What's Overbore; or Underbore For That Matter?

[Bart B]

This post in another thread got me thinking:

I've challenged myself to understand exactly how & what seating does, and I do not know. For best OTL I'm thinking results are more to do with bullet release timing affect to peak pressure.
For best ITL, I think it's about going to another peak pressure level (a pressure node), and underbore cartridges benefit predictably here. But most hunting cartridges are not competitive underbores, so full seating testing with hunting cartridges likely leads to OTL as best. That's what I find anyway.

Sp what's overbore or underbore???? Here's what I came up with.

Some years ago, I checked into what could be used across all calibers; 22 through 30. Learned that cartridges getting best accuracy for their 3000 round barrel lives in benchrest and high power competition all burned 1 grain of powder for each square millimeter of the bore's cross section. That's about 21 grains for 22 calibers and 45 grains for 30 calibers. So if the round burns more grains of powder than the square millimeter bore area number, it's over bore capacity. Underbore ones burn less powder. 22 and 6 PPC cartridges as well as the .308 Win. are right at bore capacity and are probably the most accurate one for their calibers.

Cartridges burning 40% more powder for the same bore area got half the accurate barrel life; 1500 rounds. The 100% more ones got one fourth the barrel life; 750 rounds. It's the old inverse square law.

Accuracy based on 100-yard test groups starting out averaging about .200" to .250" then opening up 50% to .300" or .375". That's been Sierra Bullets' standard for years on their test barrel's lives.

Less accurate rifles will have more barrel lives as they start out shooting bigger group. Hunting rifles may well have twice the life; 6000 rounds before their owners notice a 50% loss in accuracy. For service rifles used in combat, three time as many; 9,000 or more rounds for acceptable barrel life.

 

[bigedp51]

On my Browning superposed 12 gauge the top barrel is the overbore and the bottom barrel is the underbore.

Bazinga

 

[Mikecr]

There are a lot of ways to consider bore capacity, but IMO it's best to review the spread of capacities per cal, along with results fairly well known(without embellishments).
Trust me, a 6PPC is relatively underbore for cal, and it was designed to be so.
It's a tiny cartridge for bore, and the same can be said of the 30BR. Neither anything like a 308Win.
They reach a pressure node, by design, which is a work-around for many reloading variances.
6PPC Forgiving means 6PPC consistent, as demonstrated by less need for accurate powder measure.
But this is not viable in hunting capacity cartridges(purely out of chamber pressure per square inch applied). If you try to run a pressure node with something like a standard 243, or 30-06, your brass and barrels won't last long enough to reach any consistency.

I believe that accurate barrel life(the line) is seldom recognized because relatively few shooters reach full potential in their barrels to even begin such a measure. Much of this is because they don't choose cartridges based on accuracy potential. They go for power.
However, where they take a well designed cartridge & load to cutting edge best, maintain it, and get used to that performance, they'll know a change right off the bat. After chasing their tails a bit to recover, they'll come to the realization that it was simply time to change the barrel. Lesson learned.

I somewhat recently polled folks at BRC about loads and accurate barrel life in a competitive(today) 6PPC.
The answers were very consistent as expected. These folks shoot at levels where they recognize changes immediately. And at this measure and awareness, barrel life does present as a step change. It does not 'gradually' going away at all.
The common VV N133 load and result for competitive 6PPCs below:

 

[J E Custom]

I am a big fan of PO Ackley and his theories. Many of which he proved but in this case he states that there is no set rule to determine the proper or maximum case capacity.

Ackley felt that the limit of over bore is reached when a cartridge holds more powder than it can
efficiently handle.

Powley, ties up bore capacity with expansion Ratios. Expansion Ratio is the ratio between the total volume of the bore and the volume of the case. It is the number of times the gas will expand by the time the bullet reaches the muzzle.

High expansion ratios normally mean good barrel life and low ratios mean poor barrel life. Low ratios may produce higher velocities but as stated in earlier post, may greatly reduce barrel life.

I have always felt that there was a point where more powder did not add enough performance and velocity to make it worth while and reduced barrel life became a big factor.

Unfortunately most Long Range hunting rifles fall into the over bore category and we just have to live with it if we want the long range capability.

There are so many variables that I don't think there will ever be a Rule for what is or is not overbore for every rifle cartridge combination.

Just more comments

J E CUSTOM

 

[Lefty7mmstw]

I've always looked at the number of grains of bullet per grain of powder. I.e., a 300 rum tossing a 200 grain pill with 100grains powder is at 2 grains bullet per powder grain. a 30-06 tossing a 200 grain pill with 54 grains of powder will net you nearly a 4 to 1 ratio.
I've burned up barrels on rifles chambered in cartridges with 2 grains of powder per bullet or less, but never done in a barrel on a 4 +grain of bullet per grain of rifle powder...

Nothing scientific or anything, but it something I look at when contemplating rifles with a bit of Absynth on the rocks and a smoke... Or possibly a good Bourbon barrel stout, as I have with me tonight...

You of course have to be looking at similarly heavy for caliber bullets and equal chamber pressures for the comparison to hold any water.

 

[Bart B]

Mikecr's thumbnail of a 75,000 psi 24 caliber load predicting a 894 round barrel life is realistic. I wonder what it would be with a charge weight producing about 62,000 psi in normal pressure value ranges instead of that near proof load level.

If barrel length is part of the over/under-bore formula, can someone explain way?

If 5 identical barrels for a given cartridge at 22, 24, 26, 28 and 30 inches shoot the same load configuration, that load will perform identical pressure and velocity curves for the first 22 inches in each one. The pressure/velocity curves for each length will vary a little across each one at bullet exit for each additional 2 inches. Note both pressure and velocity are identical at 28 inches in both the 28 and 30 inch ones. So will the bullet departure angles in each barrel relative to the line of sight, but they're all repeatable for each barrel length.

 

[Lefty7mmstw]

Mikecr's thumbnail of a 75,000 psi 24 caliber load predicting a 894 round barrel life is realistic. I wonder what it would be with a charge weight producing about 62,000 psi in normal pressure value ranges instead of that near proof load level.

If barrel length is part of the over/under-bore formula, can someone explain way?

If 5 identical barrels for a given cartridge at 22, 24, 26, 28 and 30 inches shoot the same load configuration, that load will perform identical pressure and velocity curves for the first 22 inches in each one. The pressure/velocity curves for each length will vary a little across each one at bullet exit for each additional 2 inches. Note both pressure and velocity are identical at 28 inches in both the 28 and 30 inch ones. So will the bullet departure angles in each barrel relative to the line of sight, but they're all repeatable for each barrel length.

With an equal load, differing barrel lengths should mean nothing to bore life... I can see a longer barrel producing more barrel life though if one doesn't need to throttle the rifle quite as hard to reach expected velocity... Most guys will simply eat more cake though and take the extra velocity as bragging rights for the longer barrel.

 

[Bart B]

With an equal load, differing barrel lengths should mean nothing to bore life...

Which is why I don't think over/under-bore capacity has nothing to do with barrel length. I'ts caliber (bore diameter) specific. Maybe cross sectional area as SAAMI states it which is the area of a circle at groove diameter minus the area of the lands.

 

[Mikecr]

I wonder what it would be with a charge weight producing about 62,000 psi in normal pressure value ranges instead of that near proof load level.

It would be around 29.5gr, as they are into diminished returns, and a 6PPC shoots more like a 6br there.
Backing off the lands would nearly drop their pressure that much, increasing BL to ~1100 rounds.
Backing off powder to drop pressure instead increases BL to ~1700
Slowing shot rate from 10sec to 30sec, even with their hot competitive load increases BL to ~2650
62Kpsi seating adjusted load + slow shot rate increases BL to ~3200
62Kpsi powder adjusted load + slow shot rate increases BL to ~4000
Not bad, but keep in mind N133 is among the coolest of powders in existence, and amazing for burn rate. It's some kind of magical pixie powder.
For hunting capacity cartridges, we have H1000, which is as cool as N133, but of course we burn more of it.
And if someone was happy enough with '~1/2moa' from a 6PPC, and managed carbon with JBs, their barrel life would probably seem 20,000!

But melonite treatment will greatly increase all these numbers. And once credible numbers come in, for a gamut of cartridges, I'll add the melonite adjustment as 'Extending Adj' in my software.

 

[Rich Coyle]

Good to know the effect of waiting between shots.

 

[Mikecr]

I work with someone who had 22-250 that produced fantastic 3sht groups. This was a hunting gun and really just an amazing shooter. But, at what he estimated at ~850rnds, the ragged hole groups suddenly stopped. He tried everything he could think of, no luck, and then asked me about it.
There seemed an awful lot of targets brought into work, so I asked about his shot rate on these.
He said 'fast as he could shoot between conditions'..
Well, that he got ~280 groups (of which the 3rd shot in each was brutal) out of that barrel, is impressive enough. But it was burned out, and would never shoot the same again..
He could accept grouping that is not as good, or start over with a new barrel.

 

[J E Custom]

My understanding of the expansion ratio is that a longer barrel will have more volume of space allowing the powder to expand more times with a given charge (Powder expands 12,000 to 15,000 times its original volume depending on the type and chemistry of the powder).

We use longer barrels to get a higher expansion ratios, generating higher velocities with the same powder charges. with longer barrels we can increase the powder charge with more slow burning powder increasing velocity even more. This is where the large "Overbore" cartridge comes into it's own.

Erosion of the barrel at the throat is caused by the un-burnt powder leaving the cartridge cases and entering the throat before it is consumed (The military uses ball powder to minimize the effect of tubular powder erosion) Over bore cartridges typically have more powder than can be fully burned in the case so they tend to be more erosive and longer barrels increasing velocity but decreasing barrel life.

There seams to be a connection with the velocity also. The military typically try's to keep the velocity below 3,000 ft/sec to improve barrel life.

So In order to get more velocity, we use larger cases that will hold huge amounts of powder to get the velocity up without raising pressures to high, and longer barrels to consume the powder charge
with a higher expansion ratio to get the most of the powder charge.

The down side is, More powder, higher velocities = less barrel life.

Just my understanding of over bore.

J E CUSTOM

 

[Mikecr]

An attempted standard for 'Overbore' as a consideration:
Defining "Overbore" Cartridges via Comparative Index « Daily Bulletin

You can try to adjust the notion all you like, but barrel life does follow powder load to cross sectional bore area. So cartridges like 220 swift, 243win, 25-06, and 6.5x284 are relative barrel burners. All overbore.
Hardest to define is underbore. Seems easiest to at least observe these manifesting in competitive short-mid range capacities. 30br, 6PPC, 6br, 6.5x47L.
These fall into the WHITE band of table linked. I see these as underbore.

I consider the 6Dasher YELLOW band.
Where I would design a 1kyd cartridge, it would always fall into yellow. My <~600yd cartridges would always fall into white. Orange band cartridges are never really viable for LR competition.

For hunting(actual hunting) cartridges, barrel life doesn't matter directly, but reloading potential still does. This will drive the moderate white/yellow band cartridges to best again.
You may feel really powerful with a 264WM, but the truth is no ballistic attribute is more powerful than accuracy.
And accuracy is not fleeting, it's what you can maintain.

 

[Scot E]

I admit I am not an expert in this line of thinking, mostly because I shoot hunting rifles therefore I care little about barrel life, or at least it is so far down the list of priorities that it rarely comes into play when making a decision on cartridges. But I have found some things to be true in my experience and research.

Barrel length does matter if the loader makes the appropriate changes in powder and uses a slower powder in the longer barrels. Slower powder, less pressure spike, less overall pressure, so less wear on the barrel.

Also, I believe there is more to barrel life than just the static pressure created. I do buy into the flame path argument that some cases provide over others. Just seen too many examples that seem to prove this out.

The boys at GAP are adding some interesting data to this as well with their new 6.5SAUM 4S. It is well overbore but they are getting crazy good numbers out of a barrel and this is from many barrels already. Their conclusions is that it's from case design and keeping pressures low even though it is a very overbore cartridge.

I think you are going to get yourself in trouble if you try to explain this with just one baseline cause.

I'm tuning in to hear from the experts.