Well, the description of all autocannons is that they fire bursts - variants like ultras and rotaries just fire at higher rates.
In [url="http://mwomercs.com/forums/topic/2896-ammunition-loads-in-mwo/page__st__40__p__81005#entry81005"]an old thread[/url], someone had dug up an old BT reference which stated that each autocannon "round" was actually a "cassette" of several shells, intended to be fired as part of a single "burst". Presumably, the "modified feed system" of the ultra AC allows it to cycle these "cassettes' twice as fast as an ordinary autocannon, though the "redesigned ammunition" (which is not compatable with other AC models) suggests that the cyclical rate of fire is also double that of a standard AC.
It raises an interesting question, since we have single-shot tank cannons capable of delivering extremely accurate shots across long ranges, why are BT autocannons focused on firing bursts of fire at shorter effective ranges? I haven't seen an official answer in canon, but there's enough official "fluff" that I've been able to cobble something together that seems logically consistent, at least in my mind:
- Modern tank guns, such as the M256 mounted on the M1A1/M1A2, are designed to fire fin-stabilized shells at extremely high velocities. (The earlier M1 had a smaller rifled cannon, which would suffer excessive bore erosion from firing such high velocity ammunition). This is because the current preferred choice for defeating modern armor plating is APFSDS (armor-piercing fin-stabilized discarding-sabot) rounds, which fire high-density tungsten steel or depleted uranium long rod penetrators, which are kinetic kill rounds with roughly a 20:1 Length:Dia ratio, and which are designed to penetrate armor and destroy the internal systems and crew of armored vehicles through a combination of overpressure and spall damage. Since these munitions need to travel at extreme velocity, they destroy rifled barrels quickly, hence the need for fin-stabilization. They are exclusively anti-armor weapons, with little usefulness against soft targets.
- HEAT (high explosive anti-tank) shaped-charge munitions are also still in use, and while less effective against modern armor, they are more effective against a variety of targets. Shaped charge warheads use a directed explosion to push a soft metal core into a high-velocity jet of metal that behaves as a fluid and bores through armor, doing a combination of direct damage, spall, and overpressure to the contents of an armored vehicle. While HEAT rounds don't need to be fired at high velocity to be effective, they don't function as well when spun by rifling, so using fins for stabilization and firing them from a smooth bore is also desirable.
- HESH (high explosive squash-head) rounds have mostly fallen out of favor for use against armored vehicles. They were designed at plastique munitions with a delayed fuse at the base, and were designed not to penetrate armor, but to shake loose chunks of spall from the inner surfaces of armor plates, destroying the systems and occupants of an armored vehicle. Modern armor has made them almost completely obsolete, but they still function well against soft targets.
- Modern armor is made of of multiple layers of homogeneous steel, with spacing in between. These gaps are effective in defeating HESH rounds, as the spall from the outer armor layer simply stops at the next ayer down, and also help redirect the metal "jets" of HEAT rounds, and can also take APFSDS rounds off course. Sometimes these gaps are filled with other materials, like amorphous ceramics (effective at redirecting HEAT rounds due to their irregular molecular structure) and kevlar and heavy metal mesh (good for catching kinetic projectiles and spall). Additional "spall liners" inside the vehicle catch spall in the the case of an armor breach. This combination is extremely effective, and to continue with the M1 tank example, in the first Gulf War, there were recorded combat instances of tanks absorbing multiple HEAT rounds with negligable damage. Even the M256 cannon, firing high-velocity APFSDS munitions, has difficulty defeating the armor of the M1 Abrams tank, except by firing multiple shots at a low angle on incidence into the same area of flank armor.
- Reactive armor is made up of plates of steel armor with explosives sandwiched in between. They are designed to detonate on impact, propelling a HEAT round away from the armored vehicle. Some variants have been developed to defeat SPFSDS munitions by seperating with a shearing motion, breaking off the penetrators, but are less useful against HEAT rounds. Both types are problematic when functioning with mechanized infantry support, since they tend to create lots of shrapnel around the armored vehicle, so they aren't used all the time. However, they have driven the design of HEAT warheads in missiles to change. Most warheads now are made up of staged warheads, a small initial warhead to set off the reactive armor, and a larger secondary to penetrate the main armor of the vehicle.
Skipping over to the BT universe, armor technology isn't conceptually different, but its a lot better.
[quote name=']Standard BattleMech armor is composed of several layers providing various degrees of protection and support. The first layer is extremely strong steel' date=' the result of crystal alignment and radiation treatment, which is also very brittle. The second layer is a ceramic, cubic boron nitride, which combined with a web of artificial diamond fibers acts as a backstop to the steel layer. These two layers rest atop a titanium alloy honeycomb structure which provides support, and a layer of self-sealing polymer sealant which allows for space and underwater operations.[/quote']
Now, if the gun on a modern M1A1 tank can't defeat the armor of the M1A1 tank in a single hit, it can be surmised that firing a single shot at a battlemech with any sort of conventional gun, however large, is going to be equally problematic, and probably moreso, especially considering that each "point" of armor is effectively another sandwiched layer of all these materials. 20 points of armor is a lot to take out!
But maybe firing a burst is the answer to this problem, because as each successive since each shell should be able to punch out at least a layer of armor, and the subsequent shot would then be able to defeat the next layer, and so on. The HEAP rounds make sense in this context, as they could penetrate the outer steel, then detonate to shred the underlying ceramic and diamond fibers, making way for the next shell to punch through - and they'd also be effective against other types of targets. I'd also conjecture that missiles are launched as groups for the same reasons - a single large warhead might not penetrate as multiple hits, each tearing off successive layers of ablative armor. (Presumably Gauss Rifles can just produce such ungodly velocities that they can penetrate through brute force - though notably, the Gauss weighs as much as an AC/20 and penetrates less armor.)
Also, only Ultra ACs are noted as having smooth bores, which assumes the others are rifled and therefore fire at a lower velocity. That makes sense in a way, since lower-velocity shells would produce less recoil for a given size shell, and therefore wouldn't tend to disrupt point-of-aim as much, leading to tighter groupings. And tight groupings would be essential, since spreading shots wouldn't lead to deep penetration of armor. This could very well explain the reduced range of BT autocannons compared to real-world tank guns, since the maximum
effective range is the max range where shells fall in a tight group. Larger cannons would produce more recoil, and ergo they only throw tight groupings at closer ranges (maybe the AC/10 and AC/20 could be beefed up to counteract this effect, but it would probably become too massive to be useful in a vehicle or 'mech.)
Then the question of why can't an AC just fire single shots out to longer range, and why aren't they continuous feed instead of firing from these "cassettes" that have to reload? My thought is that perhaps the autocannons are designed without an interrupter - after all, they already have some jamming issues, and that additional complexity might increase the propensity for jams! In that case, having a continuous feed would prevent the gun from stopping until all ammo in the 'mech was consumed, and there would be a good chance of an internal explosion from the cannon chamber overheating and "cooking off" shells - no bueno! In the case of the RAC, this is avoided by using multiple barrels, so no individual barrel heats up as much, and in the ultra AC, the smooth bore likely cuts down on heating the barrel from friction and perhaps part of the unique ammunition design helps dissipate some waste heat - though as someone noted above, ultras do build up heat twice as fast when in rapid-fire modes. Also, both RACs and ultras have feed systems redesigned to cycle ammo "cassettes" more quickly than a standard AC.
So, coming off this lengthy segue, in terms of performance, I think all ACs should fire bursts, as they are described in canon. Ultra ACs should fire the same number of "shots" per burst, but the burst should take less time, and the next "burst" should load faster, giving them double the rate of fire of a standard AC. RACs aren't really a concern yet, but I'd figure on the same idea, just working at 6x instead of 2x. LBX autocannons would function as standard autocannons, but have better range and be able to use "cluster" munitions.
2
Felix Dante, on 22 February 2012 - 01:45 PM, said:
