Barrel Length Formula/Chart?

[GrampaMike]

Is there a formula or chart or something to recommend shortest, longest or optimum barrel length for different cartridges?  I would guess that the burning rate of the powder would be an important factor.  I was thinking of the TC Contender high volume cases and do they they get all the powder burnt before the bullet leaves the short barrels.

Mike

[hogship]

It'd be pretty tough to make up a "one size fits all" chart like that.....too many variables.

The fastest speed a bullet can go, isn't necessarily the most accurate it can be.

I can tell you that I have a Savage Striker that gets much better accuracy with faster than normal powders. I can't tell you if all the powder would be burnt at such a short barrel length, but I'd bet that it doesn't. A faster burning powder will definately be more completely burning in a shorter barrel, though.

Since most commercial rifles are somewhere in the 20" to 24" barrel lengths, my guess is that your optimum barrel length, considering accuracy, velocity, and practicality, is somewhere within those lengths....that is, when considering standard bottleneck chamberings.

[Questor]

I've never seen one that showed more than broad generalities, like "you get an increase of x feet per second for each inch of barrel added between y and z inches.  Only a few specific cartridges were specified because the data was gathered empirically by cutting down the barrel one inch at a time.  

It seems to depend also on the powder used and the pressure of the load, and the bullet weight.  

For example, my 10" contender barrel shoots 240 grain bullets just fine, but loads I've tried for 180 grain bullets (which hold more powder) have the most amazing muzzle blast I've ever seen. It's a brilliant white flash that seems to be two feet in diameter. There must be quite a bit of powder burning outside the barrel in that case.  

Another interesting factoid on the same basic topic is about 22LR. In barrels over about 16" (or is it 18"? I can't remember), the muzzle velocity actually starts to go down. The optimum length is about 16".  It is because of friction between bullet and barrel and the small volume of gas created by the burnt powder.

One way you might be able to gather some information on your own is to look at load books that have both a rifle section and a pistol section, then see what the differences in velocity are for cartridges that appear in both sections.

[Iowegan]

I doubt you'll ever find a factory made high power rifle barrel long enough where the bullet slows from friction or the powder burns completely.

There's a very good reason why you don't want a "total burn" of the powder. It became even more apparent when T/C's started getting popular.

For explanation purposes only, lets say you loaded 40 grains of powder in a given cartridge. Then lets say only 38 grains burned. If you chronograph that load, you would find the max velocity spread for 100 shots would be very low, 25 fps. Take the same cartridge and only put 38 grains of powder in it and you'll see the max velocity spread will be quite high 100 fps. Now lets load that original cartridge in a 14" T/C gun. Lets say only 30 grains of powder burned. Now your max spread is very tight, 10 fps is not unusual.  Max velocity spread has a great deal to do with accuracy. The change in velocity will cause vertical stringing and the rifling twist will cause horizontal stringing. In other words, your groups would open up a lot if all the powder burned.

Why is this you may ask? Powder is thought to be very consistent, it is not. Primers vary considerably. crimp or neck pressure varies a lot. Even some bullets are not exactly the same weight. All these variables are overcome with a little extra powder. The gun tends to burn only a certain percentage, the extra burns in the air causing a muzzle flash.

So why don't we just charge our loads with more powder? Actually, that's already built into the recipe you see in the reloading manuals. Over charging will increase the chamber pressure and could cause the gun to blow up.

This "trick" has been used for many years by ammo manufacturers. Using the right type of powder will make the gun more accurate by "wasting" a little bit so the muzzle velocity remains very consistent.

There's more to the formula than barrel length. The bore diameter, bullet weight, and case capacity all play a big roll. Too much for this post.

Bottom line: don't worry about barrel length, the gun manufacturers take care of those details.

[GrampaMike]

Thanks again guys… as usual the answers are just a “post” away.

Everything said makes sense, so I will not worry about barrel length.

Feels like I should be paying for the education.  That is a thought, are there classes where I can learn this subject matter?

Mike

[Iowegan]

Mike, gratuities are not required.  This forum has a wealth of knowledge available from some very experienced shooters and reloaders. Just keep the questions coming and someone will have the expertise to answer them. I don't know of any particular school that teaches the subjects of interest but you can start with some good books. Reloading manuals such as Speer #13, are crammed with information about every caliber. The glossary alone is worth the price of the book.

Back to barrel length. Most standard factory bolt guns have a 24" barrel. Some varment guns have a 26" bbl as do some of the heavy hitters such as a 300 Win Mag.  The shorter barrels do loose velocity but are made more for "ease of carry" than optimum ballistics.  With a few exceptions,  rifle reloading data is tested with a 24" barrel.