Home Forums Hunting & Shooting Ask The Gunwriters Why is .264 higher BC than .277 and then why is .284 higher than .277 too?

I tried to compare near identical sectional densities of bullets across 6.5mm, .277 and 7mm. For near same S.D., the B.C.'s vary by a much greater percentage?!?

SO, both the .264 shooting 130gr bullet (sd=.266) vs .277 shooting a 140 gr bullet (sd=.261). Total difference in SD is only about 2% higher for the .264 YET, the .264 has about a 22% HIGHER B.C than the .277!! 0.552 vs 0.452!

The same can be said of the .284 vs the .277. Looks like the "270 anythings" are worst performers between the two. Why? I'm confused.


6.5 MM
130 gr Match Grade VLD Hunting 0.552
140 gr Match Grade VLD Hunting 0.612

270 Caliber
130 gr Match Grade VLD Hunting 0.452
140 gr Match Grade VLD Hunting 0.487
150 gr Match Grade VLD Hunting 0.531

7 MM
140 gr Match Grade VLD Hunting 0.510
168 gr Match Grade VLD Hunting 0.617

Revealing, isn't it?

bullet companies could make a higher BC .277 bullet. for whatever reason they just don't. .257 suffers the same fate.

If one compares apples and oranges the difference might be ostriches.

SD comparisons are valid when all else is equal. .277 does not equal .284.

So, if I'm reading this right, the .277 bullets have a lower BC, which means they create more turbulence, which means an area of low pressure behind the bullet - a vacuum. So, then, that means .277 bullets suck.

It's all because the .270 sucks.....

Kaiser Norton

okbob51,
not an arguement, looking to learn here, what would bullet manifactures have to do to the 257 and 277 bullets to increase the BC?

Shrink them by 0.013" diameter.

I'm guessing then would have to increase twist first on factory rifles and still you would have a lot of rifles out there that wouldn't like the new bullets. Rifles that work just fine with what they have been shooting for years.

Change the shape of the bullet.

For example, if you check the TSX bullets on the Barnes site, the 130 gain .277 TSX has a slightly lower SD than the 140 grain .284, yet the .277 has the higher BC.

Make them heavier, add a pointed plastic tip, and a long boattail. I would love a 140gr 257 bullet. Load it in a necked-down 308 case and build light rifles with 8-twist barrels to shoot them in.

That was done quite some time ago and far more aggressively. Known as the Barnes QT cartridges which included a 150 gr .257 as I recall. Used a 1:5.5" twist.

I-phone short version: BC=SD/I I=form factor

For a given form BC is proportional to SD. For the same form and weight BC is inversely proportional to diameter. As I recall. I don't much care for I-phones and their BC sucks.

Agree'd

This ain't rocket science...6.5's and 7mm's started as military caridges of modest velocities loaded with very long bullets that required fast twists to stabilize them.Like many military cartridges,they were also use for target shooting, the heavy weights were retained,along with the faster twists.Obviously they were also used for hunting.

Bullets got better,velocities got higher; the fast twists were retained.

OTOH, the 270 had no military nor target history; the Winchester version was designed as a hunting cartridge with a 10 twist that adequately stabilizized the relatively short 130 gr C&C bullets originally designed for it. High velocity was its' forte.It also stabilized 150 gr spitzer bullets.These two weights proved fine for a majority of the hunting for which the cartridge was suited.

No one made very high BC bullets for the caliber because most 270's have a 10 twist, and the cartridge has no target or military history.Very long, sleek 270 caliber bullets of high BC could be made(one company is making them now),which would put it on a par with the 6.5's and 7mm's,although the slightly larger bore diameter of the 7mm's likely makes them the best of the bunch.It shoots heavier bullets...Even as it is,though,you gotta sift through a lot of gack to prove the 270 truly "sucks".

Given cases of equal capacity and bullets in the same weight range at similar velocities,,the 270 kills BG animals as well as comparable 6.5's and 7mm's.Anyone who thinks otherwise, is delusional, or has no experience shooting animals with it..


I stongly suspect the 270 is "better" than many of its' detractors.If you can't do it with a 270, you likely can't do it with a comparable 6.5 or 7mm either.

Not even a Mashburn?

dt: Why I said "comparable"

Honestly, we all know that certain 6.5 and 7mm bullets have very high BC's and obviously have an advantage.....but the bulk of them have BC's that are about the same as 270 bullets,which is why I don't understand all the comparisons sometimes.

Great comments as usual Bob. All of the gratuitous swipes at the 270 by the usual gang of 'haters' aside, there is an interesting point raised in the OP regarding bullet design. Your points about faster twists and heavier (for calibre) bullets are well taken, and as you've essentially said it is pretty obvious that the 270 isn't going to excel at something it wasn't designed to do well in the first place. However, let's try to eliminate the issue of heavier bullets for the moment. If we consider only the 140 grain bullets in each of .264, .277 and .284 from the OP, there is still an interesting phenomenon. Obviously, at the same weight, the .264 will have the highest SD, and so it comes as no surpise that, given the 'same' bullet design, it has the highest BC as well. However, the .277 has a slightly higher SD than .284 and yet the BC of the .284 is actually a bit higher. It would be interesting to hear some comments from those 'on the inside' with respect to bullet design on why that might be.

Take a 270 bullet a 6.5X55,30/06,6.5/06,280 bullet throw them in a bag,shake them up,and pull one out.Does not matter which one,they will all do the same job.Reading ballistic coefficients out of a Reloading Manual is meaningless as they change with velocity.