How Was Optimum Gun Barrel Length Determined?

[PhilB]

I suppose that, in hand held weapons, ergonomics were a large factor. But, in artillery and tank guns, how was the best barrel length decided? What were the pros and cons of longer barrels?

[Mattr82]

https://en.m.wikipedia.org/wiki/Caliber_(artillery)
 

Basically, a lot of it is down to best use/efficency of propellant behind a charge for the projectile. 

Edited 11 March by Mattr82

[ianjonesncl]

Barrel length is primarily concerned with range. The longer the barrel, the greater the range; contrast a Gun versus a Howitzer.

The initial start is the shell and the desired effect. So say 18 pounds, 60 pounds,100 pounds etc.  The diameter of the barrel to facilitate the shell size and the amount of propellant required to propell the shell, thus the strength of the barrel, will start to influence the weight of the barrel. Those two factors combined, together with the length of the barrel will determine the barrel weight. 

The key factor is now weight of  the trunion and carriage required to support the barrel and how that will be transported. A field  gun which is intended to be mobile needs to be relatively light compared with a less mobile siege gun.

As an example, if you require an 18 pounder field gun to be moved by six horses that gun will have a weight limit. That weight limit will include all of the gun components, so start to determine the barrel weight as one of those components.

The relationship between the strength of the barrel to accommodate the relevant propellant charge to move the desired shell weight  and the barrel length will provide a weight of barrel that can be transported by the desired methodology, thus determining the length of the gun barrel.

A 4.5 inch howitzer firing a 35 pound shell which is required to be pulled by six horses will need to be roughly the same weight as a 18 pounder. To facilitate the heavier shell, the barrel will need to be shorter to meet the weight limitations.

 

 

 

 

 

 

Edited 11 March by ianjonesncl

[PhilB]

So, if the gun is to be mechanically hauled or in a ship, tank or tank destroyer, the designer has more freedom in design as weight is a minor consideration?

Edited 11 March by PhilB

[Chasemuseum]

Light field artillery was nominally limited to being moved by a team of 6 horses and heavy field artillery by a team of 12 horses. Anything bigger (heavier) needed to be broken down into multiple loads and then reassembled before use. So flat trajectory light field guns were the 75mm to 18pr size, the howitzers wee the 105mm to 4.2inch size, while heavy field guns were in the range 100mm to 125mm (up to 60pr) and heavy howitzers up to 6-inch.

Guns like the German 21cm howitzers had to have the barrels taken out for transport. So when you watch the movie "War horse" and two horses pull a 21cm up a hill - its utterly embarrassing. Totally physically impossible.

 

For the Australian Army we are now in the program to replace the UK designed L119 105mm "Hamel" guns. The calibre, shell and characteristics of the design had all been based on the team of 6 horse transport. The Australian Army only employed motor transport artillery in combat during WW2 and transitioned completely from horse artillery transport in training in 1943, so its only taken 80 years to recognize that they can employ an alternative design criteria.

[MikB]

Another material fact in determining internal ballistic design (of which barrel length is a major component) is barrel life. The final velocity itself is, of course, a real influence on wear, but so is the pressure and temperature profile of the propellant gases during the barrel time. Early cordite propellants had a good pressure curve but a high flame temperature that tended to wash the sharp edge off the driving edge of the lands after relatively few rounds. Even in WW1 HP was commenting that practice use of sniping rifles had to be kept to a minimum as the most accurate life of barrels might begin to deteriorate after 500 to 1,000 rounds. With modern steels, fullbore target rifle barrels can remain competitive up to 6,000 to 8,000 rounds, but a dedicated target shot can get through that in 3 or 4 years.

At the other end of the scale, battleship guns might need relining every time it shot off its full outfit of ammunition - 3- or 400 rounds. One of the QEs heavily engaged in WW2 I think wore out 3 sets of linings on its 15" battery. It's clear enough that either machining or loading could go wrong - this round shows asymmetric rifling engraving has occurred to the shell body in a way that must have been damaging to the liner - you'd expect the bearing surface to ride the inside diameter of the lands and only the driving band to be engraved.

https://content.invisioncic.com/r224821/monthly_2024_03/15inshellinGenoacathshowingasymmetricengravingdamage.jpg.c0be067c709ac22d423b83ae844aa70d.jpg

In very many cases the end result is a barrel length of 40 - 60 calibres, a pressure of 18 - 24 tons psi and an MV of 2000 - 3000 fps. Beyond that weight tends to become a problem, or barrel life. 

 

 

Edited 16 March by MikB

[PhilB]

Thanks for all the replies, gents. It`s clearly quite a technical subject!

[ianjonesncl]

On 16/03/2024 at 20:47, MikB said:

Another material fact in determining internal ballistic design (of which barrel length is a major component) is barrel life.

One of the factors that has lead to the Americans cancelling their XM1299 super howitzer project

"Among the technical problems, which were the reason for the decision to discontinue funding the program, were issues with the technical aspects: the record-long barrel of 58 calibers (nearly 9 meters) was found to exhibit excessive wear during trials."

Pentagon scraps XM1299 super howitzer project amid budget constraints (msn.com)

[MikB]

1 hour ago, ianjonesncl said:

One of the factors that has lead to the Americans cancelling their XM1299 super howitzer project

"Among the technical problems, which were the reason for the decision to discontinue funding the program, were issues with the technical aspects: the record-long barrel of 58 calibers (nearly 9 meters) was found to exhibit excessive wear during trials."

Pentagon scraps XM1299 super howitzer project amid budget constraints (msn.com)

My guess would be that solutions existed (even some WW2 barrels were over 58 calibres - eg. British 17-pounder, German 75 and 88 mm PAKs and KWKs 70 - 71 cals) but were thought too expensive to justify for the XM1299 project.

Of course, I should've added that budget, especially in "peacetime", can be a very distinct determining factor for barrel length... 

Edited 18 March by MikB

[PhilB]

So it`s  case of longer barrel means higher shell velocity which means more wear?

[MikB]

2 hours ago, PhilB said:

So it`s  case of longer barrel means higher shell velocity which means more wear?

Only if matters are equal - but they're usually not. There isn't a simple relationship between velocity and wear because designers change other things to offset wear. 

Barrel materials, construction, forming or machining methods, heat treatment, internal burnishing, surface treatments (eg. plating/coating), rifling profile and twist (if used - smoothbore guns have revived since the mid-1960s in some applications). Then there are  projectile and driving band materials, profile and in-barrel mass, all affecting pressure curve and flame temperature.