Riddle me this Batman. BC applicability question
- Thread starter SteveOak
- Start date
Lol
My suggestion, pick an easy to to get cartridge that’s not insane over bore, combine good bullets with a good barrel and some load development and you’ll be smiling
I’d pick 7mm rem mag over a obscure wildcat.
Get something that brass and dies aren’t an issue.
Anything from 7prcw to 7prc case capacity is perfect.
Same with the 30’s, 300 WSM to 300 Norma is perfect
338’s, 338 Norma to 338 Lapua improved is perfect.
The real benefit of 7mm’s is the mild recoil and excellent ballistics for the cost.
That .350 to .378 G7 bullet at 2800-3000 punches far above it’s weight.
Now that's a firing solution! The rock stars start getting separated from the groupies at around 30 Mils.
When calculating drop and drift based upon inputted BC, you don’t even have to plug in weight and caliber.
While the calculator may prompt those fields, they make no difference to drop or drift, anything you type in is ignored, and the same result in drop and drift is obtained. ( Retained energy or ME, yes, the fields matter, but these are irrelevant to shooting paper).
BC is a true coefficient of drag and is therefore “net” of those two variables, /weight and diameter.
This is why BC and Velocity are so critically important to our long range shooting choices.
These two variables, BC and velocity, alone are both necessary and sufficient to tell us where to aim at any distance. They are the Red, Yellow and Blue to shooting, from which everything else derives.
Like the three primary colors, no other refinements we make to rifles and ammo can replace these. Refinements affect group “size” and only size, but BC and velocity determine “where” the group will be, and they alone determine that.
A pinnacle .22LR, unlimited and more expensive than any Fclass rifle, with the finest ammo that can be made will fall profoundly at 1,000 yard match to a factory .223.
While the calculator may prompt those fields, they make no difference to drop or drift, anything you type in is ignored, and the same result in drop and drift is obtained. ( Retained energy or ME, yes, the fields matter, but these are irrelevant to shooting paper).
BC is a true coefficient of drag and is therefore “net” of those two variables, /weight and diameter.
This is why BC and Velocity are so critically important to our long range shooting choices.
These two variables, BC and velocity, alone are both necessary and sufficient to tell us where to aim at any distance. They are the Red, Yellow and Blue to shooting, from which everything else derives.
Like the three primary colors, no other refinements we make to rifles and ammo can replace these. Refinements affect group “size” and only size, but BC and velocity determine “where” the group will be, and they alone determine that.
A pinnacle .22LR, unlimited and more expensive than any Fclass rifle, with the finest ammo that can be made will fall profoundly at 1,000 yard match to a factory .223.
One of the more horrifying realizations you'll have about ELR cartridges is that 100 fps costs about 1/3 the barrel life and only moves the target 75 yards closer.
I started putting this together about 10 years ago to give a better perspective on the tradeoffs between case capacity and ELR scoring.
The names in red are cartridges I've tried with the exception of the Cheytac cases. Those were tested by a a shooting buddy who also chambered his own barrels. The yellow background are the barrels we've run to retirement. This is a little out of date. Use the 7RM mag for the 7PRC. I've retired a 6XC barrel since I've updated this.
In general, cartridges with the same overbore ratio (case capacity in grains of water / barrel cross sectional area in square inches) will give very similar performance with the same class of bullets. The larger calibers do perform slightly better but not so much that minor differences in conditions won't completely obscure any advantages.
Most of the ELR world has decided that cartridges with overbore ratios above the 416 Barrett, 375 Cheytac and 33XC are just not worth the trouble. More or less 100 fps below that are the 300 Norma and 338 Lapua Improved. 100 fps below that are the 37XC, 338 Lapua, 300WM, and 7WSM.
From the shortest barrel life in those groups to the longest over half the barrel life is cashed out to move the target 150 yards closer. I'm using around 2000 yards as the reference.
If the 7mm stuff is picked out we have:
7rsaum - 73 grains of water capacity
7wsm and others including 7PRC between 78-82 grains
7LRM - 89 grains
7/300PRC - 94 grains
That 88-90 grain capacity in 7mm refuses to stay filled for some reason. It's been occupied by 7 Weatherby Mag, 7 LRM, and 7booboo. None of them are currently readily available.
There really isn't a wrong answer here. The biggest step is picking one and getting on with the lessons that will only come from shooting.
When 1 mile was the goal, the argument for the 7rsaum was pretty strong. Right now, for an ELR beginner that wants to minimize recoil but has 2500 yard aspirations, the 7PRC looks good. In addition to readily available brass and bullets, pick a barrel you can buy more or less on demand from a barrel broker. Currently that means 8 twist and 30". It's the cheapest place to add weight. I use heavy varmint contours. It's a lot easier to find low SD loads with a big cartridge loaded soft than a smaller case loaded to much higher pressures. When you finally figure out how to sort, cull, and prep brass for low velocity spread ammo, it's just better if it lasts more than 3 firings. The hybrids and Atips will not take the pressures that boutique brass will and still give consistent BCs.
I started putting this together about 10 years ago to give a better perspective on the tradeoffs between case capacity and ELR scoring.
The names in red are cartridges I've tried with the exception of the Cheytac cases. Those were tested by a a shooting buddy who also chambered his own barrels. The yellow background are the barrels we've run to retirement. This is a little out of date. Use the 7RM mag for the 7PRC. I've retired a 6XC barrel since I've updated this.
In general, cartridges with the same overbore ratio (case capacity in grains of water / barrel cross sectional area in square inches) will give very similar performance with the same class of bullets. The larger calibers do perform slightly better but not so much that minor differences in conditions won't completely obscure any advantages.
Most of the ELR world has decided that cartridges with overbore ratios above the 416 Barrett, 375 Cheytac and 33XC are just not worth the trouble. More or less 100 fps below that are the 300 Norma and 338 Lapua Improved. 100 fps below that are the 37XC, 338 Lapua, 300WM, and 7WSM.
From the shortest barrel life in those groups to the longest over half the barrel life is cashed out to move the target 150 yards closer. I'm using around 2000 yards as the reference.
If the 7mm stuff is picked out we have:
7rsaum - 73 grains of water capacity
7wsm and others including 7PRC between 78-82 grains
7LRM - 89 grains
7/300PRC - 94 grains
That 88-90 grain capacity in 7mm refuses to stay filled for some reason. It's been occupied by 7 Weatherby Mag, 7 LRM, and 7booboo. None of them are currently readily available.
There really isn't a wrong answer here. The biggest step is picking one and getting on with the lessons that will only come from shooting.
When 1 mile was the goal, the argument for the 7rsaum was pretty strong. Right now, for an ELR beginner that wants to minimize recoil but has 2500 yard aspirations, the 7PRC looks good. In addition to readily available brass and bullets, pick a barrel you can buy more or less on demand from a barrel broker. Currently that means 8 twist and 30". It's the cheapest place to add weight. I use heavy varmint contours. It's a lot easier to find low SD loads with a big cartridge loaded soft than a smaller case loaded to much higher pressures. When you finally figure out how to sort, cull, and prep brass for low velocity spread ammo, it's just better if it lasts more than 3 firings. The hybrids and Atips will not take the pressures that boutique brass will and still give consistent BCs.
One of the more horrifying realizations you'll have about ELR cartridges is that 100 fps costs about 1/3 the barrel life and only moves the target 75 yards closer.
I started putting this together about 10 years ago to give a better perspective on the tradeoffs between case capacity and ELR scoring.
The names in red are cartridges I've tried with the exception of the Cheytac cases. Those were tested by a a shooting buddy who also chambered his own barrels. The yellow background are the barrels we've run to retirement. This is a little out of date. Use the 7RM mag for the 7PRC. I've retired a 6XC barrel since I've updated this.
In general, cartridges with the same overbore ratio (case capacity in grains of water / barrel cross sectional area in square inches) will give very similar performance with the same class of bullets. The larger calibers do perform slightly better but not so much that minor differences in conditions won't completely obscure any advantages.
Most of the ELR world has decided that cartridges with overbore ratios above the 416 Barrett, 375 Cheytac and 33XC are just not worth the trouble. More or less 100 fps below that are the 300 Norma and 338 Lapua Improved. 100 fps below that are the 37XC, 338 Lapua, 300WM, and 7WSM.
From the shortest barrel life in those groups to the longest over half the barrel life is cashed out to move the target 150 yards closer. I'm using around 2000 yards as the reference.
If the 7mm stuff is picked out we have:
7rsaum - 73 grains of water capacity
7wsm and others including 7PRC between 78-82 grains
7LRM - 89 grains
7/300PRC - 94 grains
That 88-90 grain capacity in 7mm refuses to stay filled for some reason. It's been occupied by 7 Weatherby Mag, 7 LRM, and 7booboo. None of them are currently readily available.
There really isn't a wrong answer here. The biggest step is picking one and getting on with the lessons that will only come from shooting.
When 1 mile was the goal, the argument for the 7rsaum was pretty strong. Right now, for an ELR beginner that wants to minimize recoil but has 2500 yard aspirations, the 7PRC looks good. In addition to readily available brass and bullets, pick a barrel you can buy more or less on demand from a barrel broker. Currently that means 8 twist and 30". It's the cheapest place to add weight. I use heavy varmint contours. It's a lot easier to find low SD loads with a big cartridge loaded soft than a smaller case loaded to much higher pressures. When you finally figure out how to sort, cull, and prep brass for low velocity spread ammo, it's just better if it lasts more than 3 firings. The hybrids and Atips will not take the pressures that boutique brass will and still give consistent BCs.
I agree with your major point of marginal benefits and a steepening cost slope when increasing velocity, in other words diminishing returns.
However I think your chart, similar to or the same as others I have seen, needs to use bore volume, since cartidge “volume” is being used.
It’s roughly the difference between squaring and cubing. The 50 BMG on these charts rates as extremely over bore.
The 50 is between a scaled up .308 and 30-06, neither of which is over bore. Maybe even exactly a 30-06, I seem to recall.
Anyhow, I think it is skewed in the chart. If a quarter bore radius is denoted as “1” then a 50 is “2”. The “cross section” area of these two is 1:4. The 50 is 4 times the area in cross section. However the 50 has 8 times or roughly that, of the bore volume of a .25 caliber barrel, same length. That will no doubt change things.
Last edited:
FeMan
Gold $$ Contributor
Wouldn't using bore volume mean that barrel length was a factor in the consideration of "overbore"? Am I missing something?
The ratio of volume between the two bores won’t change, with the length selected for the barrels. The 50 will always hold 8 times the volume of the 25, at any given length chosen for them.
I admit that I’m looking for something formulaic to reconcile the chart. In my mind a 30-378 is far more overbore than a 50 BMG, with a shorter barrel life, (have both) but the chart doesn’t show that, as one example.
Steve Donlon
Gold $$ Contributor
Exactly like a 30-06. That's why I love the 30-06, It just looks right.
The overbore ratio is intended as a proxy for how long the throat spends at high temperature. I didn't invent it, I've just populated the table with data from the 2 dozen or so barrels I've been through in the last 10 years. I did a string of 3 wildcats based on the 338 lapua case with the idea of seeing how far the overbore could be taken and how to model them in QL. Using the same case and measuring the extractor groove expansion on the first firing kept the peak pressures similar from cartridge to cartridge. Then added another 6 Cheytac barrels, 3 in 338 bore, 3 more in 375, that I know were used similarly. That'd be strings of 10-15 at a 20-40 second cadence. A pretty standard ELR cof.
The temperature tracks with pressure. A larger case is a higher volume tank for the pressure, a smaller bore is a slower leak. It's not intended to be a dimensionless number like Mach. It'd have units of length if the volume and area were converted to common units. Leaving the inputs in their common format makes them more intuitive and errors easier to spot.
Another analogy is holding your hand over a candle. Larger cases in the same bore are like holding your hand over the candle longer.
This is a common entry point into a barrel life calc. It's not the final word. Single vs double base powders, peak pressure, string length, and shot cadence all heavily influence barrel life. Your 50 barrel may last a lot longer. I've spent way too much time in the woods with a 50bmg bench rest shooter and I'm absolutely certain he'd laugh at the idea of a 50bmg bench gun with the barrel life of a 30/06.
With the leaking tank analogy in mind, let's look at the 30/06 50bmg scaling thing.
Bore diameter ratio - 1.66
Case head diameter ratio - 1.71
Case length ratio - 1.57
The length ratios check.
Bullet weight - 647/150 = 4.3
This usually scales to volume, or cubed. 150 x (.510/.308)^3 = 681. Pretty close
The case volumes are (300/68) = 4.4
The bore areas scale to (0.511/.308)^2 = 2.75
So the tank grows faster than the leak does when case length and diameter are scaled up to bore.
The temperature tracks with pressure. A larger case is a higher volume tank for the pressure, a smaller bore is a slower leak. It's not intended to be a dimensionless number like Mach. It'd have units of length if the volume and area were converted to common units. Leaving the inputs in their common format makes them more intuitive and errors easier to spot.
Another analogy is holding your hand over a candle. Larger cases in the same bore are like holding your hand over the candle longer.
This is a common entry point into a barrel life calc. It's not the final word. Single vs double base powders, peak pressure, string length, and shot cadence all heavily influence barrel life. Your 50 barrel may last a lot longer. I've spent way too much time in the woods with a 50bmg bench rest shooter and I'm absolutely certain he'd laugh at the idea of a 50bmg bench gun with the barrel life of a 30/06.
With the leaking tank analogy in mind, let's look at the 30/06 50bmg scaling thing.
Bore diameter ratio - 1.66
Case head diameter ratio - 1.71
Case length ratio - 1.57
The length ratios check.
Bullet weight - 647/150 = 4.3
This usually scales to volume, or cubed. 150 x (.510/.308)^3 = 681. Pretty close
The case volumes are (300/68) = 4.4
The bore areas scale to (0.511/.308)^2 = 2.75
So the tank grows faster than the leak does when case length and diameter are scaled up to bore.
I was off on the volumetric ratio, FeMan. Pi cancels, height cancels and the ratio remains 4:1 when you double the radius from one barrel to another. 8:1 is not right; it was the ratio of the volume of a 2 inch block to a 1inch block, but the 2 inch block is twice as tall.
Last edited:
Just to be clear, the Berger website says that both of these G7 BC's are .356, not .365...
SteveOak
Gold $$ Contributor
Thank you
One of the more horrifying realizations you'll have about ELR cartridges is that 100 fps costs about 1/3 the barrel life and only moves the target 75 yards closer.
I started putting this together about 10 years ago to give a better perspective on the tradeoffs between case capacity and ELR scoring.
The names in red are cartridges I've tried with the exception of the Cheytac cases. Those were tested by a a shooting buddy who also chambered his own barrels. The yellow background are the barrels we've run to retirement. This is a little out of date. Use the 7RM mag for the 7PRC. I've retired a 6XC barrel since I've updated this.
In general, cartridges with the same overbore ratio (case capacity in grains of water / barrel cross sectional area in square inches) will give very similar performance with the same class of bullets. The larger calibers do perform slightly better but not so much that minor differences in conditions won't completely obscure any advantages.
Most of the ELR world has decided that cartridges with overbore ratios above the 416 Barrett, 375 Cheytac and 33XC are just not worth the trouble. More or less 100 fps below that are the 300 Norma and 338 Lapua Improved. 100 fps below that are the 37XC, 338 Lapua, 300WM, and 7WSM.
From the shortest barrel life in those groups to the longest over half the barrel life is cashed out to move the target 150 yards closer. I'm using around 2000 yards as the reference.
If the 7mm stuff is picked out we have:
7rsaum - 73 grains of water capacity
7wsm and others including 7PRC between 78-82 grains
7LRM - 89 grains
7/300PRC - 94 grains
That 88-90 grain capacity in 7mm refuses to stay filled for some reason. It's been occupied by 7 Weatherby Mag, 7 LRM, and 7booboo. None of them are currently readily available.
There really isn't a wrong answer here. The biggest step is picking one and getting on with the lessons that will only come from shooting.
When 1 mile was the goal, the argument for the 7rsaum was pretty strong. Right now, for an ELR beginner that wants to minimize recoil but has 2500 yard aspirations, the 7PRC looks good. In addition to readily available brass and bullets, pick a barrel you can buy more or less on demand from a barrel broker. Currently that means 8 twist and 30". It's the cheapest place to add weight. I use heavy varmint contours. It's a lot easier to find low SD loads with a big cartridge loaded soft than a smaller case loaded to much higher pressures. When you finally figure out how to sort, cull, and prep brass for low velocity spread ammo, it's just better if it lasts more than 3 firings. The hybrids and Atips will not take the pressures that boutique brass will and still give consistent BCs
A buddy ran 338 cheytac improved for a bit in light gun.
Spicy!
Similar barrel life as 7RUM at full boogie
My previous 338 barrel got to 1300 rounds before it went kaput running a spicy load of N570.
My current barrel is running 90 fps slower and with HBN coated bullets, I’ll be curious how long it lives with that.
Last edited:
The 338 Snipetac and 7/338 Lapua Improved would put jacketed bullets through a free hanging 1/4" mild steel plate at 1200 yards. Great fun while it lasted.
That table isn't intended to be a barrel life predictor for everybody's barrel. There are other significant modifiers and a given barrel might last longer than the last. Probably not twice as long, probably not half as long. It's only intended to assign a reasonably representative value using a simplified equation to that side of the case volume decision. It's centered on 300WM and falls apart much smaller than maybe 280 or larger than 26 Nosler? Thats the range of case capacity to bore ratios I'm interested in, so it's an effective model for that. I don't think it's a coincidence that the popular ELR rounds have similar values for that metric. I'm not just following the leader. I've tested that electric fence several times and the harder you push on it the more it stings.
Most people hang on to their barrels too long, particularly the first one. I've never been disappointed with a fresh barrel. Early on, I had barrels that were found to be blowing bullets up just before 200 yards. The 5 shot groups were still between 3/8 and 3/4". No lead mist on the paper either. Dropped the velocity 250 fps and it didn't help much. Maybe not pressure, maybe not rpm, I came to the conclusion it was most likely the throat.
With ELR, the barrel starts contributing significantly to BC variation as it ages. This happens long before bullets start bursting or groups open up. It's easy to have what I call a peaked in high school setup. A round that looks great from a velocity and BC standpoint, but the smaller rounds with fresher barrels just run roughshod over it beyond a mile. If the gun is shot at 1k or less, especially at the hobby level, it'll be many more rounds before the impact of BC spread is noticed.
I’ve noticed the barrel aging and growing BC variation before, it’s subtle at first then can get quite frustrating.
Kinda why I’m being a bit nicer to this barrel and backing off the gas.
2900fps and .485 G7 is still pretty fun.
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.