Reloading velocity issue: I can't figure that one out!!!

[Fox]

In the .243 Win. I prefer to use IMR 4064. Having said that it may be possible that the barrel on your .243 may have a different twist ratio then the gun used to establish the velocities in the manual, The variation in the twist ratio can result in a slow or fast barrel, which affects muzzle velocity. Projectile with longer bearing walls tend to give better performance in rifles with a fast twist, whereas projectile with short bearing wall give better performance in firearms with a slow twist. That aside, why is it important to match manual ballistics? In my mind, the important think is to find a load that gives consistent performance in your particular firearm, regardless of velocity. Actually lower velocity bullet that provide for consisted performance, will extend the barrel life of your firearm. Just some thoughts.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

His issue is the extreme difference, 800fps, over 2" in barrel length difference from the book numbers for the same load, not that he is not happy with the load.

[Gun Nut]

His issue is the extreme difference, 800fps, over 2" in barrel length difference from the book numbers for the same load, not that he is not happy with the load.

If I can find it again Fox, I'll give you a reference for the article. In the article they were comparing the performance of .270's in relation to .30-06's. The fellow selected a number of .270's for the trial but eliminated one or two rifles because they had what he termed slow barrels. Barrel speed equates back to twist ratios, in this case it would appear that you can find rifles of the same caliber that have different twist ratio. The rifles eliminated from the trial had slow barrels, and their muzzle velocity ran well below that of the other trial firearms by a few hundred fps, and as I recall they all had equal length barrels.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

[Gun Nut]

His issue is the extreme difference, 800fps, over 2" in barrel length difference from the book numbers for the same load, not that he is not happy with the load.

I found the article I was mentioning in my last posting, it was in the "Handloader's Digest, 11th. ed. In the test they started out using:
" three Remington 700s and five Winchester Model 70s, the velocity average for the same lot of ammo from the fastest barrel to the slowest one showed a loss of 250 fps. The particularly slow rifle was eliminated from our test considerations. However, such rifles do exist and are encountered not infrequently. While 270 barrels that produce higher velocities do exist, they are quite elusive. If one takes the average factory 130 grain load ( from an average barrel) and adds to it one of the frequently encountered "slow" barrels, a velocity of just over 2600 fps may be realized instead of the hoped-for 3000 fps. That's a whopping 400 fps difference."
So what determine the difference between a fast or slow barrel, the only thing I could come up with is the twist ratio of the barrel, Lyman predicates their data on a rifle with 1-10 twist/inch. but if you happen to get unlucky, you might end with a .243 with a 1-12 twist/inch, which is the twist ratio in a .308 Win. the .243's bigger brother.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

[Fox]

So what determine the difference between a fast or slow barrel, the only thing I could come up with is the twist ratio of the barrel, Lyman predicates their data on a rifle with 1-10 twist/inch. but if you happen to get unlucky, you might end with a .243 with a 1-12 twist/inch, which is the twist ratio in a .308 Win. the .243's bigger brother.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

I do not see how twist rate will change velocity, it is the pressure behind the bullet pushing it out the barrel, if it was a smooth bore that sealed up on the bullet the velocity would be the same, twist rate causes difference in stability and therefore accuracy, not velocity.

Throat length, chamber size, throat erosion, rifling depth/bore condition, all of those things can alter velocity, again you pulled an article that says a massive change of 400fps.

The throat length can be seen when you compare 30-30 rifles and 6.5x55mm rifles. The savage 340 in 30-30 has a long throat and develops lower chamber pressure than the Winchester model 94, less pressure and therefore less velocity. The 6.5x55mm military barrels are known to have long throats to accommodate long 160gr RN bullets. This lets a short bullet like a 100gr or 120gr push out a bit before hitting the lands and essentially creates a larger pressure vessel for the powder to burn in, this lowers the pressure and velocity compared to a shorter throat barrel. This being said the throat is always the same for the same rifle and does not seem to occur the same way with new rifles based around the SAAMI chamber dimensions.

[jaycee]

Does Barrel Twist Rate Affect Muzzle Velocity? (Litz Test)
The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting. That book includes many other fascinating field tests, including a comprehensive chronograph comparison.

Barrel Twist Rate vs. Velocity — What Tests Reveal
by Bryan Litz
When considering barrel twist rates, it’s a common belief that faster twist rates will reduce muzzle velocity. The thinking is that the faster twist rate will resist forward motion of the bullet and slow it down. There are anecdotal accounts of this, such as when someone replaces a barrel of one brand/twist with a different brand and twist and observes a different muzzle velocity. But how do you know the twist rate is what affected muzzle velocity and not the barrel finish, or bore/groove dimensions? Did you use the same chronograph to measure velocity from both barrels? Do you really trust your chronograph?

Savage Test Rifle with Six Bartlein Barrels
Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied Ballistics

Most shooters don’t have access to the equipment required to fully explore questions like this. These are exactly the kinds of things we examine in the book Modern Advancements in Long Range Shooting. In that book, we present experiments conducted in the Applied Ballistics lab. Some of those experiments took on a “Myth Buster” tone as we sought to confirm (or deny) popular pre-conceptions. For example, here’s how we approached the question of barrel twist and muzzle velocity.

Six .308 Win Barrels from Bartlein — All Shot from the Same Rifle
We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations. In this way, we’re truly able to compare what effect the actual twist rate has on muzzle velocity with a reasonable degree of confidence.

Prior to live fire testing, we explored the theoretical basis of the project, doing the physics. In this case, an energy balance is presented which predicts how much velocity you should expect to lose for a bullet that’s got a little more rotational energy from the faster twist. In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist (e.g. a 1:8″ twist is predicted to be 1.25 fps slower than a 1:9″ twist).

Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied Ballistics

Above, data shows relationship between Twist Rate and Muzzle Velocity (MV) for various barrel twist rates and rifling types. From fast to slow, the three 1:10″ twist barrels are: 5R (canted land), 5 Groove, 5 Groove left-hand twist.
We proceeded with the testing in all 6 barrels from 1:8” to 1:12”. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the rate of approximately 1.33 fps per inch of twist. In other words, your velocity is reduced by about 5 fps if you go from a 1:12” twist to a 1:8” twist. [Editor: That’s a surprising number — much less than most folks would predict.] In this case the math prediction was pretty close, and we have to remember that there’s always uncertainty in the live fire results. Uncertainty is always considered in terms of what conclusions the results can actually support with confidence.

Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied BallisticsThis is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.

Other chapters in the book’s twist rate section include:
· Stability and Drag – Supersonic
·

[skypilot]

Thanks for posting that J.

Interesting article. I couldn't likely see 1.33 fps change with the chrono I own anyway.
It would be interesting to see how Brian's numbers would compare with Optimum Barrel Timing and Quick Load's barrel timing.

[Gun Nut]

Does Barrel Twist Rate Affect Muzzle Velocity? (Litz Test)
The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting. That book includes many other fascinating field tests, including a comprehensive chronograph comparison.

Barrel Twist Rate vs. Velocity — What Tests Reveal
by Bryan Litz
When considering barrel twist rates, it’s a common belief that faster twist rates will reduce muzzle velocity. The thinking is that the faster twist rate will resist forward motion of the bullet and slow it down. There are anecdotal accounts of this, such as when someone replaces a barrel of one brand/twist with a different brand and twist and observes a different muzzle velocity. But how do you know the twist rate is what affected muzzle velocity and not the barrel finish, or bore/groove dimensions? Did you use the same chronograph to measure velocity from both barrels? Do you really trust your chronograph?

Savage Test Rifle with Six Bartlein Barrels
Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied Ballistics

Most shooters don’t have access to the equipment required to fully explore questions like this. These are exactly the kinds of things we examine in the book Modern Advancements in Long Range Shooting. In that book, we present experiments conducted in the Applied Ballistics lab. Some of those experiments took on a “Myth Buster” tone as we sought to confirm (or deny) popular pre-conceptions. For example, here’s how we approached the question of barrel twist and muzzle velocity.

Six .308 Win Barrels from Bartlein — All Shot from the Same Rifle
We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations. In this way, we’re truly able to compare what effect the actual twist rate has on muzzle velocity with a reasonable degree of confidence.

Prior to live fire testing, we explored the theoretical basis of the project, doing the physics. In this case, an energy balance is presented which predicts how much velocity you should expect to lose for a bullet that’s got a little more rotational energy from the faster twist. In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist (e.g. a 1:8″ twist is predicted to be 1.25 fps slower than a 1:9″ twist).

Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied Ballistics

Above, data shows relationship between Twist Rate and Muzzle Velocity (MV) for various barrel twist rates and rifling types. From fast to slow, the three 1:10″ twist barrels are: 5R (canted land), 5 Groove, 5 Groove left-hand twist.
We proceeded with the testing in all 6 barrels from 1:8” to 1:12”. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the rate of approximately 1.33 fps per inch of twist. In other words, your velocity is reduced by about 5 fps if you go from a 1:12” twist to a 1:8” twist. [Editor: That’s a surprising number — much less than most folks would predict.] In this case the math prediction was pretty close, and we have to remember that there’s always uncertainty in the live fire results. Uncertainty is always considered in terms of what conclusions the results can actually support with confidence.

Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied BallisticsThis is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.

Other chapters in the book’s twist rate section include:
· Stability and Drag – Supersonic
·

Interesting article, so if it's not twist that make for the difference in velocity between a fast barrel and a slow barrel, then it's probably throat diameter. i know when the German switched from the .318 round to the .323 round, they decided to convert the older .318 to shoot the .323 round. They stop short of re-boring them, electing instead to simply enlarged the throat portion of the barrel to allow for the base swelling of the .323 round. After the bullet left the casing and stared down the bore, the short bearing wall of the .323 spritzer bullet, allowed it to travel down the .318 bore without any significant pressure increase. This would suggest that an overly generous throat diameter in a barrel could lower the pressure of internal ballistics which in turn could substantially alter (lower) muzzle velocity.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

[jaycee]

I should have added that this is where the info came from;

We will be interviewing Bryan Litz of Applied Ballistics tomorrow at SHOT Show in Las Vegas. As a sneak preview of some of the topics we’ll cover, here are some highlights of some important, original research conducted by Bryan and his Applied Ballistics team. Bryan wanted to know how much velocity was altered by twist rate. The “real world” test results may surprise you….

The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting. That book includes many other fascinating field tests, including a comprehensive chronograph comparison.

As you can see the info came from a reliable source.

[Fox]

All of the twist rates for the 243 from Nosler show 1 in 10", Seabast, do you have a gun that has a non-standard twist rate? I assume not.

[seabast]

Hi All,
I haven't been able to check again the MV for this specific load and to be honest I haven't had time to read in details your week-end post but I'll do it all by the end of this week. Afterall, maybe the chrono was the issue that specific day.....
However, I reloaded some 70g Nosler under Varget at 100 yards and the accuracy result are alright and I haven't tried the maximum load (41g) yet. As sonn as I find the my acurate load I'll test the speed.