Barrel Length in the Smaller Gauges

[neutron619]

Hello Chaps,

I don't know whether this is a reloading / cartridges question or whether I should have posted it in whichever forum is best suited to shooting technique, but since I'm asking for thoughts probably based around the properties imaginary cartridges, I thought it would probably go in here.

Anyway - here goes:

What are the pros / cons of shorter or longer barrel length in the smaller gauges?

Let me cite a couple of examples or "facts" - feel free to correct these if I've made any incorrect assumptions.

Starting point: as I understand it, provided you haven't got a low-pressure situation and an incomplete powder burn, basically all of the powder in the average cartridge will have been burnt off by the time the shot column is 18-22" down the barrel.

Deviation: I suppose it's conceivable that in very heavy game cartridges with slow, single-base powder - perhaps on a really cold day in the depths of winter - that this distance could be larger, which might suggest why some high bird shooters like really long barrels - 32", 34" or more. They want as much "ooomph" behind the wad as they can get (see below).

Suggestion #1: .410 (and I imagine, to a lesser degree, 28 gauge) loads use particularly slow powders, especially when pushing large shot charges. This might make a longer barrel in smaller gauge guns that much more valuable that it would be, say, in a 12-gauge or 10-gauge.

Suggestion #2: Small-gauge guns tend to be of lighter weight / construction than the larger gauges. Longer, heavier barrels will therefore help the user maintain better technique if they're using a lighter gun than their usual 8lb clay-buster.

Counter-suggestion: There's so little powder in a small-gauge cartridge that it's all consumed quickly, leaving the shot column to travel most of the distance up the barrel on the residual pressure. A shorter barrel will therefore be better / more appropriate in the smaller gauges, avoiding loss of velocity through friction.

I assume that the first two suggestions are nearer the mark and that the pressure in any shotgun barrel after firing will be above atmospheric until the shot column exits the muzzle. This means, in my head anyway, that there will always be some force on the shot column whilst the system is closed (i.e. before it exits the muzzle), even if powder has all been burnt long before that happens.

Assuming any of the above is correct, what I'm trying to get a handle on is:

Let's say I like shooting a 27" 12 gauge SxS. If I switch down to a lighter small-gauge gun, do I

• get one with longer barrels (because it'll add weight / help maintain the swing / be more ballistically efficient)
• go with 27" barrels (because that's just what works for me)
• get the 24" barrel model because the "counter-suggestion" above is actually a significant consideration?

I'm not so much looking for the answer to that specific question, as much as to understand how you guys would weigh up those factors and make your own decision.

Thanks in advance.

[TIGHTCHOKE]

As long as the barrel length allows all of the powder to be burnt and contribute to the production of propulsive gas, it will have little or no effect upon the payload.

 

I use 32" barrels because they help the swing on long clays and I use short barrels if I have to carry the gun all day due to the lighter weight being more comfortable and subsequent lower number of shots too.

 

Barrels used to be 36" in order to allow the black powder to burn properly!

 

If you do go for longer barrels in the smaller gauges you will have too much forward weight.

Edited November 21, 2016 by TIGHTCHOKE

[motty]

I give very little consideration to powder burn rates etc. I buy whatever gun takes my fancy, regardless of barrel length. I chose (in the last year or so) to get a 34" barrelled o/u 12 and a 26" barrelled o/u .410.

[wymberley]

Just shoot with whatever you're comfortable with and have confidence in. With regard to higher velocity/grunt, set a velocity figure which you will think will have a noticeable affect downrange - let's assume a #6 and you want an extra 50 ft/sec - and then see what extra you need at the muzzle to achieve it. Yep, there'll always be pressure until the load leaves the barrel. Hence a ring bulge (if not two) if there's a blockage near the muzzle.

[rbrowning2]

Hello Chaps,

I don't know whether this is a reloading / cartridges question or whether I should have posted it in whichever forum is best suited to shooting technique, but since I'm asking for thoughts probably based around the properties imaginary cartridges, I thought it would probably go in here.

Anyway - here goes:

What are the pros / cons of shorter or longer barrel length in the smaller gauges?

Let me cite a couple of examples or "facts" - feel free to correct these if I've made any incorrect assumptions.

Starting point: as I understand it, provided you haven't got a low-pressure situation and an incomplete powder burn, basically all of the powder in the average cartridge will have been burnt off by the time the shot column is 18-22" down the barrel.

.

I think your starting assumption is all wrong?

 

Maximum pressure is at 1inch from the beach face when the powder is still in the cartridge case so to generate maximum pressure the goal is for virtually all the powder to be burnt then, be it fast or slow powder. Pressure graphs show this to be true.

Any powder not consumed at 1inch will play no part in the ballistics but may show up as unburnt powder down the bore.

That's my understanding.

[ditchman]

i was told or red sometime ago that blackpowder and slower burning powders give better patterns...is this the case...?

[TIGHTCHOKE]

Too much speed is known to blow patterns.

Edited November 21, 2016 by TIGHTCHOKE

[TIGHTCHOKE]

I think your starting assumption is all wrong?

 

Maximum pressure is at 1inch from the beach face when the powder is still in the cartridge case so to generate maximum pressure the goal is for virtually all the powder to be burnt then, be it fast or slow powder. Pressure graphs show this to be true.

Any powder not consumed at 1inch will play no part in the ballistics but may show up as unburnt powder down the bore.

That's my understanding.

 

I would suggest that the powder in a modern cartridge will be burnt in the first 10 inches of barrel.

[rbrowning2]

i was told or red sometime ago that blackpowder and slower burning powders give better patterns...is this the case...?

So why do we have chokes? The job of the powder is to generate a very large volume of gas to push the shot down the barrel I cannot see how the manner in which the gas is generated can impact on the pattern. The final velocity at the muzzle of the shot may have an impact on pattern rather than the way the gas is generated.

[rbrowning2]

I would suggest that the powder in a modern cartridge will be burnt in the first 10 inches of barrel.

Pressure used to be (and still can be) measured at 1inch and 6inches from the breach pressure at 6inches is already falling fast due to boyle's law and hence I think the powder is consumed way before 10inches.

[neutron619]

Just shoot with whatever you're comfortable with and have confidence in. With regard to higher velocity/grunt, set a velocity figure which you will think will have a noticeable affect downrange - let's assume a #6 and you want an extra 50 ft/sec - and then see what extra you need at the muzzle to achieve it. Yep, there'll always be pressure until the load leaves the barrel. Hence a ring bulge (if not two) if there's a blockage near the muzzle.

 

In case it wasn't clear, I'm just looking for the kind of considerations you'd make in choosing barrel length - thank you to TIGHTCHOKE, motty and wymberly for (briefly) touching on those. Regarding the above - I'm not looking for more "oomph" or extra velocity - just to see how people weighed up the various factors I listed and what other factors they might consider important. I want to know the answer to "how do you decide which model to buy?" not "what is the best barrel length?"

 

So far, comfort and confidence appear to be more important by far than any ballistic considerations.

 

I think your starting assumption is all wrong?

 

Maximum pressure is at 1inch from the beach face when the powder is still in the cartridge case so to generate maximum pressure the goal is for virtually all the powder to be burnt then, be it fast or slow powder. Pressure graphs show this to be true.

Any powder not consumed at 1inch will play no part in the ballistics but may show up as unburnt powder down the bore.

That's my understanding.

 

Respectfully, if all the powder burnt in the first inch, it would either have to be a tiny powder charge or you'd have an exploding gun. The reason the pressure is highest in the first inch is because the volume in which the gas is contained is so small.

 

There are two ways of looking at it which suggest it's more progressive. One is to say that if all the powder burns in 1" of travel, then at 2", it's half of what it was at 1". At 4" it's half again. At 8" it's half of that, and so on. If all the powder is burnt in 1" travel, that's an awful lot of wasted opportunity where, if the pressure was kept higher for longer, the muzzle velocity could be increased. I can't believe - given the UK market and it's love of higher and higher velocities - the manufacturers don't take advantage of that possibility.

 

The other approach is that we could calculate the approximate pressure inside the chamber with the shot column advanced 1" using the total mass of gas produced in complete combustion, perhaps with pure nitrocellulose substituted for the more complex powders we use today. I suspect we would find that the pressure for that mass of gas contained within that space far exceeded the maximum proof pressure of the average shotgun, by some margin. If true, this would suggest that the powder burns more slowly, only completing combustion later, which should keep the pressures within safe limits. I don't propose to work through those figures now, but I'll write them up later if you're not already convinced.

 

So why do we have chokes? The job of the powder is to generate a very large volume of gas to push the shot down the barrel I cannot see how the manner in which the gas is generated can impact on the pattern. The final velocity at the muzzle of the shot may have an impact on pattern rather than the way the gas is generated.

 

Well this goes back to the thread the other week, where some folk sent me PM's asking for more information and some argued - as usual - that physics and mathematics have no place in shooting.

 

Powder choice can conceivably have an effect on patterns in several ways. First, faster accelleration of the shot column increases the chance of cold-welded pellets, pellets damaged by the wad accellerating into the back of them and, as TIGHTCHOKE says above, by giving so much velocity that you end up with "blown" patterns - because they've smashed so hard into the walls of the choke that the pellets become damaged and adopt essentially random flight paths. The observation that black powder produces the best patterns is probably true - it produces the gentlest acceleration of the shot column over a much longer distance of travel, which should reduce pellet deformation by the wad / pellet-pellet collisions from behind.

 

Choke is of course, the significant factor in performance, but there are plenty of other variables too, I'd argue.

Edited November 21, 2016 by neutron619

[Smokersmith]

I think you're right on the 'comfort and confidence' ....

 

I was very tempted by a 32" 28g the other week ... the longer barrels certainly felt good and 'pointable' ....

 

If you love the gun, love how it fits, feels and swings, then you're more likely to shoot well with it.

 

The only thing that needs to be in your head when you pull the trigger is staring at the target.

[bartcjf]

I've been wondering about this as well, as I see it one of the main issues with the .410 is the amount of deformation caused to the shot as it travels up the barrels, so would a 26inch barrel .410 throw better patterns than a 32inch barrel if all the other factors (chokes, cartridges, etc) were the same as the shot is in the barrel for a shorter time?

or would I be better using a higher antimony level harder shot (or copper coated) to reduce the deformation in the first place?

Or is all of this over thinking a small problem.

[TIGHTCHOKE]

I've been wondering about this as well, as I see it one of the main issues with the .410 is the amount of deformation caused to the shot as it travels up the barrels, so would a 26inch barrel .410 throw better patterns than a 32inch barrel if all the other factors (chokes, cartridges, etc) were the same as the shot is in the barrel for a shorter time?

or would I be better using a higher antimony level harder shot (or copper coated) to reduce the deformation in the first place?

Or is all of this over thinking a small problem.

 

You would initially be better with a good plaswad.

[figgy]

Too much speed does blow patterns as does to much choke, don't worry about powder burning or what length for what gauge as it will make little or no difference to speed or patterns. Pick a gun up swing it around as if your shooting if it feels right thats it. If it feels to front heavy and it has long barrels look for a shorter barreled gun simples.

[neutron619]

Well - I couldn't resist. Here's the maths if we assume that powder is pure nitrocellulose. It isn't, but that's because it has stabilizers in it (which slow combustion) and nitroglycerine (which vastly increases the rate of combustion / amount of gas produced) but on that basis, I'm assuming they cancel each other out.

Trinitrocellulose (C6H7(NO2)3O5) has a molecular mass of c. 221g / mole which is approximately equal to 3410 grains / mole.

The exact mode of combustion will vary according to the conditions and the whim of the gods, but we might end up with a reaction that looks something like this (the presence of CO in the resulting mixture accounted for by the chamber / cartridge having effectively no access to atmospheric oxygen):

2(C6H7(NO2)3O5) --> 3N2 + 7H2O + 3CO2 + 9CO

This means that the reaction produces 22 moles of gas per 2 moles of nitrocellulose, or 11 moles per mole.

A powder charge might be 20 grains of nitrocellulose, or thereabouts.

20 grains / 3410 grains per mole = 0.005865 moles of nitrocellulose.

Multiplied by 11, this results in 0.06452 moles of gas produced per 20 grain powder charge.

1 mole of an ideal gas occupies 22.4 liters at standard temperature and pressure (STP).

0.06452 moles of gas at STP therefore occupies 1.445 liters (= 0.001445 m³) of volume at STP.

The volume of the cylinder created when the wad has advanced 1" down the barrel (assumed 12 gauge) is as follows:

1" = 0.00254m
0.729" diameter = 0.0092583m radius

Volume of Cylinder = PI * radius² * height = 6.83984×10-7m³

The ideal gas law tells us that if the volume of gas changes, the pressure will also change.

pV = NRT

Since we don't have a number for the temperature of combustion of nitrocellulose, we'll assume 20 degrees centigrade, even though this is patently wrong.

If NRT is constant, pV must also be constant.

At STP, p = 1 atmosphere and the volume of our combustion gases should be = 0.001445 m³.

Therefore:

pv = 0.001445

if volume is reduced, pressure will rise. The original pv, divided by the new volume will give us the new pressure:

p = pv[0] / v

0.001445 / 6.83984×10-7 = 2112.6 times atmospheric pressure = 31046 psi or 2140 bar.

This is clearly well beyond the maximum proof pressure for any shotgun.

Therefore powder must burn more slowly than to complete combustion within 1" of shot column travel.

Note that in reality, the temperature of combustion will be 500 degrees or more - which would increase these pressures by approximately a factor of 3.

Edited November 21, 2016 by neutron619

[figgy]

Whats the name of that french resident member who catches moles, think he can make a wedge with all these moles

[TIGHTCHOKE]

Whats the name of that french resident member who catches moles, think he can make a wedge with all these moles

 

He'd have to move back over here though!

[rbrowning2]

This is getting technical, with lots of interesting ideas.

 

A pressure graph I have shows the whole cycle zero to max back to almost zero pressure is all over in 2.5 milliseconds.

 

With peak pressure at 0.5 milliseconds

 

My simple crude maths Says 30inch barrel then 0.5(2.5/30) = 6 inches job done.

 

Pressure has dropped from 600bar in 0.5 milliseconds to 200bar (air gun pressure!) by 0.8 milliseconds and 100bar by 1 millisecond.

 

Cannot picture how it would be possible for the powder to still be burning past say 6 inches given the speed of such a chemical reaction, but open to ideas.

 

The initial spike in pressure is due to the primer. Also shows why we can reduce the wall thickness of the barrel along its length as the pressure drops.

Edited November 21, 2016 by rbrowning2

[neutron619]

This is getting technical, with lots of interesting ideas.

 

A pressure graph I have shows the whole cycle zero to max back to almost zero pressure is all over in 2.5 milliseconds.

 

With peak pressure at 0.5 milliseconds

 

My simple crude maths Says 30inch barrel then 0.5(2.5/30) = 6 inches job done.

 

Pressure has dropped from 600bar in 0.5 milliseconds to 200bar (air gun pressure!) by 0.8 milliseconds and 100bar by 1 millisecond.

 

Cannot picture how it would be possible for the powder to still be burning past say 6 inches given the speed of such a chemical reaction, but open to ideas.

 

The initial spike in pressure is due to the primer. Also shows why we can reduce the wall thickness of the barrel along its length as the pressure drops.

 

The effect that you're not able to account for with the numbers above is that the chamber / barrel pressure doesn't directly correlate with the acceleration or velocity of the shot column.

 

In fact, the acceleration and velocity of the shot column will be a complicated integral, and an integral of an integral, of the pressure curve, respectively.

 

The reason you hit peak pressure so fast, (but burning continues for a long time afterwards) is precisely because the shot column hasn't got up to speed. The pressure is high because the shot column has in fact barely moved at the moment peak pressure is hit - because inertia stops it from doing so.

 

An ounce of lead shot (for example) is a huge mass to displace for a relatively tiny quantity of expanding gas, which means that although the force generated is large, the acceleration is initially quite small.

 

The other thing that your numbers above will not take into account is that as the shot column starts to move, the pressure will fall, as will the temperature of the combustion gases due to adiabatic effects.

 

This means that if your powder was initially burning at a rate of (say) 10 grains per millisecond, it's not going to take 2 milliseconds to burn your 20 grain charge, because the rate of reaction will fall with the pressure. As an aside, this is why the powder burn rate charts the various powder manufacturers publish all say "this is not for the development of loads" on it: it's perfectly possible to have two powders that burn identically fast at STP, or chamber pressure or whatever, but then as the projectile moves up the barrel, one of them slows burning at a rate of 50% for a 50% drop in pressure, whilst another slows burning at a rate of 10% for a 50% drop in pressure and blows your barrel off at the end of the chamber.

 

Returning to the question in hand, at peak pressure, the shot column may have moved half an inch - but by the time you're at half peak pressure it might be 6" up the barrel and still accelerating. (In fact, with a powder geometry designed to increase burning rate as pressure falls, it could be even further up the barrel.) When you get to the "residual" pressure, you have to remember that it's still pressure over and above atmospheric pressure, which means even with 1 bar behind it, the shot column will still be accelerating, albeit, not much. (I'm answering my own question here.)

 

The real world example to think about is shooting the gun at night. If you've ever been out wildfowling or roost shooting in the dark, you've probably seen muzzle flash from a shotgun being fired. If you have, the reason you have is that powder is still burning as the shot column left the barrel. The technical stuff would eventually show why pressure and column velocity are only distantly related (and my maths would almost certainly fail before it did) but a flash in the dark is a much more obvious example.

[rbrowning2]

Are you saying all powder is initial burning at time t0 after the primer ignition or are you saying it burns bit by bit as it travels down the barrel, because the reducing pressure curve after peak pressure is smooth I would expect if the powder generates pressure bit by bit as it travels down the barrel then spikes would be observed within the curve. Hope you understand what I am trying to say.

[TIGHTCHOKE]

Modern Double Based Powders burn quickly and smoothly.

 

Many years ago when Express first attempted to get in to the British market their powder was throwing all sorts of spikes and the shells were rejected and reworked.

[neutron619]

I'm saying the powder burns progressively as the shot column travels down the barrel.

 

I would imagine - but I don't know - that it is all ignited pretty much at the same time, which causes the pressure to rise very quickly towards peak pressure. As the pressure builds, the shot column starts to move, slowly at first and then faster and faster. If the shot column didn't move, the pressure would continue to rise, but because it starts to move, the effect of increasing the size of the space in which combustion is occurring causes the pressure to drop. At some point, the rate of burning which drives the pressure increase becomes insignificant compared to the increase in volume which causes the pressure decrease. Thus, even though powder continues to burn, the pressure falls.

 

If the graph is helpful to you, try to think of it as two "component" lines on the graph: one for pressure increase as the powder burns to produce more gas; one for pressure decrease as the volume in which the gas exists expands. The sum of those two lines (the former the positive component, the latter the negative component) is the pressure in the barrel. The graph might look like an X if you drew it that way and the space under the bottom of the "X" looks a lot like the pressure curve for the cartridge overall.

[wymberley]

Being in ignorance as regards to what happens before anything actually leaves the barrel has many advantages - the biggest being bliss.

Edited November 21, 2016 by wymberley

[Old farrier]

Probably best you fire a few different ones theory is fine however practical shooting sometimes tells a different story

 

Big consideration for me on short barrels is

 

Muzzle blast

 

Perhaps not to relevant on a smaller bore gun

 

Anyway enjoy the thread

 

All the best

Of