Barrel Life...
Posted: Mon Mar 08, 2010 9:15 pm
In the Indian context this has very little meaning given the restrictions on ammunition.
However those of us lucky enough not to be limited by the laws in order to pretty much shoot as much as we want, do worry about it.
Simply put barrel life is the rough number of bullets that can be shot through a barrel before accuracy starts to deterorate.
How is it calculated?
The rule-of-thumb process, as laid out by the likes of McMillan et al is:
1. Calculate the bore area in square millimeters.
2. Use one grain of powder for each square millimeter. This is what
I call the reference, or base powder charge.
Example: .30 caliber bore = 45.6 square millimeters.
Base powder charge for .30 caliber is 45.6 grains.
A .30 cal. cartridge that burns 45.6 grains of powder should give a
barrel life of about 3000 rounds of good accuracy.
3. If a larger cartridge is used and it burns more powder, the
accuracy life in rounds for that bore size is reduced. The amount
of reduction is determined by
a. Divide the increased charge by the base charge, then square
the answer.
b. Divide that answer into 3000.
Example: .28 caliber bore has a base charge of 38.5 grains.
Cartridge burns 57.8 grains of powder.
(57.8/38.5) squared is 2.25.
3000 divided by 2.25 is 1,333 rounds.
Now what is important to keep in mind is just what one expects from an accuracy viewpoint.
For example lets say I can pretty much hold a 5 shot group at under 0.2 inches center to center consistently (I cant in reality), i.e. > 0.5 inches edge to edge. So I will swap out a barrel once my groups open up to 0.6 inches or more edge to edge which say occurs at 2000 rounds through the barrel. Now keep in mind in compititive point blank bench rest, there is often as little as 0.022 inches seperating the person finishing 1st from the one who finished 20th.
However lets consider the scenario of a hunter, who likes to shoot deer, where the accuracy window is at least 6 inches. The barrel would last a lifetime.
Also keep in mind, the average compititive shooter will go through at least one barrel a season, while a typical hunter will shoot under one box of ammunition a season.
Other factors that affect barrel life are heating. Rapid fire tends to shorten barrel life. This primarily is due to throat erosion, and fire cracking in the throat in particular. Ideally you should not exceed one shot a minute. Or in Competition, at least let the barrel cool down completely between relays.
Some calibers seem to buck the trend in barrel wear, these are generally calibers like the .30 BR, .22 LR etc, which seem to last forever. On the other end of the spectrum are calibers like the .22-250, .300 WM, .243 Win which tend to wear out very quickly.
However since in India I would hazard a guess that the average rifle is shot about two to three times a year at most, one doesnt really need to worry about it.
(I am bored and rambling now).
However those of us lucky enough not to be limited by the laws in order to pretty much shoot as much as we want, do worry about it.
Simply put barrel life is the rough number of bullets that can be shot through a barrel before accuracy starts to deterorate.
How is it calculated?
The rule-of-thumb process, as laid out by the likes of McMillan et al is:
1. Calculate the bore area in square millimeters.
2. Use one grain of powder for each square millimeter. This is what
I call the reference, or base powder charge.
Example: .30 caliber bore = 45.6 square millimeters.
Base powder charge for .30 caliber is 45.6 grains.
A .30 cal. cartridge that burns 45.6 grains of powder should give a
barrel life of about 3000 rounds of good accuracy.
3. If a larger cartridge is used and it burns more powder, the
accuracy life in rounds for that bore size is reduced. The amount
of reduction is determined by
a. Divide the increased charge by the base charge, then square
the answer.
b. Divide that answer into 3000.
Example: .28 caliber bore has a base charge of 38.5 grains.
Cartridge burns 57.8 grains of powder.
(57.8/38.5) squared is 2.25.
3000 divided by 2.25 is 1,333 rounds.
Now what is important to keep in mind is just what one expects from an accuracy viewpoint.
For example lets say I can pretty much hold a 5 shot group at under 0.2 inches center to center consistently (I cant in reality), i.e. > 0.5 inches edge to edge. So I will swap out a barrel once my groups open up to 0.6 inches or more edge to edge which say occurs at 2000 rounds through the barrel. Now keep in mind in compititive point blank bench rest, there is often as little as 0.022 inches seperating the person finishing 1st from the one who finished 20th.
However lets consider the scenario of a hunter, who likes to shoot deer, where the accuracy window is at least 6 inches. The barrel would last a lifetime.
Also keep in mind, the average compititive shooter will go through at least one barrel a season, while a typical hunter will shoot under one box of ammunition a season.
Other factors that affect barrel life are heating. Rapid fire tends to shorten barrel life. This primarily is due to throat erosion, and fire cracking in the throat in particular. Ideally you should not exceed one shot a minute. Or in Competition, at least let the barrel cool down completely between relays.
Some calibers seem to buck the trend in barrel wear, these are generally calibers like the .30 BR, .22 LR etc, which seem to last forever. On the other end of the spectrum are calibers like the .22-250, .300 WM, .243 Win which tend to wear out very quickly.
However since in India I would hazard a guess that the average rifle is shot about two to three times a year at most, one doesnt really need to worry about it.
(I am bored and rambling now).
