Technical question

[Guest]

I have asked this question several places and have never really received a good answer. Hope some of you long range shooters can provide the answer and explain why.....
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Let's say you built two long range handguns with barrels from the same blank..... identical in every way except one.....the first handgun has a 14" barrel and the second has an 18" barrel......

Then, let's say you built loads for each barrel identical in every way except one......the load for each barrel produced the same velocity in it's respective barrel......in other words, both pistols shot the same bullet at the same velocity.....

Now the question, assuming they both shot the same bullet at the same velocity.....would the added length of the second barrel have any effect on the trajectory of the bullet?

Would the added distance that the bullet traveled in the barrel of the second handgun cause the bullet to "sleep" sooner? (For example, in a 1 in 7" twist barrel, a bullet would spin two complete revolutions in a 14" barrel and 4 complete revolutions in a 28" barrel).....would the added revolutions inside the longer barrel cause the bullet to sleep sooner?

Logic tells me that, *if* the longer barrel caused the bullet to sleep quicker, then some amount of energy would be conserved and would thus cause the bullet to shoot "flatter" than the bullet that sleeps later.....but is that a valid assumption?
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Thanks, Mark

 

[Michael Eichele]

Mark,

I am not sure if this will answer your question, but the examples you give are of different bbl lengths and twists.

A bullet will go to "sleep" if it is being used in the proper application. In other words, if you shoot a 180 grain 30 cal bullet with a 14 twist, it will never go to sleep. If you shot that same bullet in a 8 twist barrel, the bullet will not sleep as well as it could. A longer barrel, to a point will help increase the ballistic coefficent which will also help the bullet go to sleep better. That last statment isnt always true though. What you need to put a bullet to sleep is a perfectly ballanced set up. What ever bullet you intend to use must have 1st the right twist, 2nd the right barrel length for that bullet and twist. A large number of factors come in to play when trying to put a bullet to sleep perfectly.

To answer your question directly, the longer barreled pistol should help the BC of the bullet and it should stay perfectly stable over a longer range than the shorter barrel. But if the twist is wrong for the bullets used, you are just chasing your tail.

Hope that helps some.

PS as a side note many bullets dont even go to sleep till they reach 300 yards or so, and that doesent meen that it doesent work well from 0-300 yards or that it hurts your BC either.

Most of the time when the perfect ballance is found, the bullet will not be asleep at the muzzle. It needs time to settle down and do its thing. That could be between 200 and 300 yards.

[ 12-31-2003: Message edited by: meichele ]

 

[Guest]

Michael

No, as I stated, both barrels are made from the same blank so they have the same twist......the only difference between the two barrels is length.....and we are assuming that the bullets leave both barrels at exactly the same velocity.....

Thanks for the input so far....I hope this thread not only answers the questions I asked, but I hope it helps me understand what determines how and when a bullet goes to sleep.....

 

[Michael Eichele]

Hi again, I think most of us are in the same boat. Same boat meaning, most of us are still trying to understand how a bullet goes to sleep.

The simple act of using a longer barrel could help the bullet go to sleep better if it helped the bullet in question to reach its optimum BC.

Most of what I write next is only un-proven theroy and mostly opinion, so take it as just that. What I belive is that the best BC's for putting bullets to sleep are .500 and .500*2 or 1.000 Between .500 and 1.000 they wont go to sleep perfectly.

Think of it this way: a bullet must be squared to the rifle firing it. A 308 win hold approx. 50 grains of powder and the bullet weight that would be squared to this would be about 150 grains (50*3) This also gives about 10 times the bullets weight in energy at the point it goes to sleep 150*10=1500 FPE at 300 yards. Belive it or not, the BC for this set up is about .500 when used in a 24-26" barrel. If a 24-26" barrel is not used, the BC is no longer .500 and the load is not "squared"

A .50 cal shoots a 750 grain bullet with about 250 grain of powder 750/250=1/3rd powder to bullet ratio. It also has approx. 10 time the bullet weight in grains in FPE at 300 yards. The BC in this case is 1.000

These are 2 examples of what many consider to be 2 of the most accurate cartridges in the world. The things they have in common are 3's and 5's and 10's (3rd powder to bullet ratio, 10 times bullet weight of bullet energy at 300 yards, .500 BC and 1.000 BC.) These are 2 weapons that are famous for putting bullets to "sleep"

Like I said this may sound absurd. Remember, this is only opinion and un-proven theory.

Does your brain hurt yet???

[ 12-31-2003: Message edited by: meichele ]

 

[chris matthews]

GonHuntin,
I don't by into the "going to sleep" theory. I have built several rifles that shoot long VLD type bullets requiring fast twists into 1/4 moa groups at 100 yards. Most people say these type bullets need 200 or 300 to go to sleep and groups tighten up.
I feel it was target resolution at the longer ranges that got them smaller groups at the longer ranges.
Darryl Cassel says his big boomers shoot one hole groups at 100 also. This indicates no need for going to sleep.
I am not a ballistics expert by any means, but I don't understand how the barrel's length affects the BC. I don't remember all the factors of figuring BC, but I think the main ones are weight, time of flight, sectional density and bullet design/shape.
I have tested 308s with the same twist, 1-11, pushing 175 MK at 2650 fps out of 20, 22 and 24" barrels. Shot the same day within an hour of each other over a Oehler 35. Different glass but all of them took 34 min elevation from a 100 yard zero to get to 1000. All were supersonic and all the rifles shot 6" groups at 1000 and 1/4" at 100.

Starting velocity is the same if it's the same, I don't care what the barrel length is.

Looking at Sierra's ballistic program, they don't ask for barrel length in any of the variable conditions. And they say BC changes with velocity. Higher fps gives a higher BC. Bullets slows down, the BC goes down accordingly.

Just my thoughts on it from my experiences.

 

[Jay Kyle]

ok, I'll take a stab at this. First lets be sure of the parameters:

A. Both barrels from the same blank
B. Same twist rate
C. Different length barrels
D. Same muzzle exit velocity
E. Same (theoretically identical) bullets
F. Fired at the same location and time

In other words all external factors being equal except for barrel length.

Second, let set some definitions.
Let's draw a line straight through the center of the bullet from tip to stern and label this line as D1, then spin the bullet and label the axis of rotation as D2.
The average difference between D1 and D2 we'll call the degree of wobble (W)

Now the solution. When the bullets leave the respective muzzles - with velocities equal - and spin rates the same, as well as no rotational instability, i.e. D1 = D2 or W = 0, then the flight will be identical.

If however due to lack of concentricity, misalignment of the round to the chamber, or some other similar factor, Wobble is introduced to the bullet as it travels down the barrel, then it is plausible that a longer barrel will reduce the degree of wobble and hence the bullet will exit with a lower wobble, than will occur in the shorter barrel.

It follows that the lower the wobble at the muzzle, the sooner the bullet goes to 'sleep'.

The only other factor that would come into play as I see it would be the gas pressure at the muzzle as the bullet leaves, the greater the pressure, the more the bullet is upset by that pressure. Hence the shorter barrel will likely have a higher exit pressure contributing to the wobble.

Is that as clear as mud?

AB

 

[Guest]

Chris

OK, assuming the "sleeping bullet" theory is a myth......do you think the bullet launched from the longer barrel would initally be more stable than the bullet launched from the shorter barrel if both left the barrel at the same velocity? I guess it's really the same question if I understand the theory of sleeping bullets correctly......

Another way to ask would be: with all other factors being the same, does a longer barrel have any effect on bullet stability??

Thanks for the input so far!

[ 12-31-2003: Message edited by: GonHuntin ]

 

[Guest]

AB

Are you assuming that the load for the shorter barrel must run a higher pressure in order to achieve the same velocity as the longer barrel, and therefore would have a higher exit pressure?

Just trying to understand how you arrive at your conclusions....

 

[Jay Kyle]

ah.. brain fart. I cannot assume the shorter barrel will provide a higher pressure due to the parameters. It's really dependent on the powder type and amount used.

A fast burning powder will produce a peak pressure closer to the beginning of the barrel, then the pressure curve drops off rapidly (pressure vs bullet distance travelled down the barrel).

A slower powder will reach peak pressure further out and drops off gradually. Hence given the same barrel length the slower powder will produce a higher exit presure - this is where I tripped on that last comment.

AB

 

[baldeagle713]

I'm gonna show my ignorance here.
Please inlighten me. What is the deffinition of butting a bullet to sleep?

 

[Jay Kyle]

baldeagle713,

As I've heard it put, it's when the wobble in a bullet settles down as it travels, has to do with gyroscopic stability, however I must agree with Chris in that I've never seen the effects when shooting at paper. I think we're splitting hairs here.

AB

 

[4ked Horn]

I would have to agree with the part about splitting hairs. The practical effect of this scenario is zero. The variables are so impossible to control that one would never be able to get the identical departure of bullet from barrel. In other words it can only happen on paper. BUT...

Since we are only playing with the theory of comparing a barrels ability to stabilize or up set the stability of a bullet then we need to bring up the topic of barrel harmonics.

A barrel, like any other pipe , has a resonant frequency. Like a wind chime. The major difference is that a wind chime tube hangs straight down and a gun barrel is largely horizontal and supported at only one end (when free floated).

When a bullet is shoved down the bore by the expanding gas the muzzle is lifted and the bore is straightened. Like when you begin to blow up an empty balloon. This causes an upward whipping motion that travels with the bullet down the muzzle. As the tail of the bullet leaves th muzzle the whip does a remarkable thing. Due to a gyroscopic phenomenon a force applied upward to the bullet will cause the bullets tail to kick sideways!!!

In practical shooting this is fine as long as the whip and kick are the same all the time. This is one major reason to free float a barrel.

To apply this to the origonal question though I would contend that the shorter barrel is stiffer and would whip and kick the bullet less and would thereby allow for a more steady departure of the bullet. A steady departure needs less time to stabilize.

BTW if you do the math you will find that the normal rifle bullet leaves the barrel at roughly (very roughly) 250,000 rpm. A .308 reaches 200 yds at .2 seconds and 300 at .3 seconds. How much quicker do you want it to stabilize? Really.

 

[baldeagle713]

Thank you!
Sleep = Stabilized ok makes sence now.
I would think that the Vapor trail off the bullet could give you a clue.

 

[Darryl Cassel]

Ghunting

You wrote;
Let's say you built two long range handguns with barrels from the same blank..... identical in every way except one.....the first handgun has a 14" barrel and the second has an 18" barrel......

DC---Lets look at something right here--4" length difference in barrels.

You wrote;
Then, let's say you built loads for each barrel identical in every way except one......

DC---The loads are the "same" in "BOTH/each" barrels.

You wrote;
The load for each barrel produced the same velocity in it's respective barrel......
DC-- If the "same" loads are used in BOTH length barrels, the longer barrel "should" produce "more" velocity.
For instance, if I shoot 100 grains of powder with a 200 gr bullet in one of my rifles and from a 30" barrel, the velocity "WILL" be slower then that "exact" same load in a 36" barrel. Same should hold true with your 4" longer barrel if you use the "SAME" load in each.

You wrote
in other words, both pistols shot the same bullet at the same velocity.....
DC---If you shoot the "SAME load in each, the longer barrel should produce "MORE" velocity which would be a better choice for downrange impacts.

You wrote
Now the question, assuming they both shot the same bullet at the same velocity.....would the added length of the second barrel have any effect on the trajectory of the bullet?
DC--- The added length should do better as per downrange impact and ballistics.

If in fact, the barrels are producing the exact "SAME" muzzle velocity, there would be NO difference to downrange impacts or drops.
Starting velocity is the key here. If both are the same to start with at point A, they will end up at Point B the same.
DC----I can relate this to the S&W 44Mags I had years ago.
One in a 4" and one in an 8". When I used the SAME load in both, the 8" barrel gave a bit more velocity which was a better hunting handgun for me.

Check your velocity over a good Chronograph and see what they are.
If the loads are the "SAME in both barrels, the longer one "should" give a higher velocity and would be a better choice for a hunting handgun.
You can get tooooooo long in barrel length to where there is more bullet drag produced and the added length of the barrel adds very little to the velocity.
I doubt if that's the case here.

Hope that helped.
DC