With all the discussion about the relative over bore cartridges

[jrsolocam]

In this forum, I thought I'd expand the list that 6mmbr.com compiled awhile back. Simply the case capacity divided by the cross sectional area of the bore. 6mm, 6.5mm, 7mm, 308 and 338 calibers chosen, with popular long lange rounds shown. Before anyone says the capacities are wrong, they are taken from QL. Yes there are other variables at work here, but this gives an apples to apples look at it. I think other measures of efficiency could be combined with this and give insight into cartridge design and analysis. Length of powder column, energy efficiency etc.

Seems, relatively speaking, the 6.5x284 isnt the barrel burner that people say...

 

[WeiserBucks]

Speed sells , many shooters aren't concerned with barrel life or efficiency.

 

[FEENIX]

I'll take accuracy over velocity any time.

 

[HARPERC]

.....I'll take accuracy over velocity any time......

I know you understand this, but there are devil's in the details. What's the minimum velocity (make it at the target), against how much accuracy you need at that target distance, target size, as well as target type.

I have a good bolt action 7.62X39, side by side with my .300 Winchester Magnum, or .30-06 it generally groups better at 100 yards. Move to 400 yards and the other 2 begin to come into their own.

Picking 2 magnums, the .300 Winchester vs say the .300 RUM, or .30-378. Most of us give up nothing with a quality .300 Winchester, but as the long range game evolves, with heavier bullets such as the rumored 245 grain Berger, the bigger cases begin to come into their own.

Rifle style and optics also play into it. I read somewhere the Applied Ballistics has a 50lb rifle, with longer barrels the big cases again begin to shine brighter. 45X scopes, useless to me, also play a role.

The flip side is, in a 6 lb rifle, 16" barrel, not only would a .30-378 not be helpful to me, I wouldn't shoot the **** thing.

Barrel life is a consideration, but not to the degree it appears to bother some folks. When I had only one rifle it was a factor, but that situation was corrected a long time ago. We live in a country where (mostly) we're allowed as many rifles, for as many games as we care to play.

Finally, to my knowledge a standard for barrel life has not been established. Neither has the amount of accuracy loss be determined. Is a 0.25 MOA rifle, becoming a 0.5 MOA rifle cause for concern? In some disciplines the 0.25 wont likely bring home any trophies.

Link didn't work for me.

 

[FEENIX]

I know you understand this, but there are devil's in the details. What's the minimum velocity (make it at the target), against how much accuracy you need at that target distance, target size, as well as target type.

I have a good bolt action 7.62X39, side by side with my .300 Winchester Magnum, or .30-06 it generally groups better at 100 yards. Move to 400 yards and the other 2 begin to come into their own.

Picking 2 magnums, the .300 Winchester vs say the .300 RUM, or .30-378. Most of us give up nothing with a quality .300 Winchester, but as the long range game evolves, with heavier bullets such as the rumored 245 grain Berger, the bigger cases begin to come into their own.

Rifle style and optics also play into it. I read somewhere the Applied Ballistics has a 50lb rifle, with longer barrels the big cases again begin to shine brighter. 45X scopes, useless to me, also play a role.

The flip side is, in a 6 lb rifle, 16" barrel, not only would a .30-378 not be helpful to me, I wouldn't shoot the **** thing.

Barrel life is a consideration, but not to the degree it appears to bother some folks. When I had only one rifle it was a factor, but that situation was corrected a long time ago. We live in a country where (mostly) we're allowed as many rifles, for as many games as we care to play.

Finally, to my knowledge a standard for barrel life has not been established. Neither has the amount of accuracy loss be determined. Is a 0.25 MOA rifle, becoming a 0.5 MOA rifle cause for concern? In some disciplines the 0.25 wont likely bring home any trophies.

Link didn't work for me.

Aye Sir, I do!

 

[AGL4now]

Why are "Carbide Forcing Cones" not used to extend barrel life....???

 

[FEENIX]

Why are "Carbide Forcing Cones" not used to extend barrel life....???

My best WAG is supply and demand and $$$$$$$.

 

[Snyper708]

Why are "Carbide Forcing Cones" not used to extend barrel life....???

Most shooters will never fire enough rounds to burn out a barrel.
Those who do are a tiny percentage of buyers.

More barrels are ruined from improper cleaning techniques than from rounds fired.

 

[RockyMtnMT]

Chart is very interesting compared to the campfire lore that is thrown out all the time. As the op said, if the 6.5x284 is a barrel burner then so is just about every other long range hunting cartridge.

Guess we all have to stick with 308 win.

Steve

 

[J E Custom]

Just though I would add a point to this thread with the definition of what "Over Bore" means.

PO Ackley Was heavy into this condition and performed many test to analyze and understand it. Here is His definition of Over bore.

The ratio between bore and cartridge case capacity is a subject about which more is heard these days than ever before. The term "Over Bore Capacity" is applied to many cartridges which is to say that an extremely large cartridge holds more powder than the bore can possibly efficiently handle.
Powley ties up bore capacity with expansion ratio. 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 mean good barrel life. Low expansion ratios mean short barrel life. Low ratios may result in higher velocities but in many instances this slightly higher velocity is more than offset by the greatly reduced barrel life, critical loading and general flexibility. (quote).

There are some cartridges that could fall above or below this condition depending on the type and amount of powder used. There are many formulas that attempt to quantifie this condition and they will get you close, but there are so many variables that they are only close.

The main thing is to completely burn all of the powder before it leaves the muzzle. An efficient case design burns this same powder/charge sooner and reduces the amount of un burnt powder forced through the throat, adding to the erosion of the throat and barrel.

Huge Case capacities need longer barrels to contain the amount of gas longer because they also need slower burn rate powders to control the pressures, the larger the powder charge the more un burnt powder will leave the case and increase the erosion of the throat and barrel.

There is no free ride.

J E CUSTOM

 

[Canhunter35]

Why did the 264 win mag have a reputation of a barrel burner and become obsolescent for many years while the 257 wby thrived?
Does the wby shoulder reduce throat erosion vs the 20(?) degree shoulder of the 264?

 

[Barrelnut]

Why did the 264 win mag have a reputation of a barrel burner and become obsolescent for many years while the 257 wby thrived?
Does the wby shoulder reduce throat erosion vs the 20(?) degree shoulder of the 264?

Seems general consensus is that the 7RM is what did the 264WM in. Just a better all round cartridge with more bullet weights for larger game. 7RM is somewhat less of a barrel burner too, but the factory 264 WM was also just a 9 twist which pretty much limited to to 130 Gr bullets.

 

[257Tony]

I didn't see 28 Nosler on the list , curious where it falls. Also would be cool to somehow work in relative burn rate of powder used to the equation, as that should have an impact also. Interesting list tho!

 

[jrsolocam]

My version of QL didnt have the 28 listed. Guessing about 99gr, would be lower than 26, and similar to 7stw. Another thing im looking at is KE=PE (kinetic energy = potential energy). An efficiency rating...if a a given charge of a given powder in a given cartridge yields x energy at a given distance how efficient us it?? We need to shake the dust off our physics books...If anyone finds this interesting please chime in.