Barrle Length...

[The Surgeon]

I have been following another thread hear and a question came to mind. This may be irrelevant but it does interest me.

We all know that a longer barrel enables the shooter to squeeze every bit from the powder in the cartridge, right. Is it possible to have the barrel to long that one begins to go backwards? That is, can the barrel be so long that the powder/charge no longer helps the projectile or even begins to hinder the performance?


If this does occur, at what point and how would one determine it on a specific caliber/charge?

In theory there should be no effect, since the gasses are trapped. The pressure is what it is, until released. But, when does the length of the barrel, if at all, become overkill?

Of course there is another theory, inaccurate as it may be, it is still theory.
If the charge is being completely used and has applied it's maximum effect on the projectile, it would be reasonable to think that the longer the barrel is, past its beneficial length, the more resistance the charge/projectile would have to overcome this hindering the performance.
What is reality?

 

[shortgrass]

In theory there should be no effect, since the gasses are trapped. The pressure is what it is, until released. But, when does the length of the barrel, if at all, become overkill?

Of course there is another theory, inaccurate as it may be, it is still theory.
If the charge is being completely used and has applied it's maximum effect on the projectile, it would be reasonable to think that the longer the barrel is, past its beneficial length, the more resistance the charge/projectile would have to overcome this hindering the performance.
What is reality?

After all the propellant has been burned and it has reached its peak in a barrel of optimum length would not a longer barrel (more volume) have a lower pressure at the muzzle? At some point the gas quits expanding because there's no more propellant to burn. It's not like a hydrolic cylinder where you have a pump to maintain pressure throughout the stroke of the piston (bullet).

 

[dmontana]

Dan liljas article on barrel length
Lilja Precision Rifle Barrels - Articles: Barrel Lenghts and Velocities in the 338/378 Weatherby Magnum

 

[TikkaShooters]

After all the propellant has been burned and it has reached its peak in a barrel of optimum length would not a longer barrel (more volume) have a lower pressure at the muzzle? At some point the gas quits expanding because there's no more propellant to burn. It's not like a hydrolic cylinder where you have a pump to maintain pressure throughout the stroke of the piston (bullet).

This has been my thought as well. Unfortunately I've never been able to test it.

I can't wait to see some of the answers.

 

[Mikecr]

Well there is diminished returns pretty quickly. And more than just w/regard to velocity.
But even with extreme barrel lengths(like 100ft), muzzle pressures would continue to drop but velocities would continue to rise. There would still be barrel pressure(~50psi)(not vaccuum).

No point here would velocity actually drop, because the bullet was engraved way back, it lost a little jacket OD and obturates less with lowering pressures, and so it's frictional resistance would remain below barrel pressure forces.
Maybe this would change with a 150ft barrel (so don't go that long)!!

 

[The Surgeon]

Looking at Liljas data it shows the the 250 gr projectile has an optimum barrel length of 42 & 44 inches in regards to velocity, any longer and the projectile begins to drop in velocity.

Now looking at the same data, the 300 gr is still gaining in velocity at these lengths. This is very interesting, because the only change in variables is the weight of the projectile.

I would expect just the opposite in velocities vs the 250 and the 300 gr bullets. But according to the data provided, it is not the case. By that I mean the 300 is still climbing in velocity at 46 inches where as the 250 is falling off. Why?

According to Lilja the increase in performance for the 300 was based on a heavier projectile, slow burn powder, and a longer barrel will equal an increase in overall velocity. Looking at the data on the 320 gr this theory does not apply.

 

[Mikecr]

I don't know what data you're referring to, but I suspect you misunderstood it.
The bullets would not drop off in VELOCITY, but instead would drop off in VELOCITY GAIN.

That is, beyond a certain point velocity gain rate decreases with increments of increased barrel lengths. But velocity IS still going up.
You want to have enough barrel that muzzle pressures are not a problem, and yet there is little gain in going more than needed for good accuracy.

 

[The Surgeon]

I don't know what data you're referring to, but I suspect you misunderstood it.
The bullets would not drop off in VELOCITY, but instead would drop off in VELOCITY GAIN.

That is, beyond a certain point velocity gain rate decreases with increments of increased barrel lengths. But velocity IS still going up.
You want to have enough barrel that muzzle pressures are not a problem, and yet there is little gain in going more than needed for good accuracy.

DMontana posted a link above.
Lilja Precision Rifle Barrels - Articles: Barrel Lenghts and Velocities in the 338/378 Weatherby Magnum



Mike, you are probably correct in that I am not understanding it fully. Some of the information was quite technical. You might understand it better than I. Give the article a look it is rather interesting.

I agree 100% that that the gains above a certain point is basically splitting hairs for example 24 - 30 inches and again from 30 - 46". Even up to that mark it's the gains are small but none the less there are measurable gains.

 

[Mikecr]

Ok, I read it and see you did not misunderstand it Surgeon. Sorry for my jest.
I feel it represents testing in another context (to show 'free velocity') and that no conclusions could be drawn from it regarding theoretical velocity 'loss'.

The velocities are sporatic with temperature affects that degrade the data, and other factors like 12 shooting sessions over about 2 1/2 weeks to affect shot timing, barrel temps and fouling.
He made good efforts to compensate, but there seems more causing the inconsistencies.
I believe this why he suggests "the reader can make his own interpretations".

Wish he had gone larger increments(like 6") to finish the testing in one day.
I am confident this would show velocities climbing more accurately, as I am confident that velocities would be higher still in barrels even longer.

 

[TikkaShooters]

One other "hair splitting" argument about his test was the significant presence of muzzle whip. Any movement of the barrel that would take the bullet of the concentric path would slow it down.

That is why we reloader are always looking for a harmonic node for accuracy. Our barrels are vibrating more concentric than non accurate loads.

I find the test very interesting as well as trying to visualize the perfect ratio of bore volume to expanding gases and bullet weight.

This is what makes this sport fun in my mind. There is always something new to learn.

 

[The Surgeon]

Mike, that was my belief with the test that Lilja performed. The 250 falls off at 46 , the 300 is still gaining velocity, and the 320 drops all around the 46 inch. The data or research he did was indeed inconclusive and I believe it was due to many uncontrollable variables, like barrel harmonics. In a field study like this there are to many variables with no absolute way of measuring them. The only absolute control he had was the charge, weight of the projectile, and the length of the barrel.

As far as barrel harmonics, if it indeed does affect velocity, is a constant variable . With factory or hand loads this variable remains and is it even measurable to the point that it can be distinguished from barrel length in regards to velocity. My guess would be, no. Barrel harmonics can not be removed from the equation, right? So all one could do is minimize it's effects by using a heavy test barrel. I feel it would be safe to say, that at 30 inches, harmonics will have minimal effect on velocity where as the length would be the bigger contributing factor in velocity. Either way, the shooter benifits in lenght/velocity, or at least up to 30 inches. At longer lengths I believe harmonics could be a significant factor but never the less it remains constant. By that I mean that the vibration should in theory increase in a theoretically predictable pattern. The only thing that changes is the length of the barrel.

 

[shortgrass]

Now I'd like to see a test done using the .22 Hornet! Would, no doubt, make for an easier to evaluate test. And for the sake of discussion, I remember from an earlier post, a statement made to the affect that "muzzle pressures would drop but velocity would increase". Just can't go there. What about a 'squib' load, for example, where the bullet never exites the barrel? Also, a reference to "going on a vacuum". I'm not sure that is possible. I know of no breeching system used today in sporting/target/tactical rifle that is 'air tight'. The brass case acts as not only the container for the propellant, primer, and bullet, it also acts as the 'gasket' to seal the breech end. And, it only remains a 'gasket' as long as there is sufficent pressure to expand that brass for a 'gasket' type fit. Even then, we've all seen some 'leakage' at one time or another. So, it is not perfect, but the best we have. There may be just way too many variables for any test to be completely valid, or, maybe I'm just not thinking about it in the right way or just plain not smart enough! I do believe a smaller case using its' smaller propellant charges would be easier to see results from. No bash intended on Dan Liljas' test.

 

[The Surgeon]

Shortgrass makes a valid point. Let me take it a bit further, as far as harmonics is concerned. Would it not be possible to have two barrels of equal specs except for the density? For instance, one barrel be a heavy target and the other a typical hunting barrel with matching specs except for density. At that point one could conclude, in theory, how much harmonics plays a role in velocity.

I feel safe in saying that with all the members posting and in watching, that some conclusive data would have been revealed. At least in the case of the Lilja test the 338 shows measurable gains up to 30+". It would be hard for me to compare this test and it's results to my 308 win, there are just to many differences in the two rifles.

It is possible to conclude that barrel length and velocity could be different for each rifle. I am sure there is data out there that deals with this very question, aside from Lilja. It's just a matter of finding it.

Shortgrass, is leakage always present to some degree?

One thing is for sure, as this thread goes on the more question arise. I am going to make a few calls to some of the barrel makers and see what info they may have, if any.

 

[The Surgeon]

Topic of the Month

Take a look at this.

It is a usually accepted fact that gun barrels of different lengths will produce different muzzle velocities with the same load. Generally, the longer the gun barrel, the higher the velocity...at least up to a point. Only a few decades ago the common length of center fire rifle barrels ranged from 26" to 30". These rifles were heavy and unwieldy to carry around in the field all day. Today, the more common lengths for sporting rifles are in the range of 22" to 24", resulting in a lighter and more portable firearm. With this change many shooters want to know just how much velocity is lost in switching to the shorter barrels or how much velocity gain can be achieved in a longer barrel.

It seems that little useful information can be gained by comparing the muzzle velocities of two different rifles, of the same caliber, but with different barrel lengths. Varying dimensions in the chamber, throat and start of the rifling affect the velocity too much for a meaningful comparison. The more obvious solution is to use a single rifle, cut the barrel off, in given increments, and carefully measure the muzzle velocity at each barrel length. Many experiments were performed using this technique on both rifle and handgun. The problem was that the variation in velocity, from shot to shot, with the same ammunition, was often larger than any effect noted by shortening the barrel in 1" or 2" increments. However, if the data were plotted graphically to smooth out the irregularities due to variations in velocity measurements then a reasonable relationship between velocity and barrel length can be shown.

Various mathematical equations were developed to relate muzzle velocity to barrel length, but one of the simplest relationships was developed by Homer S. Powley. He defined the relationship between muzzle velocity and barrel length, as one giving muzzle velocity (v) as a function of charge weight C, bullet weight B and expansion ratio (R). Here the expansion ratio is defined as the ratio of the barrel volume plus cartridge volume (total volume of the gun) to the cartridge volume. The equation relating to these factors is represented by:

v
= K[C(1- R^-.25)/ (B + C/3)]^.5
Where, v is in f.p.s., C and B are in grains and K is a constant that depends on chamber pressure and other factors in the gun. The expansion ratio is dimensionless. From this equation, it can be seen that for a given gun, with a given powder charge and bullet weight, the muzzle velocity is dependent only on the expansion ratio. By cutting off the barrel the barrel volume is reduced, thereby reducing the expansion ratio. The relationship can be represented by:

F
= [(1-R2^-.25)/(1-R1^-.25)]^.5
Where, F is the correction factor to correct the muzzle velocity at expansion ratio R1 to that of the reduced expansion ratio R2 (shorter barrel). For example, a rifle chambered for the .223 Rem. cartridge, has a 24" barrel, and fires a 50 gr. bullet at 3,080 f.p.s. when loaded with 25.1 gr. of IMR 3031 powder. The expansion ratio is 8.5. What is the muzzle velocity if the barrel is shortened to 22"? The new expansion ratio is 7.8. Plugging these values in the equation we get:

F
= [(1-7.8^-.25)/(1-8.5^-.25)]^.5
= 0.9846
v
= 0.9846 X 3,080
= 3,032 f.p.s.
The new velocity for the 22" barrel is 3,032 f.p.s. The velocity loss for removal of two inches of barrel is 48 f.p.s. (3,080 - 3,032).

The table below lists the velocity change for different barrel lengths for several popular calibers.

Refer to link to see table



Note that the greater velocity increase, per inch of barrel length, generally occurs in the shortest barrel length. As the barrel is lengthened the velocity increase, per inch of barrel length, decreases. For instance, the .223 Remington shows a 46 f.p.s. velocity increase by lengthening the barrel from 20" to 22" This drops to 38 f.p.s. when the barrel is increased from 26" to 28". Some of the magnum calibers show more variation in velocity from barrel segment to barrel segment, but the velocity change, per segment, is generally less as the barrel is lengthened.

The powder charges and powder types shown in the table are for illustration purposes only and are not to be used as load data. For details on loading for a particular cartridge the reader should consult a good loading manual or internal ballistics software program. The software program offered at this web site will quickly calculate muzzle velocity for any barrel length from 18" to 50".

Watch our web site for the next topic of interest. Until then, shoot safely and know where your bullets are going.

Sincerely,
The Ballistician