Today, someone on a non-gun forum I frequent posted the following counterfactual:
"So, I was watching the 'Lord of the Rings: Return of the King'; it got to the Siege of Minas Tirith, and it got me to thinking what battles (historical or fictional), might have been completely changed (and how) with one current Dillon Aero minigun, and enough ammunition to run it for however long it took"A'right... I can dig it, let's play...
The first thing is, Harry turtledove has written this same basic scenario into the civil war. In fact, so did a couple dozen other writers; so much so that it's pretty much it's own alternate history subgenre.
Mary Gentle did it with the novel "Grunts!" (which itself was a pretty great satire of Tolkienesque fantasy conventions, and Tolkien himself), giving Orcs the military equipment and knowledge of modern US Marines (and you should read it, it's a great book. I just wish she'd continued it as a series as was originally planned).
Then there's the various "time travelers take modern knowledge to medieval/rennaisance europe" novels/series. There's literally dozens of them.
The Grantville/Assiti shards series is probably the best of these; though I have a great fondness for the Conrad Stargaard series.
Then there's S.M. Stirlings "emberverse" series; half of which features folks from modern nantucket (including the coast guard vessel Eagle, and its crew) sent back to the bronze age (the other half of the series is set from the late 1990s to the late 2020s, in a world where all high energy interactions and energy storage are damped to uselessness; effectively reverting to medieval technology levels).
Mostly these things are mental masturbation exercises for engineers, sociology geeks, and history (particularly military history) geeks...
...and there's nothing wrong with that.
They're great fun, and they use this conceit to explore human nature in unusual situations, and unusual ways. That's the entire point of speculative fiction.
BUT...
When you actually wargame these things out, if you know what you're doing as regards military effectiveness etc... you find that equipment makes FAR less difference than training, conditioning, discipline, and tactics.
A professional soldier, in a professional army; is a professional soldier, no matter what their gear is, no matter what time they are from.
If you gave a cohort of Roman legionaires the equipment of a modern light infantry battalion, some basic training in how to use it (and in modern infantry tactics), and a little time to practice with their new gear and new tactics; they would still be among the best infantry in the world, and would likely have the same effectiveness as any modern light infantry.
Why?
Discipline, conditioning, and training.
When a man is worth his salt, the rest is just a matter of training.
... I'm sorry, it was just too perfect not to...
Now... To get down to specifics, let's talk about the inspiration for the question, the siege of Minas Tirith...
Not to say the power of a minigun (and adequate ammo for it) wouldn't be useful; it certainly would...
...But terrain and tactics are a far more significant factor here, than firepower.
The Pelennor fields are far too large a field of fire to be covered to significant effect with a single weapon, no matter how powerful; particularly given the long exposure of the Othram.
However, just covering the great gate... Well, oliphaunts are very big targets, and they clearly feel pain just like any other angry.
Also, the Witch King may not be vulnerable to bullets; but miniguns tend to be pretty effective at anti-aircraft fire, and since they'er not immune to big freakin axes... I'm pretty sure fell beasts are NOT invulnerable to large volumes of lead at high velocity.
...And hell.. if we put a woman and a hobbit on the controls, who knows, perhaps the prophecy of Glorfindel could have been fulfilled by a couple hundred rounds of 7.62 nato.
With a single properly manned and supplied minigun, well placed to defend the great gate; there would probably not have been no breaking of the gate... and a pretty frikking gigantic pile of massed bodies and brass in front of it.
Now... if we could get a bunch of miniguns out to the Rammas Echor, and particularly to the causeway forts; with interlocking fields of fire...
Well, there wouldn't have been a siege of Minas Tirith... there would have been the "bloody massacre at Pelennor fields".
Of course, where the armies of the west could REALLY have used a single minigun to great effect, would have been at Helms Deep (battle of the Hornburg).
It's only 440 yards (2 furlongs) of open field from the bend of the deeping comb to Helms dike; and another 440 yards (2 furlongs) of open field from Helms dike, to the Deeping wall...
880 yards of open field, just 880 yards wide, with a big dike in the middle (The 2 furlong measurements are in the text. Theoden says the dike "must be a mile wide at least; but the actual descriptions of it in the text, and the accounts of the battle, make it to be a maximum of 1/2 mile, and probably less)...
It takes a long time for 10,000 Orcs to cross 880 yards of open field, with a 10 yard high, 20 yard thick obstacle in the middle for them to pile up behind... and just a 10 yard wide gap to funnel through...
Legolas didn't need a hundred of the Mirkwood... what he really needed was ONE of the Dillon Aero...
At 2,000 rounds per minute (low rate, to keep the barrels from heating up as badly... and short bursts), it doesn't take very long at all to saturate that kind of field of fire.
No storming of the ramp, no ramming of the gates, no breaching of the deeping wall... Just a whole bunch of brass, and dead Uruk Hai.
So... here's MY countrefactual for y'all to play with...
Arm the 300 Spartans (along with the 400 Thebans, 700 Thespians, 900 Helots, and assorted other peloponesians. Figure 1200 shooters, and 3,000 or so tail) with the small arms and individual combat equipment of a modern light infantry brigade (a reasonable force equivalence I think).... Including light crew served weapons ( mortars, grenade launchers, and GPMGs); but no organic artillery or air.
Give the Spartans enough time to train up to proficiency with their arms and equipment, and enough train to support 10 full battle loads per fighting man (Appx 2,400 rounds per shooter. Helots are loggies only, not shooters), plus adequate rations, and medical supplies for 10% major casualty force depletion.
Assume the maximum realistic (based on logistical limits) estimates of Xerxes armies at 300,000 (with poor rations) and the same requirement to hold the hot gates for at least three days, to allow the remainder of the Peloponesian armies to reach defensive postures.
