Shewts gewd, raht?

In SHTF, you won't be able to replace barrels on your rifles very easily, because folks will be struggling just to find their next meal.

Many of the "old" cartridges that have fallen out of favor in the modern shooting world have done so because marketing is more powerful than physics.

For folks who aren't "gun people", bullets fly in an arc when you shoot them. The easier it is for a bullet to hit your target without needing to account for how wind will move the bullet left or right, or accounting for drop due to distance, the "better" a cartridge is determined to be, for largely the same reasons that people think buying a Ferrari means they're a better driver.

Now, this isn't done with just marketing magic, but through tuning the velocity of the bullet and by choosing bullets with different dimensions. This is something you can already do if you hand load ammunition, but what currently drives the gun market is not capability, but ease.

You can build a car that has a better 0-60 or skidpad rating that a Ferrari for a fraction of the cost, but if someone can just buy something off-the-shelf and "shoot better" than someone else, it won't matter if they could have done the same thing less expensively. Status, not function.

So, the first part of the "not-magic" is that when shooting a lighter bullet, the felt recoil will also be smaller, because the physics of accelerating the bullet to a certain speed require less force. F=MA, so at the same acceleration, you'll need less force if you have less mass. This is a big part of how modern rounds of a smaller caliber "reduce recoil", and do so off-the-shelf, because they're shooting a lighter bullet.

The thing is, though, that shooting a lighter bullet to the same velocity requires more powder charge, due to how inertia works. A lighter bullet is going to start moving down the barrel faster than a heavier one because of inertia. Heavier bullets will resist accelerating longer than a lighter bullet, so in order to keep generating pressure behind the lighter bullet, more powder has to be burned so that the drop in pressure as the bullet travels down the barrel doesn't occur fast enough to prevent the bullet from reaching the desired speed.

Think of it like how, the better you can keep your lips pressed together, the more pressure you can hold in your mouth.

Now, given F=MA above, it's not only easier to accelerate the bullet when it's lighter, but if you've got the same barrel length and force being applied, the final velocity of the bullet will be higher as well. Higher velocities make a "flatter" bullet arc, because the faster a bullet is the less time gravity has to work on the bullet before it finds its target.

You can also do things like use a smaller diameter bullet, because the smaller the diameter, the less that wind resistance will come into play.

The modern "hot rounds" all thus tend to be smaller in diameter than "old" rounds, lighter weight bullets than "old" rounds, and are universally praised in marketing as being "better".

Except that, you remember how you need more powder for these lighter rounds?

Each time you fire a gun, you do a very small, but still real, amount of damage to the barrel and chamber. Most firearms are capable of making many thousands of shots before that damage becomes a problem.

These new rounds shorten that lifespan considerably before a replacement barrel is required.

If you live in the 1st world, and shoot for fun, then this isn't really an issue. Just go online, order a replacement barrel, and then install it. Most new firearms are also designed to be easily serviced by the user, so you often don't even need a gunsmith or gunsmithing skills.

If SHTF though, you no longer live in the 1st world.

Instead of a controlled shooting environment like a range, you're most likely going to be dealing with suburban distances which are frequently much shorter. It won't matter if you could reliably hit a stationary gong at a thousand meters if the hostile you need to kill is moving quickly and at only 50 meters away.

Further, to be very accurate you must also deal with issues in poor reliability, because accuracy requires consistency down to very small minutia, every component needs to be doing the same exact thing the same exact way every time you shoot. Variation means things don't work exactly the way they're expected, and you won't know by how much until after you've already pulled the trigger.

Like putting the wrong gas in your Ferrari. Might still run, or it might not at all.

In evaluating your firearm choice in preparing for SHTF, you thus cannot rely on what is currently popular. The AR-15 platform is fine for its intended purpose, but as I've written before, people have deceived themselves as to what that intended purpose is, and a lot of folks think their tacticool AR-15 will give them any sort of edge in the post-apocalyptic wasteland.

The reality is that they've got a weapon which wasn't designed based on raw lethality, on efficiency of materials across the entire lifespan of the weapon, but on how well the weapon would sell in a particular market.

1st world military and SHTF are not the same market.

Prepare based on what you will encounter, not on what you currently experience.

There's a reason certain cartridges and firearm layouts were popular in the past, when technology and materials science weren't what they are today. We'll be entering such a "dark era" again, when we can't rely on modernity to be our savior, and if we can thus learn from the past, why things did or didn't work and why, then we'll actually be ready to deal with what is to come.