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On the first day of the Battle of Shiloh, some 45,000 Confederates under General Albert Sydney Johnston caught 33,000 men of Union General Ulysees S. Grant by surprise. They overran (and captured) one of Grant's five divisions, and pushed the rest toward the Tennessee River.

Grant saved the day by massing all of his artillery--field artillery, siege guns, and gunboats--at Pittsburg Landing, and reforming the Union army in front of it for a last stand. There was a letup in Confederate pressure when General Johnston was mortally wounded.

If the Confederates had massed for an all out attack against Grant's remaining troops. How would the usefulness of the siege and gunboat guns compare to field artillery in bringing about "mutually assured destruction?"

Put another way, if x is the number of pieces of field artillery, y being the number of siege guns, and z being the number of gunboat guns, is there something approaching a 1- to 1- correspondence between the value of siege artillery and gunboat guns compared to field artillery? Does the value of x +y+ z of "mixed" guns approximate the value of x+y+z of field guns? Or could it be that e.g. gunboat guns are even more valuable than field guns because they are immune to capture?

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    alternative history is out of scope for this site – jwenting Sep 6 '15 at 19:30
  • Refocused the question to ask about the relative value of x+y+z guns of different types, versus x+y+z field pieces. – Tom Au Sep 6 '15 at 19:47
  • Confederate Firepower killed Albert Sydney Johnston - his leg was clipped from behind by his own men minutes before. An old would led to him not feeling it, and he bled out into his own boot and died of shock. – Oldcat Sep 9 '15 at 0:53
  • @Oldcat: Ok, fixed. – Tom Au Sep 9 '15 at 1:04
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Is there something approaching a 1- to 1- correspondence between the value of siege artillery and gunboat guns compared to field artillery?

The problem with this sort of question is it's trying to add up a lot of different variables and come out with a "score". This would be like trying to determine who's food is better by assigning numbers to and adding up all the stuff in our refrigerators; doesn't make much sense.

Let's drag this back into reality by asking a more direct question: which guns would have been most useful to defend Pittsburg Landing against Confederate attack: siege, field or gunboat?


Let's have a look at this battle.

Frank Moore, ed., The Rebellion Record: A Diary of American Events, with Documents, Narratives, Illustrative Incidents, Poetry, Etc. (New York: G.P.Putnam, 1862), IV: 377.
source citation

The Union army is surprised on April 6th and over the course of two days falls back. Their lines were in a crescent shape with their flanks and rear protected by the Tennessee River and gunboats. The Confederate lines are in contact.

By eyeballing landmarks on Google Maps we can get a scale for the map. On the north/south scale, Snake Creek to Lick Creek is about 2.5 miles. For east/west, the Tennessee River to the road junction is about 2 miles. Thus the map depicts an area about 4 miles high by 4 miles wide. This means the lines are less than 1000 yards apart and the Union position has, at most, only two or three miles between them and the river for their artillery.

This means...

  • There's not a lot of time for artillery to set up, dig in and sight.
  • There's not a lot of space either.
  • Artillery will be doing short range butcher's work.

Now, some notes on Civil War era artillery. Today we think of artillery as big guns firing at high arcs using complicated math from far behind the lines. And some Civil War artillery did that, especially siege guns, but Pittsburg Landing was a hasty battle fought at short range. Field artillery would often be direct fired like a normal gun: see that mass of enemy over there? Shoot them. Estimate range and wind, adjust sights, fire, observe fall of shot, correct, repeat.

While much Civil War artillery was smoothbore, many were rifles (especially Union). At the ranges involved (a mile) they could be fired quite accurately, even the smoothbores. Here's some contemporary accounts.

At 1600 yards [1500 m] the Whitworth gun fired 10 shots with a lateral deviation of only 5 inches.

Our guns were 12 pound brass Napoleons, smooth bore, but accounted the best gun for all round field service then made. They fired solid shot, shell, grape and canister, and were accurate at a mile.

The Yankee three-inch rifle was a dead shot at any distance under a mile. They could hit the end of a flour barrel more often than miss, unless the gunner got rattled.

Field artillery and gunboats had more than cannon balls. They were supplied with different kinds of ammo for different situations. Solid shot (ie. a cannonball) for battering fortifications and plowing through columns of infantry. Canister/grape is like a big shotgun: great for firing at a massed charge at a few hundred yards. For longer range work, explosive shells had a fuse which would explode and send pieces of the case flying. Case/shrapnel improved on the idea by adding more pieces and fused to burst in the air and rain down on the enemy (thus "bombs bursting in air"). I don't have the details of what they had at Pittsburg Landing, but note that the standard 12-pounder Napoleon Federal artillery battery supply was 37% solid shot, 37% shrapnel, 12% canister, and 12% shell. They had choices.


Gunboats

Specially the USS Lexington and USS Tyler carrying smoothbore 10 8 inch and 3 32 pounder guns between them putting them well in the realm of siege artillery.

Pros

  • Always mobile.
    • Difficult to hit.
    • Quick to relocate for a new target.
  • Fast (5-10 mph).
  • Carries their ammunition with them.
  • Typically carries solid shot, explosive shell and anti-personnel rounds.
  • Gun crews are protected.
  • Cannot be overrun.
  • Each carries numerically as many guns as a typical field battery (six).
  • Heavy guns (8 inch and 32 pounders) typical of siege artillery.
  • Can provide flanking fire from a relatively safe position.
  • Excellent mounting.
  • Fairly accurate (they are designed to fire at moving targets).
  • Fast firing.

Cons

  • Can only move in water.
  • Heavy seas can hamper accuracy.
  • Expensive (what with the "boat" part and all).

None of these cons were a concern on the day of battle.

Of the gunboats at Pittsburgh Landing, Major General Leonidas Polk of the CSA reported...

"...[the Confederate forces] were within from 150 to 400 yards of the enemy's position, and nothing seemed wanting to complete the most brilliant victory of the war but to press forward and make a vigorous assault on the demoralized remnant of his forces. At this juncture his gunboats dropped down the river, near the landing where his troops were collected, and opened a tremendous cannonade of shot and shell over the bank, in the direction from where our forces were approaching."

General Grant stated of the victory "in this repulse much is due to the presence of the gunboats."


Field Artillery

Federal field artillery was generally 6 to 24 pound guns. Smoothbore or rifled. Long guns or howitzers. It's not clear what Grant had available at Pittsburg or even how many. Wikipedia says...

As with the Hornets Nest, the estimate of the number of guns varies widely. Grant, in his memoirs, recalls "20 or more." Daniel, p. 246, and Grimsley, p. 109, account for 41 guns; Sword, p. 356, states there were "at least 10 batteries"; and Cunningham, p. 307, cites historical accounts that vary from 42 to more than 100.

The Union Order Of Battle includes these units, but it's not clear how many were engaged in the fighting, nor how many or type of guns they possessed.

  • 1st Illinois Light Artillery
  • 2nd Illinois Light Artillery
  • 3rd Illinois Light Artillery
  • 6th Indiana Battery
  • 9th Indiana Battery
  • 1st Ohio Light Artillery
  • 5th Ohio Battery
  • 8th Ohio Battery
  • 13th Ohio Battery
  • 2nd Michigan Battery
  • 1st Minnesota Battery
  • 1st Missouri Light Artillery
  • Missouri Light Artillery, Mann's battery
  • 5th U.S. Artillery

Pros

  • Relatively cheap (not a concern once battle is joined).
  • Typically carries solid shot, explosive shell and anti-personnel rounds.
  • Very good against exposed soldiers.
  • Can go anywhere the army can.
  • Rifles can be very accurate.
  • Fast firing.

Cons

  • Immobile while firing, slow to "limber up".
    • Counter-battery fire can zero in on their position.
    • Slow to relocate for a new target.
  • Slow when moving (16 miles a day, max).
  • Vulnerable to being overrun.
  • Vulnerable ammunition train.
  • Unless given time to dig in, gun crews are exposed.
  • Lighter guns (6 to 24 pounders, typically 12 pounders).
  • Needs to feed and maintain horses (6 to 8 for a 12 pounder).
  • Smoothbores can be very inaccurate.

Siege artillery

Large rifles and mortars from 24 to 300 pounders.

I don't believe any siege artillery was available to the Army of the Tennessee at Pittsburg Landing. The previous Battle of Fort Donelson featured only light artillery and the army was lacking siege artillery during the Siege of Vicksburg a year later. They were so desperate, they made mortars out of wood.

What if Grant did have siege artillery? What would he have had in April 1862? We can take a guess. At the same time as Shiloh, the Feds were plastering Fort Pulaski using mostly 10" and 13" heavy seacoast mortars: very inaccurate, very heavy, very immobile, and probably of zero use in a mobile field battle like Shiloh. A mobile army like the Army of the Tennessee would have used smaller and lighter, but even less accurate, 8" and 10" siege mortars. They still weighed 2000 to 4000 pounds and required a prepared bed before firing.

They also had a few rifled James and Parrot rifles. While more accurate than a mortar, and at the "small" end of siege artillery, they still weighed 5000 lbs and up and could not be moved around the battlefield quickly.

Finally they had 8" and 10" smoothbore Columbaid cannons clocking in at 10,000 to 15,000 pounds.

For comparison, your typical field artillery piece is about 1000 pounds and it still required a team of six to eight horses to pull it.

The Battle of Shiloh was a surprise for the Feds and the Pittsburg Landing defense was hastily put together. With Federal units marching to the sound of battle, it's unlikely they had time to take their siege train with them nor to unload (it traveled primarily by water) it and its ammunition nor emplace the huge guns.

Pros

  • Heavy guns able to breech fortifications.
  • Able to fire from a relatively safe distance.
  • Mortars can fire over defensive structures.
  • Fires shot and shell.
  • Rifles can be very accurate.

Cons

  • Very slow to move and setup.
    • Of little tactical value.
    • Counter-battery fire can zero in on their position.
    • Very slow to relocate for a new target.
  • Can only be moved over limited terrain.
  • Unlikely to carry anti-personnel rounds.
  • Vulnerable to being overrun.
  • Vulnerable ammunition train.
  • Unless given time to dig in, gun crews are exposed.
  • Mortars can be very inaccurate.
  • Very slow to fire.
  • Expensive.
  • Often required specialty ammunition.

Few of the pros apply to Pittsburg Landing defending against a hasty Confederate attack.


Siege guns are only useful in static situations such as a fixed defense (like defending a fort or city), or a siege. In a battle such as Pittsburg Landing they would have been worse than useless. There would have been no time to move them into position nor set them up. They would have limited close range anti-personnel value. And they would have fallen into Confederate hands who desperately lacked heavy artillery.

Field artillery were fairly mobile, at least to be hastily put into position for the defense of Pittsburg Landing. They had good performance against exposed soldiers, especially firing right into the teeth of a massed charge with canister shot. However, they fired a relatively small shell and were quite vulnerable to being overrun.

Gunboats combine the best of both. Carrying heavy artillery like a siege gun, but on a very mobile platform and with plenty of anti-personnel rounds. From the Tennessee River they could engage the enemy with flanking fire from a position of relative safety. While they would often anchor to fire more accurately, they could quickly weigh anchor and steam rapidly to provide firepower at a new hot spot ("quickly" and "rapidly" being relative to having to limber up artillery to a team of horses and haul it across rough terrain).


In conclusion, if you're fighting along a river get all the gunboats you can. Otherwise, field artillery is what you want in a hasty defensive position. The value of siege artillery is less than zero unless you're conducting or under siege.

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I will not address the question which begins with "Suppose that...", because on my opinion this is out of the topics discussed here.

Considering the "x+y+z" question, and one-to-one correspondence, it is simply meaningless. Siege guns, gunboat guns and field guns are very different weapons, and adding their numbers has no meaning. A siege gun sends a very heavy projectile, which causes much destruction, but has the disadvantage of being much less mobile and has a lower rate of fire. Naval (gun boat) guns can be as heavy and powerful as you wish, because they do not have to be mobile, but their mobility is restricted to the waterways.

If siege and field guns are used in stationary position and not moved, then a siege gun exceeds in power any number of field guns, just because of the longer range and heavier projectile. But a stationary position can be taken by the enemy, or the enemy can move, etc. That's why they are called "siege guns" they are effective against a fortress (which does not move).

To illustrate what I was talking about see these Wikipedia articles:
Siege artillery in the American Civil War
Field artillery in the American Civil War
there is no general article on Civil war gun boats, but for examples see theese: CSS Forrest and USS Maiami

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    "then siege guns are actually more effective" - It's not that simple unfortunately. As Alex noted, siege guns have a lower rate of fire and are generally on carriages that are suited for battering fixed targets (and their crews would be trained in firing on that type of target). These factors would hamper their effective use against troops. So it's not a clear-cut case of "bigger is better". – Steve Bird Sep 6 '15 at 19:39
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    @Steve Bird: yes, of course. But against a fortified position, a properly placed siege gun has much larger effect than ANY number of field guns. This was my main point: you cannot trade one type of guns for another. Equation $x+y+z$ is meaningless. – Alex Sep 6 '15 at 19:51
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    @PieterGeerkens, I disagree. No one claimed that these guns had to be "fired at the maximum rate the crew could operate it" for there to be a difference. In normal, sustainable operation, a field gun would still have a higher rate of fire than a siege gun. That's because the larger gun would take longer to load and longer to reposition/aim after firing, due to its bulk and the heavier carriage. – Steve Bird Sep 7 '15 at 5:34
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    Also, siege guns would probably lack canister (and other types of) ammo. For example, the guns defending Singapore against the Japanese were largely ineffective because they had AP shells (useful against ships) but no HE shells. I doubt siege guns in the ACW would have anti-infantery ammo available. – SJuan76 Sep 7 '15 at 21:23
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    @SJuan76 Civil war era siege artillery most definitely carried explosive shells. – Schwern Sep 24 '15 at 5:34
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One difference gunboat guns had during Shiloh is that they had to fire high to pass over the bluff and thus were not restricted to line-of-sight. This indirect fire meant nobody was safe from them, but the casualties due to gunboats was correspondingly low. They did conspire to make the area so uncomfortable that the Confederates retired a mile or more to get out of the zone. This helped with the Union counterattack as they could deploy without serious resistance.

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The size difference between a 12 lb cannon ball (heaviest field artillery of the time) and a 32 lb cannon ball (common siege artillery of the time) is, because mass varies as the cube of diameter, less than 2 inches: 4.5" to 6.25". The damage done by a cannon ball is performed by physically hitting, and penetrating and then tearing off, portions of the victim's body, or even by ripping a 6 inch hole right through armour and torso alike. (This is in fact very similar to a WW2-era armour-piercing round.) Note that because drag is proportional to the square of the radius, and mass to the cube, that for a larger shot with the same muzzle velocity, the range is greater because the deceleration due to drag is less: d ~ r^2 / r ^3 = 1/r. So a 6.25" diameter shot would have had roughly 35-40% greater (maximum) range than a 4.5" diameter shot for the same muzzle velocity.

In order for a single such round shot to inflict more than a single casualty it must needs be fired so as to successively hit those additional targets after hitting the first. This was accomplished by firing the shot at a level, or very slightly elevated, angle so as to hit and skip over the ground with both minimal time and elapsed distance at a height in excess of 5 or 6 feet. Field guns of this era, firing at soft targets, were not aimed in any sense of the word but were rather leveled to fire their shot horizontal to the ground. While a battery of guns would be pointed roughly in the same direction, the need or even the desire to aim exactly did not exist - the beneficial cross-fire effect of slightly mis-aligned guns in a battery was well appreciated by artillerymen.

When the guns were being laid chocks would be set marking the placement of the carriage wheels, and after each firing the gun would simply be rolled up to the chocks again; cleaned with the damper, loaded and fired. There was no need or desire to aim or even to re-lay the gun with any frequency. Beside which the thick smoke from the black powder would have made such futile. The guns would only have been re-laid during a substantial pause in firing when clearing of the smoke permitted such.

Therefore, for siege guns employed as field artillery in a circumstance such as you envision, the main differences in battle effect would have been:

  • greater range because of decreased deceleration due to drag. which would primarily manifest as an increased rate of damage when additional troops not under cover were lined up behind the targeted units; and
  • unfamiliarity by siege artillery crews in firing very rapidly when in immediate peril, as their training was to always fire slowly enough to both not heat the barrel and allow each shot to be aimed.

There would have been negligible difference in the chance of being hit by a 4.5" or 6.25" diameter shot except as a penetrative target of opportunity; and negligible difference in the survivability of being hit by such a shot. Being hit by a 12 or even 8 pound shot was already fatal unless the shot was both spent and happened to only hit a single limb so as to take it clean off. Recall the numerous tales from the Napoleonic Period of British officers attempting to field a spent round shot as in cricket, only to lose the offending limb:

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Update - Summary:

  • A gun is a gun, is a gun, is a gun.
  • If your analysis includes range effects give siege guns slightly greater range as caliber increases.
  • If your analysis includes penetrative effects on dense formations, increase penetrative damage slightly as gun caliber increases
  • If your analysis includes crew/gun effectiveness effects:
    -- slightly reduce effectiveness of siege crews at close range to simulate slower rate of fire; and
    -- slightly increase effectiveness of siege guns at close range to simulate larger payload of case-shot (ie more balls, bigger cone of fire)

Update #2 - Gunboats:

The guns mounted on gunboats are harder to assess; because of being dependent on the (variable) height at which they were mounted. If actually mounted on a flat bottomed barge I would treat them like any other gun above. However if deck-mounted more than a few height higher their effectiveness at range would have been greatly impeded - their shot would simply not have skipped properly from that height.

In the latter case I would not even attempt to fire them at range, but would have them brought in close to shore and reserved as battery defence, to sweep the area of any threatened or captured guns of enemies with case shot. Standing ten or twenty yards off shore, firing only at ranges under about 300 yards, and then always at maximum fire rate, they would have been almost impregnable to assault, yet capable of a devastating anti-personnel fire.

Update #3

In 1809 - Thunder on the Danube III - Wagram and Znaim Jack Gill notes in endnote #98 to Appendix 13 that of Reynier's 109 artillery pieces on Lobau Island between 20 and 24 were 18 pound cannons. The number is uncertain because 4 guns of unspecified caliber, from the final reinforcement before the battle, were delayed and did not arrive in time for the fighting.

This grand battery on Lobau Island was used to cover the crossing of Napoleon's troops across the Stadtler Arm on the night of July 4th, 1809, and anchored Napoleon's right wing during the breakout from the bridgehead and subsequent battle.

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    This doesn't really address the matter of x+y+z equivalence asked in the question. – KillingTime Sep 7 '15 at 19:14
  • @KillingTime: Does my summary help? – Pieter Geerkens Sep 7 '15 at 19:47
  • This answer appears to ignore the existence of explosive shells and canister shot which greatly magnify the effect of artillery against exposed troops. – Schwern Sep 24 '15 at 5:29
  • @Schwern: As noted in your link canister shot had a maximum effective range of only 400 yds; as implied but not stated double canister had even further decreased range, in exchange for increased killing power within that range. Shells were of marginal effectiveness compared to round shot except for larger caliber guns against stationary targets, because of the small number of fragments produced by small caliber shells and the necessity for accurate aim. ... – Pieter Geerkens Sep 24 '15 at 21:32
  • @Schwern: As proved by Currie's Canadian Corps in preparing for Vimy Ridge, in order to truly achieve effective aiming of shells it was necessary to calibrate individual guns over the life of the gun, and adjust aiming every few shells, or they would start to fall short due to heating of the barrel as well as barrel wear. No- artillerist in the world made adjustments of this detail prior to April 1917, with consequent severe detrimental effect on the accuracy of shells. – Pieter Geerkens Sep 24 '15 at 21:35

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