True reason why double rifles require the barrels set to "cross" to shoot parallel.

[Ironoxide]

Every person interested in double rifles soon learns the concept of "regulation". Regulation usually means a specific load and projectile that shoots parallel. Increase the projectile weight or powder load and the barrels will shoot wide of point of aim, decrease and they will shoot more together even to the point of crossing (right barrel shooting left, left barrel shooting right). This is true for modern cartridge as well as muzzleloading double rifles.

The following image describes the concept better than any words.


The accepted explanation for this fenomenon is that recoil doesn't act in line with the centre of the stock, but acting to the right or to the left of centerline moves the gun accordingly as shown on the picture above.

The earliest record I found that mentions that "recoil" principle is a 19th century book by a very well known English gunsmith "The gun and its development" by Greener. It is also repeated in many modern books on the subject as well as online resources.

However, there is another 19th century book by James Forsyth titled "The sporting rifle and its projectiles". In that book he mentions how doubles have to be constructed, but on the reason why he sends the reader to the "Field" magazine. No doubt he didn't want to be ridiculed by questioning the "accepted science" in his book, but at the se time he read the article so he knew the real principle why the double shoots wide unless regulated near.

I'll also add the author of the article (and the experiment) is Horatio Philips. He is a person responsible for conducting many of 19th century "Field" trials experiments and he is responsible for collating the data into tables etc. He is a person that I trust is telling us truth in what he has discovered.

So with no more delay I attach the article below. It starts a third way down the page in left column. It continues in the right column and finishes on next page in left column. I'm sorry for the text quality, but it is a reprint of a 130 year old article after all.

For those that didn't fully understand the text (due to bad quality / language etc) I summarise below.

They attached a set of parallel barrels (with ribs soldered in usual way) in way that prevents them moving to a large block of wood. They backed the block of wood to prevent recoil. They shot this contraption and it was determined it shoots 20 inches wide at 100m. No recoil movement possible, and yet it does shoot wide.

Then they took a "cross eye stock". It is for people that are right hand and left eye dominant and looks like this:

They installed a set of double barrels in it expecting both will shoot to the left if it is indeed recoil not acting in line that moves the poi. Both barrels shot normally.

Finally they explain an experiment with hydrostatic pressure with witch they simulated pressure of powder combustion and noticed the barrel steel expanding slightly in diameter, at the same time the length of the barrel "being shot" got slightly shorter bending the whole assembly to the side! And this is truly why double rifles shoot as they shoot.

 

[Sam squanch]

Fascinating!

 

[Throwdown58]

It is because the solder between them effects the harmonics and the bullet will favor the other side. In practice it is almost impossible to get a double rifle to shoot to the same point of aim. You would have to experiment with soldering and resoldering, which I don't think anyone would waste time on. And even then, the harmonics are effected by where the solder attaches. It's not a constant that you can mathematically duplicate. Clamping the barrels for a test like that isn't going to duplicate an actual rifle. It is just an intellectual experiment. Barrel harmonics were not thought about back then from what I've seen. The science started to come out in the 70s.

 

[Ironoxide]

Throwdown58 I agree in a sense. It is surprising how much knowledge about weapons people had in the later half of 19th century. The problem is that this knowledge was scattered through various books, magazines, newspapers etc. There was also a lot of false myths people believed in too unfortunately. 90% was forgotten then rediscovered and given new names. I don't know when barrel harmonics was defined as a term and studied systematically , but I have no doubt people back then knew barrels bend in various elastic ways during firing. One could say their experiment and your explanation are both different descriptions of the same process.

There is however one thing I don't fully agree with. One is obviously the "time wasted" on resoldering look at my other thread and you'll see I'm on the 4th resoldering of my set of double barrels now

Second is the ability to replicate. Today there is pretty good knowledge of soldering. One can imagine design of a repeatable process that reliably joins two barrels on full length is possible (using silver solder for example like Pedersoli). Then if one knows the correct principle and all the variables like pressure curve and time the projectile spends in the barrel it is possible to simulate stretching of steel using numeric methods (FEM is used to simulate loads in architecture, including vibration analysis in machinery etc). Then manual tweaking would probably be required to just get that final bit of accuracy that is lost due to tolerances in manufacturing.

The reason why it is not done is that the market for double rifles is pretty small today. The investment in writing the right design software would run close to a million $ probably(I'm a software developer by trade) . When you have customers willing to pay $50k per rifle, but you sell only few per year it makes sense to spend some time soldering /desoldering rather than investing in software.

 

[andy52]

It's the same principle that was used on WWII fighter plane wing-mounted guns. The guns were generally adjusted to have a point of convergence of about 1000 yds.

 

[Throwdown58]

I have had an entire day long discussion with the head gunsmith at the US Army Marksmanship unit, and to this day there is little actual knowledge of barrel harmonics and how to control them. After generations of study, to this day they shoot ammo with barrels, and put them away together, because exact best performance is completely elusive as a formula. It would take a lot more than 4 resolders, and most likely, because you could not place the solder exact every time, it would be impossible. Of course if the barrels were way off you would resolder, but otherwise it's an approximation to get both barrels to shoot close to the same point of aim at a given distance, and it would bet treated, as andy52 said, as a point of convergence. An algorithmic approach would itself only be an approximation.

 

[user 48679]

Cross firing might be the reason that some made O/U config guns, likely needing much less fiddling to get the barrels to shoot together.

 

[Ironoxide]

I have had an entire day long discussion with the head gunsmith at the US Army Marksmanship unit, and to this day there is little actual knowledge of barrel harmonics and how to control them. After generations of study, to this day they shoot ammo with barrels, and put them away together, because exact best performance is completely elusive as a formula. It would take a lot more than 4 resolders, and most likely, because you could not place the solder exact every time, it would be impossible. Of course if the barrels were way off you would resolder, but otherwise it's an approximation to get both barrels to shoot close to the same point of aim at a given distance, and it would bet treated, as andy52 said, as a point of convergence. An algorithmic approach would itself only be an approximation.

From what I read and heard of barell harmonics it's effects are quite subtle.(few MOA at most of difference). For some uses those few MOA are a difference between a good rifle and a completely unusable one. If you're coming from a point of view of sub-MOA shooting precision I agree with you 100%.

However, in double barrel rifles 2~3 MOA of accuracy is considered good.

The "double barrel" rifle principle we're discussing here is orders of magnitude stronger. For example my 58 cal double rifle barrels are now set to cross at 5m and yet they shoot close to parallel with the right charge. This is exactly 17MOA change of point of impact per barrel. I think that very predictable shift of poi is caused by the reasons described in the first email. Additional barrel harmonics for sure affects the last MOA of accuracy.

It is possible to get a double barrel rifle to shoot close to parallel in accordance with accuracy expectations I stated in 4 resolders. I know it because I did it myself. Of course it is not true "shooting parallel to infinity" it is a very slight convergence or divergence, but at distances I'm interested in (100m) it is parallel enough for me

The whole reason why this topic is interesting to me is that we have barrels set to cross by a huge amount and yet they can shoot parallel with the right load. Every source repeats the old "recoil causes it" story which to say frankly always sounded far fetched to me. Now we have a record of an experiment that proves it is not recoil. I hope anyone interested in why this happens will be able to find this thread instead of having to look for 130 year old reprints.

It's the same principle that was used on WWII fighter plane wing-mounted guns. The guns were generally adjusted to have a point of convergence of about 1000 yds.

Andy52, were they actual double barreled guns with barrels set to converge at 1000 yards and therefore shooting parallel or one gun per wing both set to a common point of aim 1000 yards away? If the latter it is a completely different ball game! If the former I'm very interested to hear more(which fighter, which gun etc)

 

[andy52]

From what I read and heard of barell harmonics it's effects are quite subtle.(few MOA at most of difference). For some uses those few MOA are a difference between a good rifle and a completely unusable one. If you're coming from a point of view of sub-MOA shooting precision I agree with you 100%.

However, in double barrel rifles 2~3 MOA of accuracy is considered good.

The "double barrel" rifle principle we're discussing here is orders of magnitude stronger. For example my 58 cal double rifle barrels are now set to cross at 5m and yet they shoot close to parallel with the right charge. This is exactly 17MOA change of point of impact per barrel. I think that very predictable shift of poi is caused by the reasons described in the first email. Additional barrel harmonics for sure affects the last MOA of accuracy.

It is possible to get a double barrel rifle to shoot close to parallel in accordance with accuracy expectations I stated in 4 resolders. I know it because I did it myself. Of course it is not true "shooting parallel to infinity" it is a very slight convergence or divergence, but at distances I'm interested in (100m) it is parallel enough for me

The whole reason why this topic is interesting to me is that we have barrels set to cross by a huge amount and yet they can shoot parallel with the right load. Every source repeats the old "recoil causes it" story which to say frankly always sounded far fetched to me. Now we have a record of an experiment that proves it is not recoil. I hope anyone interested in why this happens will be able to find this thread instead of having to look for 130 year old reprints.



Andy52, were they actual double barreled guns with barrels set to converge at 1000 yards and therefore shooting parallel or one gun per wing both set to a common point of aim 1000 yards away? If the latter it is a completely different ball game! If the former I'm very interested to hear more(which fighter, which gun etc)

The planes had different gun mountings that ranged from two to four guns per wing. They were sighted in by placing the plane on a level surface and firing them at a target on the ground and adjusting them to all come together at a specific distance (point of convergence) .
Most US guns were of the . 50 BMG type, some Axis and English planes were in the .30 Cal. range.
I'm not sure of your thinking on a "Double Barrel" gun the same principle works for all multi- barrel guns that have to come together someplace down the line, 8 guns on a P-47, 6 guns on a P-51.

 

[Ironoxide]

The planes had different gun mountings that ranged from two to four guns per wing. They were sighted in by placing the plane on a level surface and firing them at a target on the ground and adjusting them to all come together at a specific distance (point of convergence) .
Most US guns were of the . 50 BMG type, some Axis and English planes were in the .30 Cal. range.
I'm not sure of your thinking on a "Double Barrel" gun the same principle works for all multi- barrel guns that have to come together someplace down the line, 8 guns on a P-47, 6 guns on a P-51.

Thanks for clarification. There has been some misunderstanding. We're taking about double rifles as those made by Merkel, Heym, Holland & Holland. Those rifles have their barrels literally soldered together (or milled from one piece of steel). Here is an example picture.

Also as this is a muzzleloading forum my interest is mostly in muzzleloader double rifles. (side by side shotguns share a lot of handling characteristics) but that's BTW.

Unfortunately those aircraft guns have absolutely zero in common with double barrel rifles. Those planes all used multiples of single barrel guns like M2 browning.

There exists a version of M2 called M2HB that looks like it has "double barrels". It doesn't. It is actually a so called twin gun. There are two M2s side by side.

Finally the most important difference. Those aircraft guns are trained on a common target a certain distance away and they shoot to that converging point of aim. Double rifles are mechanicly set for the barrels to point to a common point not far from muzzle, but they shoot parallel with correct load. That is the whole point of the thread. To present little known findings regarding why those guns shoot not where they point at.

It is a pretty fascinating subject and there are a lot of myths surrounding it. Most people think double rifles shoot to a common point of aim. Even some manufacturers perpetuate this myth by stating so called "regulation distance" and not explaining what it means. There is nothing stopping someone from making a double that shoots converging paths. It is much easier than making one that shoots parallel. The reason why quality doubles are so expensive is exactly because they can shoot parallel with the correct load.

When you buy a modern double from a manufacturer like Holland & Holland you also buy from them a lifetime supply of ammo. The exact lot they used to regulate the rifle to shoot parallel.

The "regulation distance" value came about from the fact English makers used a target set 60m away to regulate the gun(not for single point of impact, but two points of impact exactly the muzzles width apart). While German makers used 75 or 100m. Also If you feed your double a different lot of the same brand or a different brand of ammo it will likely no longer shoot parallel, but a converging/diverging way. This is where "double guns shoot to converge" myth comes from.

Perhaps I should have included that explanation in the first post.

 

[Gordoncourtney]

My my my thats a lot of brain damaging stuff and what an effort Double rifle is a kind of a disease once you own one you have to have another and another. I almost bought another at Holts uk auction the other week of course you cannot hunt with them in uk , no wild country or animals. But I love them I have a 450-400 , a 577-500 no2. A .36 ML double a cape rifle and a flint double

To me the .36 double ML by midland gun co is my .375 BP Magnum beautiful little rifle if only I could hunt with it

The Germans get around regulations. With wedges and screws , very un British, but great doubles, even pinching the Greener crossbolt I gather
My flip sights on the 450-400 go to 300 yards, it’s taken cape buff so I understand Of course for understanding regulation go to Graeme Wright’s book “ shooting the British double rifle

Great bit of work most interesting
Thanks

 

[11Bravo]

I find this thread very interesting, I have always wanted one but a little out of my league. I'll pose a silly question in theory, what if both barrels fired at once would the movement be cancelled out? I wouldn't want to be holding it when it off though! I also realize that is not the propose of a double rifle.

 

[Gordoncourtney]

That's the problem of getting old, money in the bank,but not many years to enjoy it. I bought the double 450-400 for my 70 the birthday and surviving cancer. Still got cancer now 78 I think if you fired both barrels it will just go Bang plus 8,000 lbs of energy great fun Ha ha

 

[Britsmoothy]

Barrel harmonics only really apply to single barrelled firearms and a free floating or partially free floating barrels.
The moment a barrel is fixed to another barrel it's moment of movement is completely changed.
Akin to taking three flats of steel and welding them together to form a beam. The moment of movement per flat has now completely changed, quite possibly to zero deflection!
In this case, the percussion double rifle far more influence is derived via a; the loose (relativly) wedge and b; the hooked breach. And let's not forget the flimsy (relatively) forestock!
The above variables and recoil impulses of a sxs will have more bearing on regulation requirements than ever barrel harmonics will.

Me, I would make them barrels as snug as possible in the stock and leave them there forever once I got it acceptable.

 

[Ironoxide]

My my my thats a lot of brain damaging stuff and what an effort Double rifle is a kind of a disease once you own one you have to have another and another. I almost bought another at Holts uk auction the other week of course you cannot hunt with them in uk , no wild country or animals. But I love them I have a 450-400 , a 577-500 no2. A .36 ML double a cape rifle and a flint double

To me the .36 double ML by midland gun co is my .375 BP Magnum beautiful little rifle if only I could hunt with it

The Germans get around regulations. With wedges and screws , very un British, but great doubles, even pinching the Greener crossbolt I gather
My flip sights on the 450-400 go to 300 yards, it’s taken cape buff so I understand Of course for understanding regulation go to Graeme Wright’s book “ shooting the British double rifle

Great bit of work most interesting
Thanks

Wow, I'm most interested in that .36 cal double ML. I would love to find one like that. My most accurate shooting ML is a .38(single) . I really like the fact it takes little powder and lead to shoot a lot!

I find this thread very interesting, I have always wanted one but a little out of my league. I'll pose a silly question in theory, what if both barrels fired at once would the movement be cancelled out? I wouldn't want to be holding it when it off though! I also realize that is not the propose of a double rifle.

In theory it would be impossible to time both shots to go off at exactly the same time. In practice when you do need both shots at one time you're probably shooting the gun at point blank range and you don't care the poi will shift a foot in a 100m.

Barrel harmonics only really apply to single barrelled firearms and a free floating or partially free floating barrels.
The moment a barrel is fixed to another barrel it's moment of movement is completely changed.
Akin to taking three flats of steel and welding them together to form a beam. The moment of movement per flat has now completely changed, quite possibly to zero deflection!
In this case, the percussion double rifle far more influence is derived via a; the loose (relativly) wedge and b; the hooked breach. And let's not forget the flimsy (relatively) forestock!
The above variables and recoil impulses of a sxs will have more bearing on regulation requirements than ever barrel harmonics will.

Me, I would make them barrels as snug as possible in the stock and leave them there forever once I got it acceptable.

Ah,but what to do if you have a second set of barrels practically for (almost) free? Obviously the correct answer is to make a second stock for the second barrels... Perhaps some day I'll try woodworking. For now if feel much more comfortable with metals

 

[Britsmoothy]

Well I guess two sets does change things. I certainly would still desire everything snug, that is if they are not now.
By the way, did you notice any high spots betwixt breach and breach sockets?

 

[LawrenceA]

What you say makes sense and I see the experiment however I cannot read the article (old eyes).
It makes sense that heat and or pressure would effectively stretch the side of the barrel that is thinnest effectively bending it.
But I suspect that barrel time would play a part if the bullet is in the barrel long enough.
Has anyone determined if the bullet has left the barrel by the time the stock has moved any real distance?

 

[Britsmoothy]

What you say makes sense and I see the experiment however I cannot read the article (old eyes).
It makes sense that heat and or pressure would effectively stretch the side of the barrel that is thinnest effectively bending it.
But I suspect that barrel time would play a part if the bullet is in the barrel long enough.
Has anyone determined if the bullet has left the barrel by the time the stock has moved any real distance?

Good points.
You mention " real distance".
The bullet has left before " any real distance" occurs but! There is very slight movement and as the OP has demonstrated by his very subtle adjustments it is a tiny fraction of an inch involved.
Heat would only come in to play during a rapid firing exercise.

 

[LawrenceA]

Good points.
You mention " real distance".
The bullet has left before " any real distance" occurs but! There is very slight movement and as the OP has demonstrated by his very subtle adjustments it is a tiny fraction of an inch involved.
Heat would only come in to play during a rapid firing exercise.

Understand but is the slight movement back? And if so it will push against the shoulder that will pivot which in turn will cause the shoulder to move in an arch but with resistance. Further the barrel is unlikely to be pushing straight back and as such will also start to pivot. If the bullet is fast it may leave the barrel before anything starts pivoting. A slow bullet may be more likley to be affected.
With heat the hotter right barrel may alter the cooler left barrel.
Who knows?
When you are seeking 3 moa, it is entirely academic but I am curious.
Also if you look at pistol shooting. One person can shoot to point A and the next with the same gun and ammo can often shot at 12 oclock or 6 o clock.
I put this down to grip and the effect of recoil in a 6" barrel.

 

[Britsmoothy]

Understand but is the slight movement back? And if so it will push against the shoulder that will pivot which in turn will cause the shoulder to move in an arch but with resistance. Further the barrel is unlikely to be pushing straight back and as such will also start to pivot. If the bullet is fast it may leave the barrel before anything starts pivoting. A slow bullet may be more likley to be affected.
With heat the hotter right barrel may alter the cooler left barrel.
Who knows?
When you are seeking 3 moa, it is entirely academic but I am curious.
Also if you look at pistol shooting. One person can shoot to point A and the next with the same gun and ammo can often shot at 12 oclock or 6 o clock.
I put this down to grip and the effect of recoil in a 6" barrel.

Do bear in mind any heat build up in one barrel is not isolated when soldered to the other barrel. It has a huge heat sink stuck to it! And again, there is inherent rigidity in the construction.