AMRs fire rifle rounds, assault cannons fire cannon rounds. Now guess what I think is the difference between an AC and a HVGL.

I think the generally accepted caliber barrier between rifle and cannon is 20mm? It's not that clear since there aren't many examples of rifles between 15.2mm and 20mm, but that you already knew. Now, if someone was to create a new caliber, with dimensions like 18x80mm, that would pose some problems...

The 25mm grenades of the OCSW, at ~1400ft/sec muzzle velocity, are still pretty "low velocity" when compared with most AMR or AC round. This, together with the ~2000 grain projectiles, places such weapons firmly in the GL category. The Mk19 even more obviously so. But again, if someone was to create a new caliber with dimensions like 25x70mm, 1400 grain projectile and a muzzle velocity of 2200ft/sec (don't bother checking the recoil, I know it would be HUGE), that would prove very problematic. At that point, I suppose the weapon is anything the makers say it is.

Can you make Assault Cannons fire rounds with an area of effect in SR? I know there's nothing in the BBB, but does CC mention anything like it? If it does, then that would be a good reason to go for an AC instead of the Barrett. Even if the rounds only did e.g. 8S with a Burst of -2/meter, that would make ACs a lot more versatile. Is the bastard hiding behind a meter-thick steel barrier that even your Vigorous can't penetrate? No worries, just drop a few HEs next to him and he'll be screaming to get out of his hiding spot.

Heh, 7S & 14S both come from the same round...

<inserts SR3_CANON™ Personafix BTL>

Of course! How silly of me!

Unfortunately for you, when I referred to "rifle rounds" and "cannon rounds", I mean the RL classifications, not SR ones.

No such thing. The closest you can probably come to it are

a) the smaller recoiless rifles available today, which need to be mounted
b) the 20mm tripod/turreted autoguns (my reference of choice for my players)

The launcher on an OICW is NOT an assault cannon by a long shot.

I think only one of them is described as firing an explosive shell - can't remember whether that's the Panther (which I vaguely recall fires a solid slug). In the case of the one firing an explosive shell, yeah, you'd be right in your analogy of a light direct-fire mortar.

I still refer my players to the 20mm man-portable "autocannons" (usually mounted on tripods or vehicles) which were so popular during the Dhofar war in Oman. I have a player who loves the Panther ... keeps saying, "Just wait till I come across you at two point four klicks one of these days!"

This is about as close as you're likely to get.
This might also qualify.
As would this.
And maybe this.

The whole "assault cannon" thing (a completely ludicrous concept by name) most likely came from the "Cobra Assault Cannon" from RoboCop. They were just movie-fied Barrett M82A1's.

The action of a semi-auto doesn't tend to lock up in exactly the same way every time due to the forces involved in cycling the bolt automatically. Therefore each cartridge is headspaced in the chamber slightly differently than the next, which can lead to inconsistent pressures and consequently velocity and accuracy (meaning consistency from shot to shot). The pressure variation is usually very slight, but the further away you are from the target, the more difference it makes.

There are ways to minimize the lack of consistent lock up, but generally it takes a lot more money and time to make a super-accurate semi-auto than a bolt action. Being loaded manually, bolt actions can be loaded much more consistently and less violently, which will generally allow a bolt action to stay more accurate over a longer period of time. a nicely-treated action tends to wear a lot slower.

There are a few semi-autos out there that will regularly outshoot a lot of bolt actions. But they usually cost an arm and a leg. For instance, a company called Tactical Operations makes rifles based on the Remington 700 action that are gauranteed by the factory to shoot 1/4 MOA groups (that's three to five rounds inside of 0.25" at 100 yards, 0.5" at 200 yards, 0.75" at 300 and so on). The rifles cost anywhere from $3,500 to $4,500 (no optics; tack on another grand or so for that).

A Heckler & Koch PSG-1 doesn't leave the factory until it shoots 50 rounds inside of 1 MOA (1" at 100 yards, 3" at 300, etc). The PSG-1 could likely shoot just as well as the Tac Ops rifle, but it costs upwards of $10,000 (with optics), weighs nearly twice as much, and will probably require a lot more maintenance to retain that level of accuracy.

But in both cases, it takes quite a bit of training for a person to be able to even utilize that level of accuracy. There are also tactical advantages and disadvantages to using each kind of rifle.