Smaller Faster or Higher BC

ExAgoradzo said:

Where could I start reading to learn about this subject?

I have in mind for example a 7mm projectile (for illustration it really doesn’t matter what cartridge, but as I own a Mauser we’ll use that). Is it flatter (the variable I am after here) to shoot the 120gr faster or a higher BC bullet slower (all other things equal which we know of course they never are)?

I do not have the means to do the testing myself, but I’m certain I’m not the first to think about this problem. So, where should I go to read about such things??? Is there a rule of thumb? Is the difference really worth worrying about under normal ranges (normal meaning well under 300 yards—I suspect it is not, but the thought experiment needs to start somewhere...)? Perhaps, gasp, the answer is whatever the ‘standard’ loading for that cartridge?

thanks,
Greg

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I calculate on wind and max Sonic range, my stw runs better with a high bc 162 than a 175 but this is not always the case, likewise my .243 running a 90 tgk beats many heavier offerings, download a ballistic calculation app, play with numbers and come up for air occasionally.

Go to the hornaday website and play with the ballistic calculators. Free and available

Another option here. You can compare two cartridges at the same time and input wind speed, BCs, velocity, height over bore (optic mounting) and set what total range you want information for (up to 1000yds I believe). You could put in the numbers for a 120gr in one and numbers for a 150gr or 162gr in the other and it will load up the velocity, energy, drop and drift in pre set intervals. I would venture a guess that the 120 is going to be "flatter" out to about 400-500yds.
A note of caution: A guy could waste ALOT of time on these type of websites and convince himself he needs a 280ackley or some such thing, you've been warned.

A very good question, and possibly something best found out with first hand experience.

I have loaded all my 6.5x47 Lapua rounds using either the 123gr Scenar or A-Max bullet (90% Scenar). I looked into using the 139gr Scenar as it has a higher BC and I thought this would be a more stable round for long distance shooting, but when I made up some drop & windage tables using my rifle configuartion I couldn't see much benefit at 1,000 yards. Sure there was slightly more drop and lower velocity which is expected for a heavier bullet, but there didn't seem much difference in windage at that distance. So what was is the point in using this slightly heavier round I thought?

I didn't get the chance to try it out at long distance due to range closure etc so unfortunately I don't have a solid answer for you or myself. I wonder if Nature Boy has real world experience of this...

Good responses so far. One key thing to keep in mind is that the bullet/load that is fastest at 100 yards may not be the same as the one that is fastest at 1,000 yards. Broadly speaking, those heavy, high-BC bullets will start slower, but maintain velocity better over longer distances.

But once you get out to those longer distances, you are going to have enough drop that you need to use some kind of dope to get hits. So "flat" isn't really the right word... total time of flight reduction (which is the big determinant of wind drift sensitivity) and pushing out the yardage at which the transonic threshold is reached (causes instability and consequent serious loss of accuracy when a bullet slows enough to be transonic) are the main benefits to that high-BC start-slow-stay-fast approach.

When people talk about things being "flat," what they usually are referring to is pushing out the "point blank" distance. Contrary to cops shows and movies, "point blank" actually means the distance at which targets can be hit with no holdover/drop calculation. If you define your target as being a 3" circle (maybe to approximate the reliable kill size of a certain animal you're hunting), then a gun is "flat" to, say, 300 yards if it is possible to zero the gun to keep a bullet within that 3" circle all the way from short range to 300 yards.

Lighter, faster bullets will often allow that "flat," point blank range to be further than the higher-BC bullets that will be more stable and faster at, say 600 yards. That kind of "flatness" is potentially useful if you're planning to take shots at variable distances and without the time or tools to do rapid, accurate ranging and dialing for drop.

Is the difference really worth worrying about under normal ranges (normal meaning well under 300 yards

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No. Lots of 100 and 200 yard benchrest matches have been won with bullets that don’t have the most impressive BC. They don’t really worry about “flatter” or MPBR just shooting small groups.

What are your expectations?

Might take a few different BC’s, speeds and plug them in here https://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
And you can see mathematically how they compare.

You were correct: MPBR is the right way of thinking about this for me. I just needed to get that.

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That makes it easy, what is the elevation range you want to stay inside at what ranges?

Archie said:

Two somewhat disparate concepts to consider.
During the world wars, the United States and Germany designed a load for their machineguns, even though those machineguns used the same 'caliber' of ammunition as the infantry rifle. In both instances it was heavier for use in enfilading enemy held territory. Those heavier bullet loads were slower at the muzzle but held velocity better and had more range - distance from firing point - than the infantry round.

Extreme range does not matter if the game, terrain, or shooting ability limit one to 200 meters or less. (Which is most of us mortals.) That is the big difference between long range target shooters and hunters. Maximum Point Blank Range is the answer. I'm a heavy bullet aficionado, by not to attain range.

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You must have realistic expectations while shooting off hand

At 300 yards there isn't enough difference in trajectory to worry about. But BC is about more than flat trajectory. Granted this is an extreme example, but if we shoot a 180 gr bullet with the worst BC from a 300 WM @ 3000 fps, and shoot one of the highest 180 gr BC bullets from a 30-06 at 2800 fps the 30-06 bullet will surpass the 300 WM speed at only 75 yards and hit harder. The greater the distance the bigger the edge goes to the 30-06. But the 300 WM will still shoot a tiny bit flatter until past 300 yards.

Shoot the same high BC bullet from a 308 and its faster than 300 WM at 175 yards and is hitting at much greater speed and energy out at 300. The advantage of high BC bullets is that it allows you to shoot smaller, lighter recoiling rifles without sacrificing bullet impact speeds downrange.

A more realistic comparison would be to compare your 270 to the 6.5 CM. There is only .3mm difference in diameter and in the same bullet weights 270 is about 200 fps faster at the muzzle. But the higher BC 6.5 bullets will catch up in speed at around the 300 yard mark. While hitting the shooter with about 1/3 less recoil. Yes, the 270 has more energy inside 300 yards, but either has more than enough.

I haven't run the numbers, but your 270 with modern high BC bullets would probably pretty much match 7mm Mag performance at 300 yards and beyond if you were comparing old school bullets fired in the 7 mag.

I'm not a fan of MPBR. I think it is an outdated method. I zero at 100 yards. At 200 yards bullet drop from anything I own (308, 6.5 CM, 30-06) will be around 2". At 300 yards if I can see a sliver of daylight between the top of any game animals back and the horizontal cross hair I'll still hit the kill zone. No real hold over necessary.

MPBR doesn't extend usable range much farther anyway. But if you start zeroing so that your rifle is 2-3" high at close ranges you just complicate things at the ranges where you will likely take 99% of your shots while gaining no real advantage at 300. If you're going to try to shoot much beyond 300 yards bullet drop begins to be pretty steep with even magnum rounds. You need to know the exact range and have a scope with a method of compensating for the drop. Either twisting dials or with multiple aiming points. And all of that is calculated with a 100 yard zero.

When guys start trying to extend MPBR out to 350+ yards they end up with a rifle that is hitting 6-7" high at close range. It is a lot easier to compensate for a bullet hitting 6-7" low than 6-7" high. It just isn't natural to have to remember to hold low at close range.