Home Forums Hunting & Shooting Ask The Gunwriters Fast and slow barrels

I have read comments on here about fast and slow barrels, in the same caliber.

Does anyone know what would cause a barrel to shoot the same identical load faster than another barrel of the same length?

Pixie dust... .

I took three 22"-barreled .280's to the range last summer with three loads. Velocities varied up to 150 fps among the three. The consistently slowest barrel also features the largest chamber dimension, as observed from resizing brass. Neck-sized brass fired in it will not chamber in the other two, for example.

Other factors that can play a role include bore dimensions and polishing or lack thereof in the bore. Stack tolerances properly and you can end up with some observable differences.

Dennis

"Fast" barrels that will allow you to shoot 150-200fps faster than the new reloading manuals tell you it's safe to go don't exist. If you're going a couple hundred fps faster than everyone else it's not your brilliant reloading skills it's that you are running higher pressures and might be lucky to not have a gun blow up on you eventually.
There are factors that will cause a barrel to shoot slower than standard at the same pressures including loose chambers, rough bores, oversized bores etc..

I like to take a survey of all the latest pressure tested manuals for a given bullet weight and cartridge and set the average highest velocities between them as my maximum, if I'm going above that I know my pressure is getting too high. Reloading isn't magic and there isn't a truly safe way to go faster other than buying a more powerful chambering.............................DJ

It should have occured to me that some might misunderstand my point.

The loads that were the "fastest" in my test noted above were within standards. The "slow" load was running, in at least one case, almost 200 fps below maximum levels in various books.

Only pixie dust can make loads run faster than safe without being unsafe . And it's such a nuisance to find a goat to sacrifice every time you want to try some new loads with it ...

Dennis

I imagine twist rate, number and size of grooves, bore diameter, bore smoothness, etc would all be factors affecting whether a barrel was slow or fast relative to other like barrels.

+1

Fast barrels always amaze me because they're usually accompanied by powder charges in excess of those published by those rascally lawyer proof manuals..............

Methinks you guys have too much faith in manuals! None of them are perfect, and if you want an extreme example of that simply compare Nosler's Reloading Guides 6 & 5 in the 35 Whelen section. Note: same barrel (Lilja 24", same case (Remington)and same primer (Fed 210). But with the 225gr Noslers in No.5 56.5grs RL-15 is shown as max at 2525 fps. Now, some of us knew that was NOT correct. In No.6 it's upgraded to 59grs of RL-15, all other components being the same, at 2789 fps. NOW THAT'S MORE LIKE IT! The 350 Rem Mag will equal that from a 22" tube!

So, would you guys suppose that say a 308 with a 10 twist vs a 308 with a 12 twist one or other should be on average going a bit faster?

Everything being apples to apples cept the twist.

Your thoughts please, I'd be for guessing that the quicker twisted one would go a bit faster...right?

Thx
Dober

Flip that.

So, you think that the slower twist will be faster?

Less twist means less rpm which means less of the available energy from the powder charge is used for spinning and more for pushing.

For the scenario you mentioned the effect will be negligible.

There are barrels that give normal,safe,but upper end velocities;and there are barrels that give substandard velocities with pressures that are "max".

There are lots of factors involved,(as has been pointed out)including throat length, number of lands and grooves,tighter vs "looser" groove diameter,bore finish,material (stainless vs chrome moly),etc.

As muledeer points out,all or any of these factors can "stack" to lead to some pretty odd results. Two of my 7 mags,fired the same day,with the same load,etc,showed over 200 fps velocity difference. In the "faster" barrel, I dropped the load to a velocity level I knew to be "normal" and everything has been fine ever since.

Sounds logical,makes sense in theory,but have never seen it.

Sure.

For a given bullet and powder, muzzle velocity is practically completely determined by friction, the area under the pressure curve and the cross sectional area of the bullet.

The area under the pressure curve in turn is practically completely determined by chamber dimensions, throat length, projectile mass, barrel and ammo temperature, and engraving force.

If you are getting more than standard muzzle velocity for the powder you are burning, you are running more than standard pressure. If energy is to be conserved, it's either that or people in the ninth dimension are getting much colder or warmer. I vote for more pressure.

I believe mathman's right- less twist means less energy imparting rotational velocity to the bullet.

I've gotten the same info in the past from one of the premium barrel makers, who recommended I go with a slower twist than I had originally requested- I didn't need the faster twist, and the slower twist would give me slightly higher velocities.

The angular momentum of a bullet soaks up only a fraction of 1% of the energy. The effect is close to negligible.

Yep.

After reading the posts on this subject I have some concerns regarding some loads that I developed a few years ago. I worked up some loads in my winchester M70 classic 300wsm using IMR 4831 powder and 165 gr swift scirocco bullets. I worked up to 67.5 gr and stopped, new brass expanded .0003, I got an average of 3112 fps from a ten shot string, extreme spread was 54 fps, standard deviation was 15. Reloading data that I got from Swift shows a max charge of 71 gr and a velocity of 3070, I contacted Swift about this and they said that every gun is different. I went out a few weeks ago and chronographed these same loads, I only fired two rounds, they read 3062 and 3056 fps, when I fired the ten shot string I was using a different chronograph and it was 90 degrees, it was 60 degrees when chronographed the two rounds. I believe I have a slightly fast barrel in my rifle because the factory winchester power point ammo has an advertised velocity of 2970 fps, I chronographed a ten shot string and got 2987 fps,temperature was 85 degrees, I chronographed those same rounds a few weeks ago with a different chronograph, temperature was 59 degrees, average velocity was 2961 fps. I have about 50 of those Swift hanloads loaded up, I don't feel like pulling them, from the information that I have just posted, do you think these loads are ok, any advice is appreciated.

Freddy...

A lot depends on how good a job Swift did in developing their published loads. If they used piezo equipment rather than copper crusher, that's a good sign.

Assuming that they did a good job, I don't think I would worry excessively, but I may be wrong.

SAAMI specifies testing at 70 F. And we all know that not all rounds will be fired at that temperature. So your 90 F is probably not out of line with what you might get at 70 F.

It is not unheard of for some handloads to safely exceed the MV of factory loads. My favorite 30-06 load exceeds SAAMI's spec for commercial ammunition by a bit over 100 FPS. I'm using a straight book load from Hodgdon, and my pressure is verified with a strain gauge on the rifle. The rifle shoots commercial ammo right at spec.

You can pretty much disregard brass expansion in the range you mention. CHE is so loaded with random noise that you can barely reliably distinguish a plinker load from a barrel buster.

So, all that said, let me be a little more careful in stating what I said: If you have reloading data from a reliable source, using piezo or strain equipment, and if you are getting a higher MV than was published, with the components specified, you are running higher pressure than the publisher was.

While a 1% difference in bullet speed is truly negligible - people have "improved" rifles and even resorted to buying new cartridges to achieve that insignificant difference.

I noticed, that in the latest BARNES Reloading guide, the difference between individual rifle barrels - was more than what a 6% more powder increase could accomplish. Lookat the data comparing the 375 H%H, and the new 375 Ruger. Even when a custom-built barrel is individually load tested, and when the pressure is tested in those loads!

Look it up - most loads, had the 6% smaller case-capacity 375 H%H either equal to - or faster than, the larger 6% larger Ruger case.

That just goes to show that even with the finest custom barrels - the differences in speed between individual rifle barrels can be greater, in some cases, than what can be accomplished with as much as 6% more powder capacity.

Then think about how many re-loaders get all excited about an extra 100 fps or so!