9th Infantry Memoir - 75mm artillery question - ??

Greetings artillery experts, I'm publishing my grandfather's fantastic 500 page war journal. He was in the 9th Infantry Division Mine Platoon under Patton. I have run into a specific problem.

According to the journal his Lieutenant was struggling with laying his half track mounted 75mm guns and approach him to help with the problem. Joe was in the mine platoon but had surveying experience and could use an aiming circle. He noted that these guns were French surplus from WWI.

My specific question: Why did the these guns have no gauge to set the azimuth for the gun? Was it because of the retrofit to the half track? A bad design?

Please read in the text below. Still in the editing, so please bear with me. Having trouble making paragraphs indent here too.
Thanks in advance!
From the book:

[SIZE=13.333333333333332px]Lt. Buchanan came over to my slit trench. I found it comfortable to sit on the ground, my feet in trench, while working on my drawing board. He sat down beside me, " Joe what do you know about a clinometer?' It sounded like an instrument a surveyor [/SIZE][SIZE=13.333333333333332px]should[/SIZE][SIZE=13.333333333333332px] know something about, but I didn't. I replied, "Nothing."[/SIZE]
[SIZE=13.333333333333332px]He persisted to show me a brass instrument that looked like a bastardized Abney Hand Level - something I understood. He explained that it was an instrument designed to measure the vertical angle of any level surface. When placed appropriately on the breech block of a 75mm gun it measured the vertical angle of the bore.[/SIZE]

[SIZE=13.333333333333332px]He had a book of tables. We pored over them while he explained what he knew about the instrument. The tables converted vertical angles into distance for specific types of 75mm ammunition. It soon became clear with the clinometer we could direct the gun to drop its shell at any given distance within its range. Help from an observer could directed it to a specific target after the first volley. "That's fine," I said, "we know how set the distance, how do we lay the gun in the proper [/SIZE][SIZE=13.333333333333332px]direction[/SIZE][SIZE=13.333333333333332px]?"[/SIZE]

[SIZE=13.333333333333332px]He said, "That’s why I am here. Captain thought you might be able to help with that problem."[/SIZE]

[SIZE=13.333333333333332px]He outlined a possible conversion of our company into an artillery unit if I could sort this out. It seemed like a logical mission for the Anti-tank Company - this secondary capability was feasible if properly equipped. We had the survey skills and ordnance handling experience. We weren't placing many mines in Sicily anyway.[/SIZE]

[SIZE=13.333333333333332px]I explained how the aiming circle worked. Set it up and turned several azimuths to show the Lieutenant. Arms crossed and two fingers on his chin, he’d heard of it, but never seen one in operation. I showed how to compute the azimuth between two points on a map, then set the aiming circle to that figure — showing the line of sight.[/SIZE]

[SIZE=13.333333333333332px]On several possible firing solutions, we had no problem laying a gun for trajectory or determining the azimuth. Stumped, we still needed to know, how do we to transfer this azimuth to the gun? [/SIZE][SIZE=13.3333px]**I would like to further explain here why there is no instrument on the gun to set the azimuth? - mdpphoto**[/SIZE]

[SIZE=13.333333333333332px]Lt. Buchanan and I wracked our brains trying to solve this problem. One of the first possibilities was to set up the aiming circle in the half track, logical enough. Upsetting this solution, I pointed out the aiming circle had a magnetic needle. Its azimuth would be distorted by all the metal in the gun and the armored sides of the half track. A crazy idea, but it led to possibly setting the aiming circle away from the weapon. Sighting a prominent skyline object, say a mountain, then bore-sighting the gun on it, would roughly achieve the target azimuth. The gun would be subject to a small error due to the distance between the aiming circle and the pivot point of the gun -[/SIZE] lines of sight wouldn’t be perfectly parallel, but close enough to range the guns. Possible, but inefficient, and impossible in the dark.

 

Hello and welcome.

I don't have an answer to your question but for further research reference the half-track was the M3 Motor Gun Carriage.
The US used the both French and American manufactured versions of the M1897 75mm. Check out M1897A3, A4 and A5.

https://en.wikipedia.org/wiki/M3_Gun_Motor_Carriage

 

The Anti-Tank and Tank Destroyer guns, both vehicle mounted and towed went through a series of exercises in England to be used as artillery if needed, instead of their normal direct line-of-sight role. In effect, as indirect fire supporting the standard 105 and 155 arty. They were almost never used in that role, were never intended to be used in that role, and as replacements came and went and new units came in without getting that training, it wouldn't surprise me if a particular unit had troubles when called upon to perform that function.

It sounds like somebody up at Division or Corps decided that since these guns *could* be used that way, let's do it. Overlooking the fact that the concept had been ignored and never used and no refresher training offered to keep those skills intact.

 

Weeeeell...

Given that the M3 was an anti-tank weapon - and certainly was in this particular episode - they'd have been equipped instead with a direct sighting tube...direct sight with graticule!

I've come across this before - in the British Army the MkI Churchills, the ones with the 3.5in howitzer fitted in the front of the hull glacis? Offical load-out for the howitzer was smoke, was occasionally leavened with SOME HE...and as the MkIs' users gained battle experience in Tunisia, on some occasions and for some objectives more HE than others!...

But the gun was NOT equipped with anything other than the very same direct sighting tube used "upstairs" in the turret for the 2pdr antitank gun! The MkI's lading diagrams list ONLY a direct sighting tube on each gun, and a spare sighting tube carried in each gunner's location

The howitzer was built into the MkIs for the same ideal use as the often-debated "CS" tanks - the handful of tanks allocated to each British cruiser tank unit to provide smoke for the squadron's tanks to manouver into, or to lay around enemy guns or possible gun positions. Apart from the CS tanks' chemical smoke, British tanks ONLY had their little cut-down Lee-Enfield rifle-based smoke dispeners, which really only laid smoke where they were, not where they wanted to manouver into...and certainly didn't have the range or the smoke capacity to be thrown out at range to blind an enemy up close.

So ideally the howitzers would only be throwing smoke at locations/targets the tank commander could see, and order his hull gunner's attention onto...and thus the howitzer gunner didn't need to do more than be able to see the same targets!

Sadly, information on the performance of these howitzers in the MkI, and other types in CS tanks is more than sparse....in fact downright non-existent at times! BUT the use of the SAME sighting tubes on the hull howitzer as the 2pdr turret AT gun would make ME for one think that the ballistic characteristics of the two weapons were round about the same! I.E. the same maximum ranges, and they'd drop on the same places at shorter ranges as set by the graticule

It's the ONLY way that using the same sight and graticule for two completely different weapons - high-angle howitzer, relatively flat trajectory AT gun - could work!

 

Wow, gentlemen, Thank you for the thorough detail. Combined, this should provided adequate material to keep the reader from getting lost. I really appreciate it, and will be posting questions later on other blurry details.

 

Other posts have explained why there was no indirect laying sight on the anti tank gun (though the French 75mm M1897was used extensively as an artillery piece in both world wars.) An indirect laying sight is just another aiming circle (less the compass) clamped to the sight bracket. But an anti tank gun doesn't have one of those

There is a quick and dirty method for laying guns for azimuth using a compass, standing at least 50 metres behind it and in line with the gun. A compass bearing is accurate within 20 mils (20 metres for each km) and would get the first round where the observer can see the first adjusting round. (You aren't likely to hit the target with the first round. There are 14+ variables that affect the trajectory. You may not know the gun or target location and certainly won't have a meteor correction for a 75mm gun)

You can use an anti tank sight as an improvised indirect laying sight if you have to.

1. Lay the gun in the centre of arc - roughly where you expect to shoot, using a compass. Suppose your targets are Due NE on a bearing of 45 degrees/ 800mils

2. Send soldiers with a couple of posts in front of the gun sight one at 50m and the other at 100m make them move left and right until they are in line when the gunner looks through the anti tank telescope. Whenever the tank sight is layed on the posts, the gun is pointing NE. That is where you point the gun before any indirect fire mission

3. Use the traverse handwheel to change the azimuth left (less) or right (more) of the centre of arc. The maximum traverse the M3 GMC is

• M3: 19° left, 21° right
• M3A1: 21° in both directions

Using this you can calibrate the handwheel to calculate how many turns you need to point the gun in the right direction. Within an arc between 24 or 26 degrees and 66 degrees.

4. Obviously the gun needs to be layed for elevation using the gunners quadrant.

5. if you fire a mission and hit the target you can put out more sticks to record the direction of known targets.The gun layer can be ordered to lay on that point.

Actually, your grandfather's unit could have aligned the guns using the aiming circle. It is slightly more complicated than standing behind the gun with a compass.

a. Work out on the map what the centre of arc or zero line will be.

b. Bring the guns into action roughly facing the direction where you expect to shoot.

c. Set up the aiming circle in front of the guns where you can see their sights, and orient the aiming circle.

d. Tell the guns to lay their sights on the aiming circle (or director in britspeak.). Take a round of angles - work out the bearing to each gun and write it down. Calculate the back bearing (add or subtract 3200mils or 180 degrees) You can tell the gun that is the angle to the director. (In the example above with the guns roughly pointing NE the aiming circle might be slightly away from the centre of arc and record an angle of 210 degrees to the gun. If the gun is pointing at the aiming circle it is therefore on a bearing of (210 - 180) = 30 degrees.

e. The gun layer then traverses the barrel right by 15 degrees (using the calibrated traverse wheel) it will be pointing at 45 degrees - the centre of arc/ zero line. The prudent gun number one then deploys aiming posts and should select a second aiming point in case the posts were moved.

f. Carry this out for all guns and you have a battery which should fire parallel. but check the alignment with a compass.

If you have a self propelled gun or a tank you can paint marks on the hull and turret to orient the barrel quickly.
like this.

 

Welcome on this great forum:! This is the place to be.

 

Here's an ancilliary qustion...

When the "American 75mm"s arrived in the UK in the summer of 1940, on their nice new single trail/balloon tyre chassis...they were initially distributed as indirect fire artillery; for a few months mixed batteries of 75mms and 18/25pdrs etc. were not uncommon (must have been a logistical nightmare tho')...

But very soon they were being dug in as protection for the Emergency batteries, were given over to airfield defences and dug in - all as A/T guns.

My question therefore is...and now it'll become obvious why this is relevant to the OP's intitial query...

What sight was fitted to them in the indirect arty role - and was it THEN swapped out for a direct fire sighting tube in their A/T role???

 

The original post mentions the 9th Infantry Division mine platoon, which I presume would have actually been the mine platoon of the antitank company for one of the infantry regiments in that division. Are we sure that the vehicle in question is the M3 GMC with the 75mm gun and not the T30 HMC with the 75mm howitzer? The T30 was used by the organic cannon companies of US infantry regiments in North Africa. I do not know if any were still in service in Sicily, but I was just wondering.

...Of course, if the weapon in this case was a howitzer, then presumably it would have had sights for indirect fire and a crew trained to use the same, so maybe I just answered my own question...

 

In Summer 1940 the British Army was grateful for what they could get. I am not sure exactly what sights these came with.

Here is the user handbook. http://usacac.army.mil/cac2/cgsc/carl/wwIItms/TM9_1305_1942.pdf

and the Gun Drill book - In American English "Serving the Piece"
https://ia801304.us.archive.org/0/items/Fm6-50-nsia/Fm6-50.pdf

In Ww1 the equipment used a french collimeter/dial sight.
http://www.lovettartillery.com/French_75mm_mle_97_.htm

The 75mm guns used in Crete by the Austrlian and New Zealand gunners did not have proper sights. TYhe History of the Hertfordshire Yeomanry says that 86th Field Regiment received eight guns in the UK in Autumn 1940, without any sights, gun stores or range tables. It seems that the Americans just supplied the gun equipment.