Best Bullet Weight/Bullet Diameter or Caliber

[Catpop]

I'm old school and was always lead to believe a certain bullet weight was best per a given caliber. This was "somewhat" substantiated by the offerings by ammo and bullet manufacturers.
Example: 30-06 best with 150 grain, 270 130 grain and 25-06 100 grain.
Does this have anything to do with Ballistic coefficient?
Is there any substantiation to this, or is it just something I maybe imagined or read into the picture?
Thanks, Catpop

 

[suemarkp]

It is not per caliber but per cartridge. There is a sweet spot for given bullet weights where you get the best balance of weight to velocity. The choices can be rather broad if you change powders, as you generally need a slightly slower powder as the bullet goes up in weight. It would probably be difficult for everyone to agree just where this sweet spot is or how to define it. I'd do it by comparing power factor (weight x velocity / 1000), with the highest power factor in the cartridge being the sweet spot.

If you look at the table in the back of every Gun Digest, you'll see a summary of most factory loads available. Looking at that chart you can see where the sweet spot is. In 30-06, it is around 180gr. In 308 it is around 168gr. In 270 Win it is 130 to 140gr.

Things like Hornady Light Magnum rounds can throw these tables for a loop, as they use a special loading technique to get more powder in the case and not go over pressure. If they offered the same technique in a wide range of weights you'd see a similar sweet spot, but they tend to only make one or two bullet weights per cartridge in the Light Magnum ammo line.

Ballistic coefficient (BC) is something that matters after the bullet has left the barrel (external ballistics) whereas the sweet spot is something that occurs in the gun (internal ballistics). BC tells you how fast a bullet will slow down, so once you find the best power and bullet combination to hit the sweet spot, you can choose the bullet of that weight with the highest BC to maximize downrange ballistics.

 

[rskent]

Yes and No

Your example of a 150 grain round being the best in 30 06 is great if you are an M1 guy. I love my M1. If you are a hunter with a bolt gun, it's probably not gonna be your first choice.
It would be hard to find many people that think that a 230 grain round is not best for a 45 ACP. I do have at least one gun that vastly prefers 185 grainers.
Again. Most people I talk to think in nine millimeter 124 grains is the way to go. I shoot 115's because I'm cheep.

So, yes and no.

 

[tightgroup tiger]

I was always under the impression that it was more the length of the bullet per given caliber that was most optimal for the rifling used in the most common barrel for that caliber. Like 124gr for 9mm, bolt action 30-06 was 168gr, 170gr for 30-30, 158gr for .357mag, and so on.

Length of bullet equates to weight of bullet of a given caliber. Their not going to grow in diameter by increasing the weight.

Again, that what I was led to believe when I was starting to reload 40 some years ago.

 

[gamestalker]

I'm old school also Catpop, and I was also taught about the general co-efficiency weights as they apply to each common caliber rifle bullets.

Two bullets, one heavier than the other, being driven at identical muzzle velocities, will produce two different trajectories at the same distances. This is because the heavier bullet will maintain a higher velocity longer than the lighter bullet, thus the heavier bullet does not slow down as quickly as the lighter bullet. thus it produces a better BC.

But there is a point in muzzle velocity, in which the lighter bullet, if driven at high enough velocity, it will produce a flatter trajectory, but only out to a certain distance. As an example, this is not an actual confirmed external ballistic estimate, as I don't have my ballistic charts in front of me right now. But for the sake of example, lets say a 40 gr. BT is leaving the muzzle @ 3900 fps, but a 55 gr. BT is delivering a muzzle velocity of 3500 fps., the 40 gr. BT will produce a superior, or flatter trajectory over the 55 gr. BT out to say 400 yards. But once those two BT's go beyond 400 yards, the 55 gr. BT will eventually reach a matched trajectory, as far how much it has dropped, that might be at 500 yds.. Then as the distance increases, the 40 gr. will have dropped more than the 55 gr. BT at the same distance.

Early on in my reloading hobby, I began to realize that the much higher obtainable muzzle velocity of a lighter bullet isn't always going to deliver the most desirable trajectory. Yes, out to a certain distance the lighter projectile is going to drop significantly less, but beyond that barrier, the heavier bullet becomes superior. And even though there are some high dollar range finders that deliver BDC estimates in either hold over, or are intended to correlate with the turret moa adjustment, it's absolutely essential to confirm all three, the load, range finder BDC estimates, and the BDC system on the optic in order to reap the full benefit of all those tools.

GS

 

[Bart B.]

What's best for a cartridge? If its accuracy, consider the following.....

Most commercial barrel rifling twists are based on age-old ideas of what works best or what the original bullet was for the cartridge. Best example is the .30-06.

In the beginning, the .30-03 used a 220-gr. bullet and a 1:10 twist so its mediocre muzzle velocity would stabilize it reasonably well for all ranges used. Same twist was used when the new version came out in 1906 with a 150-gr. bullet leaving faster. And again with the 172-gr. bullet starting in the middle 1920's. When the standard military bullet was changed back to 150-gr. bullets because the heavier recoil of the 172's was too much for the average soldier, the 1:10 twist was maintained.

Proper tests showed that the 150-gr. bullets at .30-06 muzzle velocities shot most accurate with a 1:13 twist. And the 172-gr. match bullets did best with a 1:12 twist. So did 200-gr. match bullets. 1:11 twists were popular if 168 or 180 grain match bullets were used in reduced loads as well as max loads with 190's or 200's.

That same 1:10 twist was also carried over to the 30 caliber magnums in commercial rifles because most customers felt that a 1:10 twist was best for 30 caliber rifles. Match rifles using these cartridges did best with 1:12 twist barrels.

Along came the 7.62 NATO round shooting the same bullet weights as the .30-06 some 100 fps slower and it's standard 1:12 twist was excellent for all bullet weights. Even 190's shot from 7.62 cases in 24" barrels with 1:12 twists was probably the most accurate long range load used in 7.62 NATO match grade service rifles. And with longer barrels, 150-gr. bullets leaving near 3000 fps were most accurate from 1:13" twists; some used 1:14 with great results. Then there's Sierra's 240 and 250 grain HPMK bullets shot from 1:8 twist .308 Win barrels leaving at 2150 fps; very accurate long range load.

I had a .300 Win Mag hunting rifle with a 1:13 twist barrel that shot 168's and 180's into MOA groups at 1000 yards.

Most interesting to a lot of folks is the benchrest cartridge 30BR shoots 115 to 120 grain bullets out at about 3000 fps from a 1:18" twist barrel for best accuracy.

http://www.6mmbr.com/30br.html

My point is, most commercial barrels have twists fast enough to stabilize the heaviest bullets customers will use; at the expense of less accuracy with lighter bullets because they're spun too fast.

 

[suemarkp]

As you can see, there are many different aspects to "best". Do you mean best accuracy, best terminal performance, best for very long range, best for penetrating armor, best for penetrating large animals, most efficient, best price to performance, ...

I always hated it when someone would ask "what's the best XXX". It depends on what aspects or requirements are most important to you. Once you know those, you can find more optimal solutions.