Another question about "grains" and ammunition

K

Kimio

New member
I believe I asked this question in the past or at least something similar. Anyway, what I remember is that "grains" is a type of measurement, and does not specifically relate to the powder load in a cartridge.

That said, I have my AR15 which has a 1:9 twist rate, and if I'm not mistaken, I need a "heavier" bullet (75gr bullet if I'm understanding everthing correctly) for my rifle.

Lighter bullets if I understand correctly need a faster twist rate to stableize in flight, or they may end up "keyholing" the target, at the same time, if the bullet is over-stableized, that may cause issues as well such as "shearing" (am I using the right term?). I believe that is when the copper jacket of the bullet actually starts to seperate from the lead core.

Can someone please clarify and or correct me if I'm wrong?
 
A grain is a weight measurement equal to 1/7000Lb.
I'm not familiar with the AR so I cant help with the exact rifling question, but in general you need to match twist rate to bullet for best accuracy. Under spun will keyhole but I've never come across the jacket shedding thing.
 
You're understanding is bit backwards.

Yes, grains is weight. What it's referring to depends on the context. Unless you're specifically talking about powder, it's normally referring to bullet weight.

Faster twist stabilizes heavier bullets but it also stabilizes lighter bullets. (Although it's technically length that matters more than weight, since the caliber is fixed, weight and length generally go together)

The only problem that's possible with a fast twist and a light bullet is spinning it too fast and introducing wobble from any tiny imperfection in it's balance. This is rare and only at issue with extreme examples and when concerned with extreme accuracy. You'll most not see it.
 
Oh okay, so a 55gr bullet should shoot fine out of my AR15 then? It's an M16A2 style AR15 with a 20" barrel, but if I want to fire heavier rounds I'll want to get a faster twist rate.

May I ask what exactly is the benefit of having a heavier round? Do they typically retain their energy or accuracy out at longer ranges or something?

Most of the shooting I do with open sights at roughly 100-300 yards (300 is reeeeeally pushing it for me)
 
Heavier bullets drift less and drop less over distance because they have more momentum and are less effected by wind and take longer to slow down. In short hand, you get an idea of how well a bullet does by looking at it's BC. (Ballistic Coefficient)

However, it depends on the distance and starting speed. A very fast, light bullet can very often beat a slower, heavier bullet in the first several hundred yards.

Use a good ballistics calculator (like JBM Ballistics, free online) to determine trajectory. It's certainly irrelevant at 100.
 
Okay, that makes sense. You had mentioned that weight and length typically go hand and hand, could you please elaborate?
 
As I said, the width is fixed. The bullet is .224" (in the case of an AR15) wide not matter how much it weighs. That means the only way to change is weight (other than making it out of different material) is to make it longer if you want it heavier or shorter if you want it lighter.

Since traditional bullets are all made from lead cores and copper jackets, only length can effect their weight if they're all the same caliber (.224 in this case).
 
The faster twist barrels need "longer" bullets. They are usually heavier, but not always. A 1:7 twist would be considered fast and would work best with heavier bullets weighing 55 gr and up. Most bolt rifles have rather slow 1:12 twists which shoot better with bullets 55 gr and lighter.

Your rifle with a 1:9 twist is a good compromise that should be acceptable with most any bullet weight. You just don't know for sure until you experiment but it will probably be best with mid weight bullets in the 55-60 gr range. With the heavier 70+ gr bullets or lighter 40 gr bullets it might not do as well. Might do fine.
 
"Since traditional bullets are all made from lead cores and copper jackets, only length can effect their weight if they're all the same caliber (.224 in this case)."

Not entirely true. Nose and base profile will also have an effect on weight.

In two bullets of the same length, the one with the flat base and round nose will be heavier than a boattailed spitzer.

How much heavier depends on bullet diameter. It might only be a few grains in a small bore bullet, but it could be 20 grains or more in a larger diameter bullet.
 
kimio said:
Oh okay, so a 55gr bullet should shoot fine out of my AR15 then? It's an M16A2 style AR15 with a 20" barrel, but if I want to fire heavier rounds I'll want to get a faster twist rate.

May I ask what exactly is the benefit of having a heavier round? Do they typically retain their energy or accuracy out at longer ranges or something?
Click to expand...
The original M16 was designed to shoot 55-grain bullets and it had a 1:14 twist. That quickly proved to be so inadequate that the weapons were unusable in cold weather, and the twist rate was steepened to 1:12. That's what it was when I carried it in Vietnam, and that's what it remained for many years thereafter.

Current military issue (other than for certain Spec Ops units) is a 62-grain bullet, and the 1:12 barrel is not capable of handling that; it needs a 1:10 twist or better. Anything heavier requires a faster twist. For "standard" (55-grain or 62-grain) ammo, your 1:9 barrel would be considered a moderately fast twist.

The answers to all your questions about AR-15 ammo and twist rates (including the questions you haven't even thought of yet) are found here:
The Ammo Oracle http://www.ar15.com/ammo/
 
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Aguila, thanks for posting that link, there is some good info there, definitely going into my bookmarks. It should help you out a lot Kimio.
 
On a side note, my 'budget' S&W Sport has what's called 'gain twist' rifling. It starts out slow, probably 1:14 then steepens to 1:8. Or perhaps steeper but it states 1:8 which could be an average.
 
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