Remington ceased production of 5mm Rimfire Magnum ammunition in 1982. When Aguila Ammunition announced its availability in 2008, thousands of hunters and shooters who owned firearms chambered for the cartridge shouted with joy and rushed out to stock up. When that production run was bought up, fans of the cartridge wondered if the dry spell would ever end. The answer came in 2018 when Aguila announced the second coming of its 5mm Remington Rimfire Magnum ammo with the first shipments slated for departure from its Cuernavaca, Mexico, factory in early 2019. A recently completed plant in Conroe, Texas, will increase ammunition production. Before getting to the results of my punching paper with the new ammo, let’s hop aboard my time machine and visit the roots of the cartridge.
In the Beginning
The 5mm Rimfire Magnum was introduced with great fanfare at the Remington writer’s seminar in 1968, but no ammunition or rifles chambered for it were on hand. Its case was formed by shortening the .22 WMR case about 0.040 inch and necking it down for a 38-grain Power-Lokt HP bullet measuring 0.2045 inch in diameter. As the sales pitch went, a maximum average chamber pressure of 33,000 psi would push the bullet out the muzzle at 2,100 fps. In comparison, Winchester’s .22 WMR, which had arrived on the scene nine years earlier, was loaded with a 40-grain bullet at 22,000 psi and 2,000 fps. In addition to shooting a bit flatter, the Remington bullet, with its higher ballistic coefficient, was said to deliver about 30 percent more energy at 100 yards. Shortly thereafter, all who had attended the announcement party were informed that the project had been cancelled until further notice.
What evil monkey wrench got thrown into the machinery? The original plan was to chamber the 5mm Rimfire Magnum in the Model 580 series rifle. It had proven to be quite successful when chambered to .22 Long Rifle, and with six hefty locking lugs positioned in the middle of its bolt, the action was certainly strong enough to handle the new cartridge. Overlooked was the fact that the high pressure to which the cartridge was loaded demanded that the rim of the case be fully supported, and the breech lockup of that action was not designed to do so. As a result, the rims of 5mm Magnum cases sometimes ruptured during firing, usually at the extractor slots of the chamber. Luckily, the problem was discovered before any rifles were shipped.
Remington’s solution to the problem was to give the barrel of the Model 580 a counterbored chamber, and that along with a rather complicated, two-part, two-stage case extraction system gave the rim of the case enough support to prevent it from blowing. The model designation was changed. In 1970, the Model 591 with a detachable magazine, the Model 592 with a tubular magazine, and 5mm Magnum ammunition became available. A 50-round box of ammo cost 32 cents more than a box of .22 WMR ammunition.
The two 5mm Remington Rimfire Magnum loads introduced by Aguila in 2008 proved to be what Remington’s 1968 version should have been. Maximum average chamber pressure was 33,000 psi, the same as for Remington ammunition, but velocity was increased to 2,300 fps by reducing bullet weight to 30 grains. Giving the new bullet a higher ballistic coefficient flattened trajectory, and there was no sacrifice in downrange residual energy. Rather than falling short of the 2,300 fps velocity rating as sometimes happens when advertising claims and actual chronograph readings collide, both Aguila loads exceeded it. The jacketed hollowpoint bullet expanded explosively out to 200 yards or so, making it an excellent choice for chipmunks, flickertails, and other small varmints. Expansion of the jacketed softnose bullet was more controlled, making it better at punching the tickets of larger varmints.
The Cartridge Today
According to an engineer at Aguila, today’s ammunition is loaded to the same chamber pressure as the 2008 batch. The 30-grain JHP load is back, but the 30-grain JSN load has been replaced by what the company describes as a semijacketed hollowpoint, or SJHP for short. In addition to a hollow cavity, it has a tiny bit of lead core exposed at its nose. The JHP is still the one to use when thinning out populations of wee critters, while the SJHP offers deeper penetration on foxes and coyotes called in close. The SJHP also might be better for bumping off groundhogs. When both were fired in the 23-inch barrel of my test rifle, muzzle energy was about the same, but due to the more streamlined shape of the JHP, it will deliver a bit more energy out where varmints are munching on grass.
There has been a bit of confusion on velocity. Printed on the boxes of both loads is 2,200 fps, but the 2019 Aguila catalog indicates 2,300 fps. The fact that both current loads exceeded that from my test rifle told me that the printing on the boxes of ammo is incorrect. When bringing this to the attention of Aguila officials, I was told the velocity rating on future packaging will be corrected to 2,300 fps.
When I was asked to write up Aguila 5mm Remington Rimfire Magnum ammo, the first thought that came to my mind was that I no longer owned a rifle chambered for it. J.D. Jones of SSK Industries was first to come to the rescue with the loan of a 10-inch T/C Contender barrel wearing his own T’SOB scope mount. He also sacrificed 10 precious rounds of 1960’s-vintage Remington ammo for velocity comparisons. My luck continued, and not long after receiving the pistol barrel from J.D., another friend, who has a 23-inch-barreled Contender rifle chambered for the cartridge, generously allowed me to include it in the project. His rifle came with a Thompson/Center 3-9X scope already mounted, and I attached a Nikon 2.5-8X Monarch pistol scope to J.D.’s barrel. I shot the rifle with a Lyman Bag Jack up front and a bunny-style sandbag at the rear. The pistol was nestled in an MTM Pistol Rest with the butt of its grip resting on a flat sandbag.
As you can see in the accompanying chart, the ammunition was more accurate in the rifle. That may have been me, but it was more likely due to a difference in barrel weights. The thin octagon barrel of the pistol was only 0.650 inch across its flats at the muzzle, while the round barrel of the rifle measured 0.810 inch. Both Aguila loads exceeded 2,100 fps in the 10-inch barrel and were faster than their advertised velocity in the 23-inch barrel. The Remington load did not quite reach its 2,100-fps rating in the 23-inch barrel, but Remington used a 24-inch test barrel, so it was close enough for ammunition that has to be at least 36 years old.
Other Fine Aguila Rimfire Loadings
Several Aguila .22 Rimfire loads arrived with the 5mm Rimfire Magnum ammo, and I enjoyed checking them out as well. With the exception of the 40-grain Interceptor, which exceeds 1,400 fps without a hyper-velocity rating, most loads are standard issue, so I won’t use up space describing them. My Cooper Model 57M Western Classic has a match chamber, and seven of the loads were fired in it. Then we have the .22 Short, one of my favorite cartridges. During my youth, I bagged far more cottontails and gray squirrels with .22 Short ammo than .22 Long Rifle, and I could not resist shooting the Aguila Super Extra ammo in my Volquartsen Firefly built on the Ruger 10/22 action. Its match-grade barrel has a .22 Short chamber.
Aguila Supermaximum also caught my eye. Just about every company that loads the .22 LR cartridge has produced one or more hyper-velocity loadings, with the CCI Stinger pushing a 32-grain bullet to 1,640 fps by far the most successful. In the spirit of one-upmanship, Aguila came forward with the Supermaximum loading with its 30-grain bullet at an advertised 1,700 fps. The CCI load has been my favorite .22 LR varmint medicine for a very long time, so a shootout between it and the upstart from south of the border seemed appropriate. Neither cartridge should be used in a match chamber, so I fed them to my trusty Kimber Model 82 that has a sporting chamber. The Supermaximum load averaged 0.92 inch for five-shot groups at 50 yards and a velocity of 1,611 fps, while the Stinger averaged 0.68 inch and 1,602 fps.
Colibri is Spanish for hummingbird, and the image of one on boxes of Colibri .22 LR ammo and on the head of each cartridge indicates tiny in both size and noise. Who could resist shooting the little cartridge? Like the old .22 BB Cap, the standard version contains no propellant, and its 20-grain bullet is pushed to 420 fps by the gas produced by its priming compound. The Super version has a small powder charge (a peek inside revealed half a hummingbird feather less than 0.1 grain). With a velocity rating of 590 fps, it is Aguila’s answer to the .22 CB loading available from CCI on the .22 Short and .22 Long cases. Like the CB and BB cartridges introduced by Frenchman Louis-Nicolas Flobert in the 1800s, the lead bullet of Colibri ammunition has a pointed nose. The absence of recoil makes it safe to use in rifles with tubular magazines.
Overall length of Colibri ammunition is between the .22 Short and .22 Long cartridges, so it would not feed in any of my bolt-action rifles with detachable magazines designed for the .22 LR. It works fine in a rifle capable of handling .22 Short, .22 Long, and .22 LR cartridges interchangeably, and the old faithful Marlin 39A Mountie used in my tests is an example. Super Colibri is also quite happy in revolvers. It was a bit faster from the 4.0-inch barrel of my Ruger Bearcat than from the Marlin. My guess is the small volume of gas generated by the tiny powder charge begins to run out of push before the bullet reaches the muzzle of a long barrel.
Back to the 5mm Remington Rimfire Magnum: If you own a rifle in this caliber, stocking up would be wise. And while doing so, try a box or two of Aguila .22 Rimfire ammo; when that is used up, you will likely head back for more.