Moving the point of impact to the right.

[Sunbeam]

Hi,

I have a 50 cal 12" barrel on my mini cannon, and we shoot it in a mini cannon postal comp via our club, so we have no cannon shooters to ask this question to. The comp has hardly any shooters and I can't contact them anyway.

The bore of the gun is perfect, it's straight and like a mirror. The comp rules say you can not use the fuse as a sight, but for the purpose of sighting up and testing accuracy, we do leave a bit sticking up. In doing this we have found that the cannon groups consistently well (13 balls in a 3" group at 25m).

However, every time we carry out this test, the group is ALWAYS 6" to the left, even when we shoot blind (without checking shot to shot). So we have concluded the cannon mush be shooting 6" to the left and it's not bad aiming.

Is there any way to pull the shots to the right 6" so we are not forced to sight up on the very edge of the black scoring ring as we are finding this type of sighting quite difficult.

I have checked all the wheels (ships cannon, 4 wheels), they turn freely.
I have checked the barrel alignment to carriage over a 6 foot distance, everything stays and runs straight.
The barrel face and crown are perfectly square.

I was thinking I could oval the crown so at 3 o'clock the crown has a slight oval in it. My thinking is that the gasses will escape the muzzle a fraction before the patched ball on the left and force the ball over to the right as it leaves the bore.

This would have to be done in VERY small steps and a constant monitor of each adjustment, or we could end up with a right shooting cannon !!!!!

I have been told it's because we live in the northern hemisphere, the Southern hemisphere would shoot to the right, BUT I don't believe that for a second!!

Any thoughts?

 

[Griz44Mag]

Hi,

I have been told it's because we live in the northern hemisphere, the Southern hemisphere would shoot to the right, BUT I don't believe that for a second!!

Any thoughts?

Hemisphere is not a factor.
POI is influenced by earth rotation, however, it is dependant on the direction you are shooting.
It will not be noticeable over short distances, say 500 yards or less.
For longer ranges, use a good ballistic program and let it do the math for you. (It's a very complex array of factors)
Depending on your latitude (how far from the equator,) the earth is rotating at a fixed speed. The closer to the equator, the faster it spins(approximately 1,000 mph)
In Texas where I live the ground speed is approximately 940 mph.
If you face rotational north, your target is moving to your right, if you face South, to your left.
If you face East, the ground is moving away from you and due to the curvature of the earth, your target is dropping.
If you face West, the ground is moving toward you and due to the curvature of the earth, your target is rising.
If you are facing any other direction then the target is moving up\down\right\left - depending on the exact angle you are shooting.
If you are shooting artillery or cannon at distance - then flight time of the projectile and direction of flight will determine the offset of aiming point to account for spin.
You MUST know the projectile weight, drag coefficient, velocity, flight time, direction of travel, air density, temperature, wind speed and direction.
If you really want to see the true effects, download the Phone\tablet software BallisticArc. It will gather the appropriate data for you and do the math.

 

[Sunbeam]

Thank you Griz44mag
Sounds very complicated and I'm sure earth rotation is not my issue

I've had a very hard long look at my crown under a very powerful magnifying glass and I was SURE I could see more bevel on the 9 o'clock position than 3! So I have addressed this issue and now the crown looks totally even. I will know within the first 5 shot group if the P.O.I is getting better or even fixed. If it's improved, but not fully fixed, I know I'm on the right path and will address the crown a little more. Fingers crossed.

I'll report back in a few months when this Covid thing passes and we can go back out and play!

P.S, it's the cannon in my avatar

 

[ppg1949]

Is your canon mounted square on the caisson? At 25m it doesn’t take much to move your POI.

 

[Sunbeam]

Is your canon mounted square on the caisson? At 25m it doesn’t take much to move your POI.

So even if the barrel is lined up correctly, but the gun is not totally square in the carriage, the P.O.I will be off?

 

[Zonie]

Is the vent hole "exactly" pointed straight up when the gun is sitting level on a surface?
If the vent is pointed to the left, it will result in the barrel being aimed to the left as well.

Then again, it would take quite a bit of tilt of the vent hole from verticle for it to cause the cannon to hit 6" to the left at a distance of only 25 yards.

If the vent hole was 11" from the muzzle, the hole would have to be .073" to the left of vertical to cause it to shoot left 6 inches. That's just a bit over 1/16" off center.

If the vent is off by that distance, about the only way of fixing it would be to remount the barrel with the vent hole straight up.
If that was not possible, drilling out the vent and installing a threaded plug would allow you to drill the new vent hole in the right place. Of course, that would put a somewhat unsightly threaded plug right on top of the barrel for all the world to see.

 

[ppg1949]

So even if the barrel is lined up correctly, but the gun is not totally square in the carriage, the P.O.I will be off?

Is the barrel sitting squarely on the caisson. In other words, not tilted off center by where the barrel trunnions are resting on the carriage.

 

[Artificer]

However, every time we carry out this test, the group is ALWAYS 6" to the left, even when we shoot blind (without checking shot to shot). So we have concluded the cannon mush be shooting 6" to the left and it's not bad aiming.

Is there any way to pull the shots to the right 6" so we are not forced to sight up on the very edge of the black scoring ring as we are finding this type of sighting quite difficult.

Any thoughts?

Adjusting and filing front sights was a BIG part of my job as a NM Armorer, though in this case I have to add my experience as an Ordnance Officer for an Artillery Battalion.

OK, since the groups are impacting to the Left, you have to move the front sight (or in this case the muzzle) slightly to the Left to center the groups on target.

Now, it is difficult to see in the picture in your avatar, so I have to ask is the barrel held in place by its trunnions as is normal in a cannon? If so, can you put a spacer/shim between the RIGHT trunnion and the barrel? This would move the barrel sort of catty cornered to the LEFT, which is what you need to do.

Gus

 

[Sunbeam]

Humm??

You talk about shims, I was told that the barrel should be under some tension and not allowed to tip loose. So I made the brass adjustable pressure pads to adjust the amount the trunnion keeps can clamp the barrel. I thought it was a great idea and they work well at holding the barrel. Maybe they are giving me the problem.

Her are some pics of the pressure pads, shot target at 25m and the cannon overall.

The low shot out of the group was the first shot and then all the rest were shot without touching the elevation adjustment at all (it locks). Every shot looked perfect looking over the top of the cannon. As you can see, we were not too far off as the group is fairly tight, but way over to the left! To get the group in the 9 and 10 ring, you have to line the muzzle up at 3 o'clock in the 6 ring. Yes, the group could do with being a little lower, but it was an experiment to see how good the group could be.

https://ibb.co/yPyppLP
https://ibb.co/C7wx65c
https://ibb.co/fn88jx5
https://ibb.co/7g5mRWx
https://ibb.co/MZj3QSc

The rope is for looks and is not wrapped around the barrel when shot.

 

[Zonie]

Adjusting and filing front sights was a BIG part of my job as a NM Armorer, though in this case I have to add my experience as an Ordnance Officer for an Artillery Battalion.

OK, since the groups are impacting to the Left, you have to move the front sight (or in this case the muzzle) slightly to the Left to center the groups on target.

Now, it is difficult to see in the picture in your avatar, so I have to ask is the barrel held in place by its trunnions as is normal in a cannon? If so, can you put a spacer/shim between the RIGHT trunnion and the barrel? This would move the barrel sort of catty cornered to the LEFT, which is what you need to do.

Gus

I'm confused I guess.
I was always taught that the rear sight should be moved in the direction that you want the shot to move on the target.
Since the cannon is shooting to the left, I think the rear sight should be moved to the right to move the impact in that direction.

Course, I've been wrong before.

 

[Sunbeam]

There are no sights on this cannon as per the rules of our comp.
You sight it using the very top of the muzzle as it 'sunsets' over the rear of the barrel.
Then adjust the whole cannon so the very tip of the sunset is directly under the centre line of the target.

You can 'cheat' a little and leave the very tip of the fuse showing for group testing, (or at least we do )

 

[ppg1949]

Here is more useless advise from the far side. With your cannon a level surface, put a small level across your trunnions. If it is level it is not your shims.

 

[Sunbeam]

Here is more useless advise from the far side. With your cannon a level surface, put a small level across your trunnions. If it is level it is not your shims.

That's a great tip, thank you

 

[Zonie]

There are no sights on this cannon as per the rules of our comp.
...

You can 'cheat' a little and leave the very tip of the fuse showing for group testing, (or at least we do )

When I made my comment about the vent hole, it was with the idea that you were doing exactly this. Using the fuse as a rear sight.

If the vent hole is too far to the left and you are using a fuse stuck in it as a rear sight, the gun will end up shooting to the left.

 

[Sunbeam]

No, not normally using the fuse as a sight.

 

[Col. Batguano]

That was a great piece describing direction and Earth rotation affecting POI. I wonder if that goes in to very long range artillery sighting calculations, like battleships that shoot at things from 10 miles away? Then you have to add in the direction and speed your ship is moving too.

 

[Nyckname]

I wonder if that goes in to very long range artillery sighting calculations, like battleships that shoot at things from 10 miles away?

Yes, it does. In fact snipers working at extreme ranges even need to take it into account.

There are ballistics calculator apps available for your smart phone.

 

[Artificer]

I'm confused I guess.
I was always taught that the rear sight should be moved in the direction that you want the shot to move on the target.
Since the cannon is shooting to the left, I think the rear sight should be moved to the right to move the impact in that direction.

Course, I've been wrong before.

Zonie,

I'm not talking about a rear sight, as there is no rear sight on the OP's barrel.

Howevever when you move a rear sight, you are correct about how to move the sight as you mentioned.

My intention of a shim or shims in the trunnions is to cause the muzzle (and actually the whole gun) to be angled slightly off center from side to side. This may require angled shims, btw. That way when you use the front sight to sight normally, the gun will actually be firing at more of an angle to compensate for the windage or where the group hits side to side.

Gus

 

[John May]

I shoot an Ardesa 'Old Ironsides' in .69 cal mostly in a UK National Postal League. I've had the cannon for about 3 years and during that time I have made a lot of modifications A) to make it more competitive and B) to make it less prone to recoil as I use 60 grains of Swiss 1 powder for a charge.
I have found that to get it to group consistently in the centre of the target I have to aim off to the right? Like Sunbeam I have checked, replaced and adjusted almost every variable on the carriage, I have honed the barrel and laser-bore aligned it and still it needs a right bias to be successful. I fully agree with the comments that mm's of horizontal movement can cause inches of variation at 25 metres but interestingly, after some testing, the POA is virtually identical at 50 metres with only a slight difference in the horizontal adjustment, again with a bias towards the right side?? Just out of interest my personal best at 25 metres is 91 with the top Division 1 shots averaging 93. All this without sights!

 

[Sunbeam]

Hello John,

I'm in the same posal League as you, unsighted, but I'm in Div 3, my daughter is in Div4.
We have tweaked the cannon after my daughters first two scores of 74 and 72.
Both our scores went straight to 91 a piece
We have to aim at the 6 ring at 3 o'clock to hit the 10.