• This Forum is for adults 18 years of age or over. By continuing to use this Forum you are confirming that you are 18 or older. No content shall be viewed by any person under 18 in California.

Barrel length question

Ranger188

Silver $$ Contributor
I have seen some tests with 6 and 6.5 calibers that anything longer
than 28 inches the bullet "sometimes" starts to slow down in a longer barrel.
I know this has many variables. (bullet weight, powder and bore, etc.)
And with the F class shooters, some are as long as 32 inches.
Do you feel that 28,30 is optimal?
Has anyone tested to see if there 32 inch barrel was indeed
faster than 30 or 28?
 
Years ago, we did a test with the 6BR and 108 grn bullets in my rail gun.

we started with a 31 inch blank and recorded the velocity. We then sawed 1 inch from the barrel and shot 5 shots to get an average. We used a Port-a Band to cut the barrel.

we continued this until we saw a velocity change.

from 31 inches to 28 inches, there was no significant change. With Reloader 15, it was hovering around 3000 FPS.

after 28 inches, the velocity started to drop, but it was only about 25 FPS per inch average down to 24 inches. We left the barrel at 24 inches.

in short, a 6BR loaded with enough Reloader 15 to hit 3000 FPS with a 108 Berger in a 31 inch long barrel only lost an average 125 FPS at 24 inches.

28 inches would seem about optimum.

by the way, we shot some 1 1/4 groups at 600 yards with that barrel at 24 inches. I also shot a five shot five group agg of .184 at 200 yards.
 
Last edited:
The answers to these questions are not as simple as they may seem. For example, a 2" longer barrel doesn't necessarily mean any increase at all in velocity. Sometimes it might be only a few fps faster. The barrel will tune in where it tunes in.

One thing that can be definitively stated is that bullet weight, barrel length, and achievable pressure (velocity) are closely linked. For example, in .308 Win there is a very nice accuracy node for 185 Juggernauts over Varget at ~2725 fps or so from a 30" barrel. Trying to hit that same accuracy node with a 28" barrel will require higher pressure than a 30" barrel, even though the final velocities from 28" versus 30" barrels aren't much different. Likewise, hitting the same node with 32" barrel would require even less pressure.

In terms of velocity, with commercial ammunition you will generally find the trend to be somewhere in the neighborhood of 15- 25 fps increase in velocity per inch of barrel between lengths of 16" to 26" or so. Once you start using a barrel longer than about 26" with commercial ammunition, the velocity gain per inch of barrel typically drops noticeably. In fact, with barrels in the 28" to 32" length you may not even observe much of an increase in velocity. The reason for this is likely to be the powder/primer choice. Commercial ammunition loads are not necessarily optimized for 28" to 32" barrels. The powders used may effectively be completely combusted long before the bullet exits the bore.

To some extent, reloaders can compensate for some of the undesirable factors observed with commercial ammunition and long barrels by careful selection of powders and primers, and charge weight testing. However, there is a limit to the number of good powder choices available within a given burn rate range. As such, there is no guarantee that a 32" will provide velocity that is significantly higher than a comparable 30" barrel with the same bullet, primer, powder, etc. In fact, any detectable increase in velocity may be less than the ES/SD values for the two loads. Alternatively, a 32" barrel in just the right circumstance might give you an extra 10 to 20 fps over a 30" barrel.

The more important question is whether such a modest increase in velocity from a 32" barrel will make a measurable difference in a shooter's score. The answer is that even very highly skilled shooters would find it difficult, if not impossible, to accurately quantify the effect of a 20 fps increase in velocity in terms of reduced wind deflection and higher scores. My point here is that the main reason some have chosen to go with a 32" barrel length is not about increased velocity and a possible performance gain, it's more about the fact that you can usually hit the same velocity at lower pressure with the longer pipe. That can represent a significant increase in brass life under the right circumstances. It may also allow loading a higher weight bullet that would cause excessive pressure issues when attempting to hit a certain accuracy node with a shorter barrel.

Of course, an obvious trade-off with longer barrels is the added weight. So many shooters will go with the minimum barrel length and chamber specs that allows them to load their bullet of choice without having to run the pressure up too high. Barrels of 30" to 32" length are probably more common, but 28" or 29" barrels are certainly not unheard of. Quickload reloading software is actually an excellent tool for making predictions such as the effect of barrel length, in silico, as an aid to the decision-making/buying process.
 
Last edited:
Velocity "per inch" is not a good measure because it changes wildly with loads and with which "inch" you are investigating. If that inch is 17" vs 16" you get a very different result than 27" vs 28".

The difference in pressure for the same MV from a 28" to a 32" is likely to be very small. The reason is that the pressure curve has flattened so much at the end of the barrel that you are adding very little area under the curve by extending it.

There's also the potential downside of additional jacket damage. It seems the longest barrels have higher incidences of blow ups more than 26"-28" tubes do.
 

Upgrades & Donations

This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.


Click Upgrade Membership Button ABOVE to get Gold or Silver Status.

You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.


To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.

Forum statistics

Threads
165,634
Messages
2,199,992
Members
79,028
Latest member
Stanwa
Back
Top