Barrel Life: A Screed of Old Standby & the Math of the MatterHow about something I bet you don’t have but you would like? Yeah, I’m talking about new and free stuff. Yeah, that’s pretty exciting. So let’s dive in. Warning: There will be math.
People ask me probably 10 times a day, “How long will a barrel chambered for X last?”. I hate that question. Makes me want to bash my head on a desk because they’re asking me to predict the future and naively hoping for my answer being exactly correct. It can’t. I try to make that clear every time I give an answer. What I can do is get a useful estimate by making a horrifying number of assumptions, and so can you. What is needed is a mathematical model that describes reality within an acceptable level of imprecision. What's acceptable? In this case something to represent ideal conditions.
Tada: Below is the equation that I use (NOTE: I did not invent it, I found it years ago) to give me an idea of my accurate barrel life. It seems really useful dealing with match rifles where 1MOA groups are simply not going to cut it. This is also not something that’s necessarily scientifically precise so please see and use it for what it is, a model. I’ve shot rifles well past the numbers produced by the equation below but they all would soon enough tell me or had already started telling me they’re giving up the ghost. That said, no firm statement of usefulness or accuracy in any particular use case is made. It's just a model.
Powder Heat in KJ/kg (kilojoules per kilogram): H
Peak Pressure in PSI: P
Bore Capacity = (Bore Diameter^2 * 1000) / 2: B
Powder Charge Grains: C
The Formula for Non-Moly Loads:
The Formula for Moly Loads:
((3800/H)^5)*3600/((C/B)^2)*55000/(P - 10,000)
So let’s do the non-Moly math for my .243AI (which is a known barrel burner). Conventional wisdom is something just north of 1000 round barrel life. Let’s see what our model says…
First replace all the variables with numbers.
Next do the insides of the parenthesized sections (done in two stages here for clarity):
(.95^5)*3600/((1.524)^2)*55000/55000 = (.7737)*3600/(2.3716)*55000/55000
Once all parenthesized sections have been reduced, run the equation left to right:
.7737 * 3600 / 2.3716 * 55000 / 55000 = 1199 rounds of accurate bore life.
So yeah, the model is looking pretty accurate so far. What it cannot account for is how long of a delay between shots or how hot the barrel gets and a ton of other little things. Those things all vary radically based on the shooter, thermodynamic properties of the barrel and the effect of environmental conditions among other things. I’ve used this model for everything from .223 to .30-06 with substantial success. Try it out and see how it compares to your experience.
Meccastreisand is a long time competitive and recreational shooter, wildcatter, computer geek, exterior ballistics geek, inventor, outdoorsman, writer, husband and father. With over 20 years of experience in local and regional airgun, handgun, rifle and shotgun competitions of all sorts he competes currently in high power and smallbore metallic silhouette in the western states and long range precision and tactical matches throughout northern and central California. In his free time he wishes that he had enough free time to do anything other than wish for more free time.Jul 31, 2017 | Updated: Jul 31, 2017