust more for different distances than the others adds to it's training value for understanding bullet flight.

There are many pros and cons of each, best solution is to have all three!

10. Weight, Rim Thickness, or Diameter sort?

Which is the best? In my experience, Rim Thickness sorting appears to have advantages at shorter ranges (75 yards or less). The thickness of the rim acts as a headspace, and if that varies, accuraccy is lost.
Weight sorting has given me better results 75 yards and further. Through the equipment I've used at further ranges the wieght sort seemed to be the most productive and consistant.

Bullet Diameter also shows to be a good way to "weed out the flyers" within that 75 yard range. It is also a good way to match ammos to your chamber, and make a more positive contact with the rifling of your barrel.
All three of these types of sorts can help to tighten groups and increase accuraccy / consistancy, weed out flyers, and in some cases reduce misfires or "squib" rounds. However, referring back to #1, it must be tried in your particular rifle. There is no definate answer as the ammo and the rifle should be mated properly for the best results. Different manufacturer ammos, velocities, and bullet designs should be tried to see what works the "best".

I hope that this post has addressed, at least generally, topics that are often at the core of rimfire ammo questions. Hopefully it has given somewhat of an overview of the subtle and major differences between the different chamberings, and some general things that can be done to the ammo itself to increase accuraccy. At the very least I hope it points out a few things that makes shooting rimfires the interesting addiction that it is.
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Old 09-17-2011, 08:20 PM
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Re: Thoughts on Rimfire ammo - Q & A part 1

Thanks for taking the time to write this and for posting it. Very interesting reading.
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