Just because some dangerous item will never amount to a large proportion of killings, doesn't mean it should be legal.
I mean an eccentric millionaire could conceivably own a howitzer, if it was legal. But we probably shouldn't let them.
I'm interested in the description of assault rifles as Poodle Guns, since it reminds me of the dismissive attitude towards relevance of clip size. Why did anyone bother developing such things, then? Should we go back to giving all our soldiers bolt-action rifles?
Some people own canons (usually muzzle loading).
In general, possession of their ammunition was banned as destructive devices (they're really bomb hurlers), and then sometime after WW-II we banned anti-tank rifles, which were becoming popular with collectors. I'm pretty confident that the number of homicides committed with canons or anti-tank rifles is zero.
In any event, to address your edit, virtually every military had settled on rifle cartridges in the narrow ballistic range of the 7mm Mauser, .303 British, to the .30-06 (a slightly upscaled 7mm Mauser). All of them had two or three times the kinetic energy of Civil War muzzle loading rifles, but most nations were pretty sure such cartridges were required for modern combat. That was probably in part because smokeless powder and better bullet shapes made such guns very effective at much longer ranges, and if naval combat and artillery were proper examples to go by, out-ranging your opponent would produce victories.
By WW-I we were making fully-automatic rifles to fire such high-power cartidges, such as the Browning Automatic Rifle (BAR) which had a 20-round magazine. However, physics being a bitch, such guns have to be quite heavy to be controllable with full-auto fire.
, or full-powered automatic rifles are heavy.
The BAR weighed close to 20 pounds. Full-auto weapons also eat a lot of ammunition, and full-powered rifle ammunition is heavy, too. Standard .30-06 ammunition weighs about 17 rounds per pound, and armies liked soldiers to carry 200 or 300 rounds of ammunition, which would weigh 12 to 17 pounds, respectively, even without magazines, boxes, or pouches. The empty BAR magazines weigh about a half pound each (and hold 20 rounds), so the all-up weight if you wanted to carry 300 rounds fully loaded into combat would be: 20 pounds for the gun, 17 pounds for the cartridges, 7 pounds for the magazines, or about 44 pounds. Add pouches for the magazines and would hit 50 pounds. That works great if other soldiers carry the BAR gunner's tent, socks, flashlights, insect repellent, and whatnot, but if all the soldiers carried BARs they would need gun bearers following along behind them.
Better mechanical designs can lighten the gun, but lightening the gun starts making it uncontrollable in full-auto fire, since conservation of momentum combines with human anatomy to cause high-powered light full-auto guns to spray nothing but the sky. In late WW-II, in a desperate need to stop massive Soviet advances, the Germans got around this problem the only way the problem can be solved, by coming up with a wimpy, low-powered cartridge that doesn't have as much recoil. The cartridge was the 7.9mm Kurz (short), and it was in between a pistol cartridge and a rifle cartridge. The low powered cartridge allowed the full-auto rifle that fired it to weigh only 10 pounds instead of 20. Hitler called the rifle "Sturmgewehr" (assault rifle) because he wouldn't admit that it was a desperate measure to try and stop advancing Soviet armies that were rolling toward Berlin.
Meanwhile the Soviets made the same innovation, coming up with the 7.62x39 (still wildly popular) in their SKS rifle. Some say they found the 7.92mm Kurz and adapted it for .30 caliber barrels, but of course the Soviets invented everything, so they'd never admit such a thing. They might even be right about that. Both cartridges fire a 125-grain bullet at about 2,200 feet per second. (There are 7,000 grains in a pound, if you want to crunch numbers). A standard battle cartridge like the .30-06 fires a 150 to 170 grain bullet at 2,700 to 2,900 feet per second. Using the formula for kinetic energy (KE=1/2mv^2) shows that the assault rifles have to fire a bullet that has only half the ballistic energy of a regular rifle cartridge.
The US military was never going to do something like that, and insisted that only a full battle cartridge would be acceptable. When it came to coming up with a standard rifle round for NATO, they shortened the .30-06, very slightly reduced its muzzle velocity, and introduced it as the .308 Winchester. It's a little shorter than the .30-06 so machine guns can cycle faster (less bolt travel). Then they tried to make a regular-weight full-auto rifle for it, to replace the M-1 Garand, and failed spectacularly with the M-14. At the demonstration in front of a reviewing stand full of generals, the soldier firing the M-14 landed flat on his back with the barrel aiming straight up.
So the US still didn't have an answer to the Soviet AK-47. Eventually an inventor came up with a very light weight .22 caliber rifle firing a modified .222 cartridge (normally used for hunting rabbits and foxes). The gun was small and light so the Air Force adopted it as a gun their aircrews could use if they bailed out. It was fancy, looked high-tech, and used lots of aluminum, the Air Force's favorite metal, so they bought lots of them. Someone showed it to President Kennedy and he thought it was nifty. The US Army despised it. They worked for Kennedy, and he told them to adopt it, so they did, after trying everything in their power to keep using a real combat rifle.
Then Vietnam heated up, and even though the rifle was thought wholly inadequate for the European battlefield, it would probably do well in close-in jungle combat. So we tried it out. The guns jammed all the time and lots of American soldiers died as a result, to the extent that Congress had to conduct hearings on the gun's worthlessness. The Army made fixes, lots of them. But even today, the gun jams a great deal of the time, and the .223 cartridge is still so underpowered that the military and industry spend a lot of time pushing for its abandonment. Almost anything you read about combat rifles will center around the question of what cartridge the military needs to use instead of
what it's using.
But they're fun and cheap to shoot, so they're wildly popular for target plinking.
ETA: Oh, and there's a fascinating story about why
the US Army wanted a full-auto rifle. During the war some PhD justified his existence by claiming he was doing "battlefield research", and wrote scholarly looking papers showing that US soldiers simply would NOT fire back at the enemy unless they had real machine guns. He claimed he was doing on-the-scene post-action interviews with them, using questionaires and all the other tools of modern psychology, and that soldiers who only had ordinary rifles felt that they simply couldn't contribute anything toward success because their guns weren't awesome enough, so they'd just sit there and do nothing.
The Army brass fell for it hook, line, and sinker, and became adamant that whatever new rifle was developed, it had
to be full auto or soldiers wouldn't bother pulling the trigger. The belief became so pervasive that it overrode consideration like having an effective bullet.
Much later it turned out that the PhD had made the whole thing up. He'd never gone to any battlefields, he'd never conducted any interviews, and his conclusions were completely wrong. Subsequent checks with people who were actually in
the battles he claimed to have studied said the American problem was that soldiers would open up with everything they had on anything that moved. Even enemy soldiers complained about it. One German noted that if he fired one shot, the whole US front would erupt in a wave of gunfire and artillery that would take forever to die back down.
So the whole full-auto thing was a red herring, and newer M-16 derivatives don't even offer full-auto fire. So really all we've done is junk the M-1 Garand and give everyone the rough equivalent of M-1 carbines, a lightweight rifle that fired basically a pistol cartridge, used for tank crews and such because it was small and could fit behind a seat.