How to Choose a Rifle Cartridge for Any Purpose
By Chuck Hawks
Any rifle shooter needs the correct rifle and cartridge for whatever purpose they have in mind. This is obvious. The key is choosing an appropriate cartridge wisely, which will then greatly simplify finding a rifle to shoot it.
Even governments face this problem when adopting a military cartridge for a service rifle, although they often make poor selections for political, economic and other reasons. The US, for example, has locked itself into a particular, very short, rifle action (AR-15) and its shortcomings have negatively impacted cartridge development and selection ever since. They have put the cart before the horse, so to speak.
Realistically, one should first decide on the parameters of the cartridge that will fulfill the intended purpose before worrying about a rifle to shoot it. To do this you must know and understand the purpose for which the cartridge is intended to be used and you must have a reasonably accurate idea of what is needed to accomplish that purpose. Only after deciding on a cartridge (or a list of cartridges) should a rifle to shoot the cartridge be selected.
For example, let's say you want to hunt all Class 2 game, including deer, antelope, sheep, goats, black bear and other creatures weighing, on average, between about 50-300 pounds. To achieve clean kills you will need adequate penetration (sectional density) and energy on target (a function of bullet weight and velocity). You also need to consider the maximum range at which you may be shooting (trajectory and maximum point blank range), which will be determined by bullet shape (ballistic coefficient) and muzzle velocity (MV). Finally, your cartridge must be tolerable to shoot in a rifle of reasonable weight in the field.
A government issuing specifications for a new military rifle cartridge has the most latitude in this process, as they can commission the design of a completely new cartridge to meet the desired specifications. This is what the US Army did before adopting the very successful 7.62mm NATO (.308 Winchester) in 1954.
Wildcatters have more latitude in selecting a cartridge for a particular purpose than normal recreational shooters, as they are willing and able to modify, essentially re-make, existing cartridges to fit their particular needs and desires. Since there are hundreds of existing cartridges to use as starting points, wildcatters have a lot of latitude.
Many recreational shooters are also reloaders. Reloading means that, within reasonable limitations, the ballistics of factory loads can be modified to achieve a particular purpose or purposes. Although there are exceptions, maximum pressure (safe) reloads for most cartridges do not exceed the velocity of factory loads, but reloaders can reduce velocity to lessen recoil, or use bullet weights/types not available in factory loads.
When it comes to cartridge selection, the poor sap with the least options is the recreational shooter who must rely on factory loaded ammunition. However, there are a great many cartridge and loads on the market today, so even the shooter limited to factory loaded ammunition has a lot of choices.
Let's look at the factors involved in cartridge selection (or development) one at a time, assuming for this exercise our purpose is to hunt general Class 2 game. This means, for example, we need enough power to anchor a 300 pound black bear in the woods, or give us a reasonable chance at a 100 pound pronghorn on the plains.
1. Bullet sectional density (SD)
2. Bullet form (BC)
3. Trajectory (MPBR)
4. Energy required at MPBR
5. Recoil (must be acceptable in a rifle of normal weight)
Bullet SD is a good place to start. Sectional density is defined as the ratio of a bullet's weight (in pounds) to the square of its diameter (in inches). Note that bullet shape has no influence on SD.
SD matters, because it is an important factor in penetration. All other factors being equal (bullet design/expansion, impact velocity, etc.) the bullet with the greater SD will give more penetration. Many shooters think, incorrectly, that bullet weight (irrespective of caliber) determines penetration, but they are wrong; it is SD.
To achieve clean kills, the bullet must reach the animal's vitals and disrupt its vital processes. This means adequate penetration. "Adequate" does not mean the bullet has to shoot clear through the animal lengthwise. Responsible hunters are not going to shoot animals in the ass, attempting to break them down, but the bullet should be capable of penetrating the animal's heart/lung area from the front or side and of breaking bones to get there.
We have over 120 years of experience hunting with smokeless powder rifle cartridges. For example, 150 grain .30 caliber bullets (SD .226) from cartridges such as the .30-30, .30-06, .300 Savage and .308 Winchester have proven deadly on all sorts of Class 2 animals. It is hard to argue with success, so let's use .226 as our minimum desired SD.
Hornady defines ballistic coefficient as, "an index of the manner in which a particular projectile decelerates in free flight . . . represents the bullet's ability to overcome air resistance in flight." Ballistic coefficient has an increasingly important effect on trajectory (and hence MPBR) as the range increases. Pointed bullets, for example, retain their velocity better than blunt bullets as they travel downrange.
Woods cartridges typically shoot round or flat nose bullets. Long range shots are seldom required in the deep woods and such bullets are likely to deflect less if they accidentally encounter brush and twigs on their way to the target, unfortunately a common occurrence in heavy cover.
Very long range bullets, such as those used in F-Class match rifles, typically have a very pointed (often plastic) tip, long ogive and a boat-tail base. The boat-tail (tapered heel) primarily serves to reduce air drag after the bullet's velocity has fallen below the speed of sound (1125 fps at sea level). Such factors matter less at typical hunting ranges, since most hunting bullets will fail to expand properly when the impact velocity has fallen close to or below the speed of sound. No one should be shooting at valuable game animals at such extended ranges.
For a general purpose, Class 2 game hunting rifle, we should probably assume a flat base spitzer (pointed) bullet. This is the most common type of hunting bullet for most calibers today. For example, the popular Hornady Spire Point .30 caliber, 150 grain bullet (SD .226) has a BC of .338 (Hornady figure).
Maximum Point Blank Range
For hunting Class 2 game animals, an acceptable bullet deviation from the line of sight is generally regarded as +/- 3". This allows the hunter who has zeroed his or her rifle to take advantage of the cartridge's MPBR (+/- 3") to hold in the middle of the animal's heart/lung area (which is normally at least 10 inches deep) and be confident of a fatal hit. No fiddling with the scope or holding over the animal's back is required out to the MPBR.
Since we are looking for a general purpose Class 2 game cartridge, useful at both woods and plains ranges, let's say we want a MPBR (+/- 3") of at least 250 yards. This is well beyond the distance at which the average hunter should shoot at game animals from field positions.
Of course, we are much better than average shots (at least in our own minds), since we regularly practice at our local rifle range. On the other hand, there are no bench rests in the field and we may need to shoot quickly from unsupported positions (standing, kneeling, sitting, etc.). 250 yards is a long way in the field!
Energy at MPBR
To avoid a long discussion about killing power, for the purposes of this article, let's use kinetic energy on target as an indication of killing power. Actually, as long as you are comparing similar cartridges with bullets of similar SD, energy has a pretty good correlation with reality.
Long experience has shown that about 800 ft. lbs. at bullet impact is adequate for Class 2 game (assuming an adequate bullet). For example, the traditional .30-30/150 grain FP factory load (MV 2390 fps) retains 858 ft. lbs. at 200 yards (Remington figures). There is no question this is an effective killing load, so let's set 800 ft. lbs. (or more) at MPBR as the desired energy for our Class 2 game cartridge.
Most reasonably experienced shooters can handle 15 ft. lbs. of recoil energy without developing a serious flinch. The effects of recoil are cumulative and less recoil is always preferable. Anyone can shoot more accurately with a rifle that kicks less and it is bullet placement, not raw power, that results in clean kills.
Recoil is directly proportional to rifle weight. Double the rifle weight and you cut the recoil energy in half. For a given amount of recoil, a hunter willing to carry a nine pound rifle in the field can shoot a more powerful cartridge than a person who insists on carrying a seven pound lightweight rifle.
If we choose 15 ft. lbs. as the maximum permissible recoil energy, we need to state the rifle weight. I would guess that most general purpose hunting rifles weigh between 8.0 and 9.5 pounds in the field. This means loaded, with a scope and a sling. Let's estimate our rifle will weigh at least eight pounds in the field, a reasonably conservative estimate.
Note that, while it can be difficult to lighten a factory built rifle, it is relatively easy to add weight. You can choose a heavier riflescope, steel rather than aluminum scope mounts, add lead to the stock, and so forth.
Choosing a Cartridge
To summarize, per our example, we are looking for a cartridge that shoots a spitzer bullet with a minimum SD of at least .226, has a MPBR (+/- 3") of at least 250 yards, delivers at least 800 ft. lbs. of energy to the target at the MPBR and recoils with no more than 15 ft. lbs. of energy in an 8.0 pound rifle. This should be a good, general purpose, Class 2 game cartridge.
Having defined these parameters, we can now identify cartridges that will meet our requirements. The recreational hunter who shoots only factory loaded ammunition can use the ammunition companies and bullet suppliers published data and the many resources available on the Guns and Shooting Online Tables, Charts and Lists page. These include, among many others, the Expanded Rifle Bullet Sectional Density List, Expanded and Improved Rifle Ballistics Table, Rifle Ammunition Ballistics Table, Expanded Rifle Trajectory Table and the Expanded Rifle Recoil Table.
For example, some candidates that can qualify as our general purpose Class 2 game hunting cartridge with typical factory loads and bullet weights include the .240 Weatherby Magnum (100 grain bullet), .257 Roberts (115+ grain bullet), .25-06 (115+ grain bullet), 6.5x55mm (120+ grain bullet), 6.5mm Creedmoor (120+ grain bullet), .260 Remington (120+ grain bullet), 7x57mm (139+ grain bullet), 7mm-08 Remington (139+ grain bullet), .300 Savage (150+ grain bullet) and .308 Marlin Express (150+ grain bullet). One or more of these cartridges are available in new rifles from every major rifle manufacturer and there are many good used rifles in these calibers on the market.
The hunter who is also a reloader can load less than maximum loads in such popular cartridges as the .270 Winchester, .280 Remington, .308 Winchester and .30-06 and stay within our MPBR and recoil limits.
The basic procedure described above can be used to intelligently choose a rifle cartridge for almost any purpose. Just alter the parameters to fit your personal needs, intended purpose and anticipated conditions.
Copyright 2017 by Chuck Hawks. All rights reserved.