I recently came across an article at American Outdoor Guide with the title, "Prepper Rifles, Past And Present" written by Jonathon Kilburn and published in May 2020. Prepper weapons is a subject near and dear to my heart, so I gave it a read.
The “prepper rifle” has received many names over the years and many still call these rifles “brush guns.” Essentially, it’s a rifle intended to be easy to carry, having enough ammunition to handle most immediate situations and offering reliability. As the world changes, so does the apparent purpose and design of this always-ready firearm.
I'm going to have to stop right here for a bit because I think Kilburn meant "bush gun" and not "brush gun." The term "brush gun" is not used to denote a rifle that is handy to carry and use in the field (which is a "bush gun"), but has taken on a more specific meaning as a gun that is effective for shooting and taking game in heavy brush. Since the amount of deflection that a twig or leaf can impart to a bullet seems related to bullet weight (mass), calibers like .45-70 are typical for a "brush gun." That "brush gun" was a typo seems to be supported by later comments, including Kilburn explaining that "[i]n the American North and West, a bush rifle was often carried by men who were hunters, trappers or just woodsmen who needed a rifle for hunting or defense from animal predators."
Frankly, a bush rifle is probably as good a criteria for a prepper weapon as most I've heard--particularly if you are limited in the number of weapons you can own--but without getting into the specific criteria of what constitutes a "scout rifle" as advanced by Jeff Cooper. 
Going through the list of historical weapons that meet the author's definition of a prepper or bush gun, the oldest listed is the musket so widely used in 18th Century America. (Although not discussed in the article, the smooth bore musket offered a good compromise of speed and accuracy for the time, with between a 74% and 86% hit rate at 100 yards; greater if using buck shot).
He also discusses lever-action rifles and carbines, including the perennial favorite, the .30-30. While good as a bush gun, however, the author contends that the lever action leaves much to be desired in a modern combat rifle, particularly for room clearing! Setting aside the inherent problems with trying to clear a room by yourself and/or without training, I'm not sure why a lever action rifle would be problematic for room clearing and the author does not elucidate. I suppose that magazine capacity might be an issue, but in calibers like .44 Magnum, .45 Colt, or even .500 S&W, you will have between 7+1 and 10+1 shots. Even the modern .30-30 will hold 5 rounds, with older models holding 6 or 7 rounds. I don't think that I would be so quick to dismiss the lever action rifles and carbines for CQB.
As to the post-WWII era, the author explains:
Many would argue there are three main firearms from the late 1930s to 1960s that would fit the role of a prepper rifle. While there are numerous options to choose from, those three would be the SKS, M-1 Garand and M-14. They are all semiautomatic (selective fire in some), reliable and utilize decent ammunition.
They are also, IMHO, too long and heavy to fill the role of a bush gun whatever their merits as a combat rifle. But if our troops in WWII could slog through the jungles of the Pacific or engage in house-to-house fighting in Europe using the Garand, I suppose the Garand or an M-14 style rifle could do, especially if you laid in some quality hunting ammo.
The .30-06 is a highly effective game killing cartridge, its strengths lie in the availability of a wide range of bullet weights and styles and combined with good velocities, makes the .30-06, extremely versatile. It is however important to match bullet weights and bullet styles to the job at hand. Readers need to be aware of this point and not fall into the trap of concluding that the .30-06 is versatile or emphatic with one, do it all load.
Light 125-130 grain frangible bullets, although ideal for light game in theory, are not any more spectacular, nor more thorough in killing performance, as heavier frangible .30 caliber bullets. Nevertheless, light, frangible bullets can be put to a variety of uses in the .30-06, including light recoiling loads. Loaded with stout 125-130 grain bullets, such as the Barnes TSX and GS Custom, the .30-06 can be used to deliver great shock, wide wounding and very deep penetration on tough game, out to extended ranges of around 400 yards.
Loaded with 150 grain bullets, the .30-06 is a fast, often spectacular killer of lighter medium game. Many bullet designs boast excellent performance out to ranges of around 650 yards while the 155 grain A-Max produces wide wounding out as far as 800 yards.
Loaded with 165-168 grain bullets, the .30-06 has both high velocity and high down range killing power. Hunters can choose stout through to highly frangible bullets which cover a wide range of body weights and hunting situations including both woods hunting and long range hunting, out to 800 yards.
Loaded with stout 180-220 grain bullets, the .30-06 is an excellent performer on heavy body weights. Heavy bullets and mild velocities ensure deep penetration. On the heaviest of game, the .30-06 cannot be expected to produce wounding of the same diameter and depth as the big bores, however the .30-06 is entirely adequate in extreme situations where care is taken with shot placement.
Loaded with 178-210 grain high BC, frangible bullets, the basic 24” barreled .30-06 produces wide wounding on both light through to large medium game out to ranges of around 900 yards (1400fps), becoming subsonic at around 1170 yards. Although this cartridge is not as flat shooting as the large capacity sevens, it must always be remembered that the .30-06 can carry a heavy pay load to large bodied game at long ranges.
The SKS is an interesting case, in my mind. An SKS was actually the third rifle I ever bought (I was in my mid-20s and cash was scarce) and I bought it specifically for hunting. I used it only once for that purpose. I agree that the SKS is basically "bomb proof". In fact, I would rate it above the AKM style rifles in that regard. But the rifle is heavy for caliber and has poor sights. Worse, however, for a weapon intended as a bush gun, the 7.62x39 is a very marginal cartridge for hunting. The guys at Ballistic Study have this to say about the 7.62x39:
As a hunting cartridge, the M43 round is adequate for close range hunting of lighter medium game but it’s performance is generally poor to fair. Using either military FMJ or commercial sporting ammunition, kills with a chest shot are often delayed while animals shot beyond 200 yards may show no sign of being hit, in extreme cases going so far as to continue grazing (in a confused state) after moving to safer cover.
Full metal Jacket ammunition generally produces pin hole wounding unless bone is struck. Fast kills can therefore only be obtained with select shot placement.
Loaded with soft point (expanding) bullets, the 7.62x39 produces somewhat better performance but due to a combination of low muzzle velocities and low BC’s, bullet expansion is limited, the cartridge quickly losing the ability to create disproportionate to caliber wounding. At around 100 yards, width of wounding begins to taper dramatically. At around 200 yards, most soft point bullets have lost the ability to create fast killing wounds. Beyond 200 yards, retained energy is extremely low along with excessive wind drift. As suggested, the 7.62x39 is best suited to the hunting of lighter medium game at close ranges.
While people will often compare the 7.62x39 to the .30-30, if you look at the bullet weights, energy, and so on, the .30-30 actually outclasses the 7.62x39 in most everyway even using the poor BC bullets used in the tubular magazines. But load it with the Hornady Lever Evolution bullets and even that disadvantage disappears. If you want to read more about the SKS, I've a post on it called "Survival Weapons: The SKS."
Moving on to more modern rifles, the author brings up the AR-15, AR-10 style rifles, and AKM. The AKM uses the same 7.62x39mm cartridge as the SKS which I have already discussed. (See also my article, "Survival Weapons: The AK Rifles"). Kilburn writes:
It’s overpriced but it offers performance and ease of maintenance that is arguably better than its AR-15 and AR-10 counterparts. It’s hard to omit the AK as a modern prepper rifle, but it might be more economical to chamber a 7.62×39 in an AR-15 platform.
Again, the combination of poor sights and poor ballistic performance makes it less desirable. On the other hand, there is no denying that it is very reliable and easy to maintain. There are plenty of options available for mounting optics, although I would be very careful of these as many are not very sturdy or suffer from other defects.
If I wanted an AK style rifle, though, I think I would get an AK-74 using the 5.45x39mm ammunition or perhaps one chambered in the 5.56 NATO to get the better terminal performance offered by those rounds. Even without going to the smaller caliber, I would probably use a gas-tube mounting system in combination with a red-dot; and add the Magpul handguard for mounting a flashlight and/or vertical foregrip.
The AR-15 was theoretically designed as a wounding weapon, i.e., drain the enemy’s resources by wounding their soldiers. With the low weight of ammunition, it’s incredibly easy to produce a follow-up shot. This is significant whether hunting deer or defending oneself.
Again, I am going to have to disagree with the author. The whole reason for adopting the 5.56 round was because of its greater lethality at common combat distances than the .30-06 or .308 of the time. Initial combat testing in Vietnam using a 55 grain projectile from 1:14 twist barrels gave incredible results, including limbs being blown off, heads completely blown apart, and large, gaping exit wounds. Even reducing the twist rate to 1:12 as the Army required still left you with a weapon with significantly more wounding at short engagement ranges than the .30 caliber projectiles then used by the U.S. military.
Now that we have switched to heavier bullets and even faster twist rates, the explosive yawing seen in earlier rounds is much reduced. Nevertheless, with the right ammunition selection, you can still achieve rapid yawing and bullet fragmentation after striking the target. As for a hunting weapon, going back to Ballistic Research, we read:
Like the .222, when using 40 to 55 grain projectiles, the .223 produces a broad but shallow wound channel at ranges less than 100 yards. Beyond 100 yards, especially between 200 and 250 yards wound channels tend to be much narrower than cartridges of 6mm upwards. A major difference between the .222 and .223 is that both Federal and Winchester produce loads for use on medium game. Nevertheless, if using the .223 on medium game, it is very important to try and avoid major shoulder bones to aid bullet penetration and ensure that wounding occurs exclusively within vitals.
As a varmint or target shooting cartridge, the .223 is an outstanding performer, inexpensive and capable of great accuracy. As a medium game cartridge, the .223 is under powered if fast killing is to be expected with ordinary chest shots. The one exception is when using tumbling FMJ ammunition which completely and utterly changes the performance of this cartridge on medium game.
The tumbling 55 grain bullet is truly violent and fast killing and is the most effective medium game hunting load for the .223. Exit wounds on medium game are often as wide as 3”. However, it must be stated that when full metal jacket ammunition tumbles, the bullet also very gradually falls to pieces due to the unsealed base of the bullet allowing jacket core separation. Because the process is gradual, wounding occurs through vitals and bone, rather than on impact resulting in adequate penetration for all but tail on shots on medium game.
For many years all .223 caliber sporting rifles featured barrels with a twist rate of 1:12. Recently, some manufacturers have increased twist rates to 1:9. When 55 grain FMJ ammunition is used in either 1:12 or1:9 twist barrels, wound channels are extremely small, about 6mm in diameter with mild bruising around the wound (the former T44/7.62/.308 Winchester is no better in this respect). The one brand of 55 grain FMJ ammunition that does tumble is Norinco although the mechanism which initiates yaw on impact is difficult to ascertain.
In some cases, rifles with a 1:12 twist will produce tumbling on impact with 62 grain FMJ ammunition. The decision of whether a hunter can adopt such a load must be based on whether the rifle is capable of producing adequate accuracy. Many rifles can shoot around the 1MOA mark with 62 grain FMJ ammunition. A second factor is the stability of the load because in some cases, a projectile can be so carefully designed and engineered that it retains stability and fails to produce excessive yaw on impact despite an incorrect twist rate.
Hunters who wish to experiment with these loads are highly encouraged to do so but should first ascertain their barrel twist rate, either by studying the revolutions of a ram rod and cleaning patch or by consulting manufacturer’s data. The use of FMJ projectiles can be significantly more effective than any available sporting .223 load.
Although the .223 can be used to ring steel at ranges out to 800 yards, its effective range is generally considered to be 300 to 400 yards, and the range at which it will violently yaw is under 200 yards.
As the author observes, "[t]he AR-15 and AR-10 are unique in their ability to utilize different calibers in the same system." In the AR-15 platform, this results in the ability to make use of calibers such as the .450 Bushmaster, 6.8 SPC, 6.5 Grendel, .224 Valkyrie, and others that offer greater performance as to in-flight ballistics and/or terminal performance at longer ranges than the standard 5.56, although often with a slightly lower magazine capacity. (See also, "Survival Weapons: The AR-15 Family Of Weapons").
Finally, the author discusses the AR-10 style rifles observing that "[s]ince the AR-15 was designed as a scaled-down version of the AR-10 (or LR .308 DPMS) the AR-10 might be a more attractive option. Using the .308 caliber provides a longer effective range with a lot of power."
The .308/7.62x51mm ball is certainly no better than the .30-06 ball, and possibly not as good. But turning to hunting loads, Ballistic Research relates:
The .308 is a highly effective medium game cartridge, supported by a huge range of ammunition and projectile designs that enable it to achieve optimum performance at varying ranges and on varying body weights.
Loaded with conventional soft point bullets, many bullet brands lose the ability to produce hydrostatic shock at impact velocities below 2600fps and in such cases, dead running game can be a common occurrence when using the .308 at ranges beyond 50 yards. In fact with some bullet brands, its as if a magic button has been switched off right at the 2600fps mark. Several bullet brands do however have the ability to produce hydrostatic shock (instant collapse) of game down to velocities as low as 2400fps, depending on target resistance and relevant factors. Regardless, hunters can manipulate speed of killing by matching bullet construction to the job at hand and in this caliber, there are some excellent options, capable of extremely fast killing via wide wounding. The 2600fps parameter and the gradual reduction in shock with conventional SP bullets below this velocity is common throughout the small bores, up to the .338 caliber.
In the .308 (actually all bores 7mm and above), a simple rule of thumb for best results on deer is to use either a stout 150 grain bullet or a soft/ frangible heavy bullet, as a means to effect wide wounding combined with adequate penetration. This may seem an overly simple rule but it can be used with great success prior to load selection. Of course, tough game call for a different approach.
The .308 can be loaded with 110 to 130 grain bullets and used with great results on varmints and light bodied game however; heavier bullet weights can in many cases give better results than their lighter counterparts regardless of reductions in muzzle velocity. In this regard, light bullets are often better utilized, down loaded, for training new hunters.
Loaded with 150 grain bullets, the .308 is immensely effective across a wide range of game species. Hunters have a choice of fast expanding through to stout, deep penetrating projectiles. This bullet weight can be used to produce clean kills on medium game out to ranges of around and exceeding 600 yards.
The 165 to 168 grain bullet weight in .308 is, generally speaking, somewhat more effective on game weighing above 90kg, than on light bodied game. Performance of this bullet weight can be altered by matching bullet construction to the job at hand. Soft, frangible bullet designs work extremely well on a wide variety of game while the vast range of semi stout projectiles work well on tough animals. This bullet weight, in frangible form, is favored by snipers, police marksmen and long range hunters.
The 180 grain .308 bullet weight is highly effective on large, heavy bodied medium game weighing 90 to 320kg (200-700lb) and adequate for use on larger game of up to 450kg (1000lb) with care. Frangible bullet designs can also be used to promote fast killing on lean animals, adding a great deal of versatility to the .308.
The .308 is immensely effective when loaded with 200 grain bullets for use on large bodied game at close to moderate ranges. This cartridge is pushed to its limits on 450kg game (1000lb). On the largest of animals, readers must understand that the .308 caliber cannot produce wound channels as large as that of a wider bore, even though penetration is quite often outstanding. The .308 can be used reliably and with a great deal of satisfaction on game weighing as much as 600kg (1300lb), but produces best results with head and neck shots.
In other words, the .308 makes a good all-round caliber so long as the correct bullet weight is chosen, but does not equal the .30-06.
However, just as the AR-15 is available in different calibers, the AR-10 style rifle scan be found in other calibers which may provide better long range ballistics, even if less effective at taking the larger game. The most popular of these is the 6.5 Creedmore cartridge.
I see the AR15 style rifles as being best as a defensive rifle and/or hunting rifle for use in a built-up area or in woodlands. Open savannah or desert or mountainous terrain calls for longer effective ranges than offered by the 5.56 mm. As we have seen in Afghanistan, engagement ranges may even be at ranges beyond that of the .308. There are two approaches to dealing with this: a more powerful weapon (e.g., .300 Win. Mag.) or something designed for accurate long range shooting (e.g., the 6.5 Creedmore). To the best of my knowledge, AR-10 style rifles are not offered in .300 Win. Mag.,  but the 6.5 Creedmore is available in the AR-10 platform and comes with the added bonus of having less recoil than .308.
 The only rifle that I know of that fits Cooper's criteria (although finding a scope mount may be problematic) is the Enfield Jungle Carbine. Although Cooper settled on the .308, his original requirement is only that the rifle use a cartridge capable of taking large game up to 1,000 lbs which I'm pretty sure can be accomplished by the .303.
 Falkor Defense offers an AR style .300 Win. Mag. rifle (the "Petra"), but the receiver is longer than the AR10 receivers, so not technically an AR10. The same goes for Noreen's AR style rifles in .270, .25-06 Rem., .30-06, 7 mm. Rem. Mag., and .300 Win. Mag., or the rifles offered by Nemo.