Barrel Length,point of no return?

[Lucky]

Not sure where to post this;
I know there must be a point of diminished return in terms velocity related to length of barrel but what is it? Does it vary with caliber, powder, bullet type or weight? I am specifically interested in 6.5 caliber 130-140 gr VLD type bullets using H4831sc or comparable powder. I know the rule of thumb is 25fps per inch of barrel but also understand that at some point all of the propellant is completely burned & at some point the pressure would begin to fall off.Is that point within the practical range of barrel length,say 24" to 30" or would you continue to achieve an additional 25fps per inch for an additional 3"? 5"? 10"? As i said, i am now working on a 6.5 cal but would appreciate any general info that could be applied to other calibers if it it does in fact vary from cal to cal.
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[40X Guy]

Generally speaking, heavy bullets for caliber require a bigger case and slower burning powders in a longer barrel. My 6.5X47L set up for 123-130 class bullets is 28.5 inches long. But I have been told a caliber like a 6.5-284 may need up to a 30 inch tube to take full advantage of 139-142 class bullets. I have read on the board that the 6.5X47 may not have the needed boiler room for 139-142 class bullets but plenty of shooters including me are using them. But there are also velocity nodes in each caliber and particular rifle. I guess what i am trying to say is find what your gun likes and run with it. Your barrel will tell you. If building a 6.5X284 or a 6.5-06 AI then you would probably need a 30 inch barrel for heavy bullets. You can get by with a 26-28 inch tube for a .260 or a 6.5X47. My other 6.5X47 Tactical rifle based on a BAT action is only 23 inches in barrel length but i can shoot 142 MK's in it at 2650fps with 40 grains of H4350.


Frank

 

[Turbulent Turtle]

Not sure where to post this;
I know there must be a point of diminished return in terms velocity related to length of barrel but what is it? Does it vary with caliber, powder, bullet type or weight? I am specifically interested in 6.5 caliber 130-140 gr VLD type bullets using H4831sc or comparable powder. I know the rule of thumb is 25fps per inch of barrel but also understand that at some point all of the propellant is completely burned & at some point the pressure would begin to fall off.Is that point within the practical range of barrel length,say 24" to 30" or would you continue to achieve an additional 25fps per inch for an additional 3"? 5"? 10"? As i said, i am now working on a 6.5 cal but would appreciate any general info that could be applied to other calibers if it it does in fact vary from cal to cal.
Logged

When you say "point of no return", I think you mean the point at which the velocity no longer increases in a barrel.

For centerfire cartridges, that can be several feet of barrel, so anything you can lift and handle unassisted has not reached that point. In actuality, all the powder that is going to burn does so before the bullet has traveled much more than a few inches at most. A longer barrel does not allow more powder to burn, it allows the hot gases from the consumed powder to push the bullet faster. As long as the gas pressure behind the bullet exceeds the friction of the bullet in the barrel, the bullet accelerates.

Cartridges that have a larger charge compared to the bullet size benefit from longer barrels because they have so much more pressure to push the bullet. A good sign for the amount of useful gases that you waste because of a short barrel is highlighted, some might say punctuated, by a big-a$$ muzzle blast. When said muzzle blast is so large because of the large amount of wasted hot gases, it will actually ignite on contact with the cool atmosphere and its 21% oxygen content. The greater the muzzle blast the more hot gases you have wasted because your barrel is too short.

 

[M-61]

Bayou shooter is dead on. Point to ponder: For about the first 18 inches or so as the bullet leaves the muzzle it is travling "Down Wind". The gases escaping behind the bullet pass by it.

 

[wboggs]

I have heard the the velocity of the hot gas is around 6000fps and that would limit velocity in a frictionless bore, something that does not exist. I believe the tanks use a smooth bore with a sabot that allows very high velocities with the AP rounds. Would appreciate hearing from some of you that know the physics. It is a very interesting subject.

 

[M-61]

WBOGGS: If you havn't already you might take a look at the work of one Gerald Bull (1928-1990). Off the subject somewhat but fasinating. Opened a whole new line of thinking. Talk about thinkng outside the box!

 

[Turbulent Turtle]

Gerald Bull. Now there's a name I haven't heard for a while. I went to school with his sons and met him once. He was a certified ballistics genius. He was killed in Belgium just before the first Gulf War, probably because he was working on Iraq's supergun. The man would do just about anything to play with super cannons. Look up Space Research Corp. and HARP. An enigmatic figure he was.

I'm not convinced that gasses blow by the bullet for the first 18 inches, I would think that proper obturation occurs quite rapidly and cuts off that flow very quickly right when the bullet engraves fully into the lands.

The reason modern tanks use smoothbore main guns is because the required spin for the long skinny projectiles they use is so high that the rifling would have to be very fast indeed. And as we know with a fast twist comes higher pressures. Instead, they use fins on the projectile that provide the required spin and the barrel is smooth which increases the velocity of the round. The reason behind the thin and long projectile is they have very high BC values and they also concentrate all the energy on a very small point which enhances the penetrating capability of the projectile. That principle has been around since the crossbow bolt.

The flight characteristics of the two main rounds used in the Abrams, APFSDS vs HEAT are different enough that the ballistics computer has to be told for which round it is providing the firing solution.

 

[Catfish]

If you are planning on shooting out the barrel you want to go with the max lenth barrel for another reason, that is so you can set it back and rechamber it. I did that with a 2-6mm and have had it set back and rechambered. I got abt. 1000 rounds down the tube before I shot out the throught and don`t know how long it will shoot with this chamber, but it`s alot cheaper than a new barrel every time, and it seems to shoot as well as it ever did.