Hi,

I've been lurking on this site for a number of months and caught the reloading bug. I've had a lot of fun loading and shooting my 22-250.

I just bought a .308 (a Winchester Stealth II) and have some questions about loading for it. It has a 26" barrel with a 1-10" twist. Now, the loading manuals I have all used either a 22" or 24" barrel to develop the loads.

I know a longer barrel can increase velocity and therefore range. That was one of the main reasons I bought this rifle. But my concern is pressure. Maybe this is a dumb question. Does this velocity increase come from higher pressures? Will I still be ok starting at the minimum loads in the manual?

Many thanks.

BZ,

The higher velocity from a longer barrel is due to use of the pressurized gas behind the bullet to drive the bullet for a longer period of time, like keeping your foot down on the accelerator for an extra bit.

this effect, however is more pronounced with large-capacity cartridges which due to their greater powder capacity and use of slow-burning powders have considerably more pressure trapped behind the bullet down at the muzzle end of the barrel, right before the bullet exits.

An extreme example is the .22 LR, which actually slows down after 18-20" of barrel length, since the remaining pressure is not sufficient to overcome bullet friction.

It's not likely that your 26" .308 will demonstrate a big velocity advantage over a slightly shorter barrel - probably about 25fps/inch with the slowest useful powders. The difference from barrel to barrel of the same dimensions is frequently greater than this.

Good Shooting,
BV

Maximum pressure occurs when the bullet just leaves the case, that's why the barrel is thicker there.Velocity starts at zero when the bullet is just leaving the case but accelerates as it moves down the barrel as the powder continues to generate hot gases.When the bullet leaves the barrel velocity starts to drop.Start at minimum loads and work up. Maximum accuracy is often a grain or two less than maximum.As you work up remember that you are dealing with your gun not the laboratory one used by the powder company.

quote:

Does this velocity increase come from higher pressures?

No....pressure is totally unrelated to barrel length

quote:

Will I still be ok starting at the minimum loads in the manual?

Yes and for the very same reason.

Thanks, everyone, for your quick and helpful responses.

/

Umm Alf, can you splain sumptin to me?

There is a finite amount of gas @ pressure, as the bore volume increases (due to a longer barrel), the pressure would need go down as the bullet progresses further down the barrel (P1V1=P2V2), true/false?
The longer barrel, allows a longer TIME period for the gas pressure to work IOW "work under the curve", true/false?
At any given time/distance, wouldn't the "spot" pressure curve be the same, regardless of barrel length? Ex: when the bullet is at 18" ahead of the breach face, wouldn't the pressure, at that instant, be the same, regardless of weather the barrel is 20" or 36" long? True/false?

quote:

Does this velocity increase come from higher pressures? Will I still be ok starting at the minimum loads in the manual?

No, it doesn't. Barrel length has no effect on PEAK pressures whatsoever. Peak pressures are reached at a point when the bullet is about (depends on cartridge capacity) 4"-5" down the bore.

The higher velocity from longer barrels comes from the fact that the pressure gets to push on the bullet base for a longer period of time, thus accelerating it to a higher speed, that's all......

ALF's answer is confusing, and does not address your questions in the least! Just start with the starting loads, check your accuracy, and work up if you wish to, just as if your barrel were 16" or 22", or whatever.......

/

Alf.....you're just plain full of shit!

/

Let's toss in a ballistic complication here:

Propellants are nitro-cellulose and nitro-gylcerines. They generate working pressures dependent upon the sorts of pressures they're under as they combust.

If you put smokless propellent out in a dish, unconfined, and ignite it the result is a relatively slow burn because the charge is not confined and develops no working pressure.

It is the pressure resulting from confined combustion that generates both more pressure and increased combustion. Essentially, increased pressure results in ever more increased pressure.

There are graphs available online showing combustion profiles and pressure spikes. It gets complex -- which is why a slow burning powder producing less maximum pressure can result in a higher bullet velocity.

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But back to the question at hand.

Modern firearms are designed with an ample safety margin relative to working pressures and catastrophic pressures. Two inches, or even an extra foot of barrel length in a rifle is not going to begin to reach the end limits of this margin.

Rule of thumb in hand loading is to reduce the maximum load charge weight 10% and work up if desired -- checking for pressure signs.

Reducing the charge weight by 10% decreases the working pressure by significantly more than 10% because of the pressure/combustion characteristics of smokeless propellants.

Peak pressures are found at a point just inches in front of the chamber and drop from there out to the muzzle. The fact they drop doesn`t mean they no longer are high just that they likely aren`t as high at 12" as they were at 10" and definately are lower at 22". The pressure at the muzzle may still be in the 10K psi range and pushing on the base of our bullet at 26" but it won`t be the 60K it was at peak.

I am not saying the powder charge isn`t still producing gas farther down the barrels lenght from the point of peak psi, but the volume of the bore disapates the gas faster then it is produed and pressure are not kept up.