bullet
Handloader
- Dec 26, 2007
- 4,973
- 8
Is There a Magical Barrel Length for Rifles? By: Mike Price (bullet)
There is not a single, magical barrel length that is optimal for all cartridges. I believe it is firearm and load specific that determines accuracy no matter what the length of barrel with in reason. It's not just length, but how the barrel is mounted, it's style, bearing pressure points - many factors but mainly the time it takes the shock wave to move up the barrel to the muzzle and back down again to the chamber end as the bullet is exiting the barrel that really matters - not so much the length (this is all assuming we have a properly bedded rifle, a well made barrel and good barrel receiver fit).
There are many products out there for folks to try that are suppose to help you find the magical group by controlling the time the shock wave reaches the end of the barrel and back down to the chamber end of the barrel – so that the bullet is exiting the barrel at the optimum moment. Browning uses the Boss as one example. It has to do with the barrel's weight, mass, caliber, the load, the load components, etc... affecting the timing of the shock wave that moves up and back down the barrel. There are way too many variables for a common barrel length solution, for all rifles. But there is a common solution for all rifles and barrel lengths that I will get to in just a moment for good accuracy, and it has to do with the length of the barrel.
The barrel weight (thickness) is part of the equation. A stiffer barrel moves less - no matter what the vibration node pattern looks like. Remember that you sacrifice velocity as you lose barrel length (in the shorter barrel the shockwave gets to the end of the barrel and back faster); especially true of the larger capacity cases as you approach a more "overbore" condition. Some very thin contoured barreled hunting rifles are very accurate (case in point - my M48 TGR in 7mm08) - and some slim barrels are at least for a few shots. It makes no more sense to me to say, “x" barrel length for best accuracy in all rifles, a pet load being great for all rifles, or expecting any manual's load to perform for us as it did for those who put the manual together.
It has to do more with the load used that creates the shock wave that goes from one end of the barrel and back no matter the length. The initial shock wave, generated by any charge of powder, will travel at the speed of sound in steel (just at 18,000 fps or there about), from the chamber to the muzzle, then back, in a repeated pattern. What we try to do with our hand loads is get the speed of the load to time the bullet’s exit just as the shock wave reaches back to the chamber end of the barrel - before it starts back up the barrel again.
Of course we have to experiment with different load chain combinations, because we don’t have the lab equipment to help measure or see when the shock wave is at the muzzle or at the chamber end of the barrel. When this wave is present at the muzzle, there is naturally a great deal of turbulence and obturation (obturation refers in this case to the flaring under the pressure of the shock wave and bullet upon exiting) of the bore at muzzle. However, when this main shock wave has reverberated back to the chamber end, the muzzle is relatively stable allowing for much less turbulence at the muzzle. This window in timing, gives the bullet the best opportunity, to exit the muzzle with much less turmoil and on a more consistent and repeatable bases. The barrel is basically straight, and relatively calm, allowing for tighter groups.
If the bullet is leaving the end of the barrel when the shock wave is at the muzzle, this creates a “scatter node effect” (in other words it will produce a scattered group and even cause random flyers). It is interesting that many times all I have had to do is add one or two grains of powder to a charge that is causing the opening of groups and flyers for it to settle down (do not use more grains if already at max) and you will be right where you need to be a lot of the time. Some times we give up when the group is not a tight group when just a little more work with the load will match the timing needed for our barrel length. When having the right load to push the bullet at just the right speed to exit the muzzle when the shock wave is at the chamber end of the barrel, usually produces good results.
Once you find this load for your specific bullet weight and powder charge that causes the wave to move at the right speed, for the wave to get back again to the chamber end of the barrel, and the bullet leaves the muzzle at that moment – then barrel length does not matter. You will generally have a great load and small consistent groups. As long as, the relationship between the time it takes the bullet to exit the muzzle, and the oscillating shock wave to be out and back to the chamber end of the barrel, you will more than likely have good groups. It is important to know that the shock wave travels at the same speed, no matter what length of the barrel is being used, so since this is the case there is no such thing as a magic barrel length for all cartridges and rifles, just good loads, that get the timing right.
There is not a single, magical barrel length that is optimal for all cartridges. I believe it is firearm and load specific that determines accuracy no matter what the length of barrel with in reason. It's not just length, but how the barrel is mounted, it's style, bearing pressure points - many factors but mainly the time it takes the shock wave to move up the barrel to the muzzle and back down again to the chamber end as the bullet is exiting the barrel that really matters - not so much the length (this is all assuming we have a properly bedded rifle, a well made barrel and good barrel receiver fit).
There are many products out there for folks to try that are suppose to help you find the magical group by controlling the time the shock wave reaches the end of the barrel and back down to the chamber end of the barrel – so that the bullet is exiting the barrel at the optimum moment. Browning uses the Boss as one example. It has to do with the barrel's weight, mass, caliber, the load, the load components, etc... affecting the timing of the shock wave that moves up and back down the barrel. There are way too many variables for a common barrel length solution, for all rifles. But there is a common solution for all rifles and barrel lengths that I will get to in just a moment for good accuracy, and it has to do with the length of the barrel.
The barrel weight (thickness) is part of the equation. A stiffer barrel moves less - no matter what the vibration node pattern looks like. Remember that you sacrifice velocity as you lose barrel length (in the shorter barrel the shockwave gets to the end of the barrel and back faster); especially true of the larger capacity cases as you approach a more "overbore" condition. Some very thin contoured barreled hunting rifles are very accurate (case in point - my M48 TGR in 7mm08) - and some slim barrels are at least for a few shots. It makes no more sense to me to say, “x" barrel length for best accuracy in all rifles, a pet load being great for all rifles, or expecting any manual's load to perform for us as it did for those who put the manual together.
It has to do more with the load used that creates the shock wave that goes from one end of the barrel and back no matter the length. The initial shock wave, generated by any charge of powder, will travel at the speed of sound in steel (just at 18,000 fps or there about), from the chamber to the muzzle, then back, in a repeated pattern. What we try to do with our hand loads is get the speed of the load to time the bullet’s exit just as the shock wave reaches back to the chamber end of the barrel - before it starts back up the barrel again.
Of course we have to experiment with different load chain combinations, because we don’t have the lab equipment to help measure or see when the shock wave is at the muzzle or at the chamber end of the barrel. When this wave is present at the muzzle, there is naturally a great deal of turbulence and obturation (obturation refers in this case to the flaring under the pressure of the shock wave and bullet upon exiting) of the bore at muzzle. However, when this main shock wave has reverberated back to the chamber end, the muzzle is relatively stable allowing for much less turbulence at the muzzle. This window in timing, gives the bullet the best opportunity, to exit the muzzle with much less turmoil and on a more consistent and repeatable bases. The barrel is basically straight, and relatively calm, allowing for tighter groups.
If the bullet is leaving the end of the barrel when the shock wave is at the muzzle, this creates a “scatter node effect” (in other words it will produce a scattered group and even cause random flyers). It is interesting that many times all I have had to do is add one or two grains of powder to a charge that is causing the opening of groups and flyers for it to settle down (do not use more grains if already at max) and you will be right where you need to be a lot of the time. Some times we give up when the group is not a tight group when just a little more work with the load will match the timing needed for our barrel length. When having the right load to push the bullet at just the right speed to exit the muzzle when the shock wave is at the chamber end of the barrel, usually produces good results.
Once you find this load for your specific bullet weight and powder charge that causes the wave to move at the right speed, for the wave to get back again to the chamber end of the barrel, and the bullet leaves the muzzle at that moment – then barrel length does not matter. You will generally have a great load and small consistent groups. As long as, the relationship between the time it takes the bullet to exit the muzzle, and the oscillating shock wave to be out and back to the chamber end of the barrel, you will more than likely have good groups. It is important to know that the shock wave travels at the same speed, no matter what length of the barrel is being used, so since this is the case there is no such thing as a magic barrel length for all cartridges and rifles, just good loads, that get the timing right.
Nice article. CL
