Why bullets do what they do

[gamestalker]

My years of reloading and shooting experience has taught me a lot about various elements of firearms and the ammunition the utilize, however, one particular element that I have never been able to decifer is bullet drift. Why do light weight bullets shoot high and right from most barrels that have the same twist. And although the groups they produce are usually nice and tight, they often climb hard to the right?

The other day I was running some 140 gr. Sierra's at 200 yds. nadthey were holding nice tight groups zeroed dead center. And then I ran some 120 gr. Hornady, 110 gr. TNT's, and some 100 gr. Sierra down range and each had a graduated climb to the right with the 100 grainers poi 4-1/2" high 2" right, 110 gr. 4"-ish high and just under 2" right, and the 120 gr. were about 2-1/2" high and 1-1/2" right. Can someone explain what is happening with the barrel to bullet contact that causes this? It's nothing that is typical of the specific rifle or cartridge, I get the same effect with any of the barrels I shoot light weight varmit bullets from.
Thanks,
GS

 

[chrisf8657]

I probably shouldn't even be replying, as others know WAY better than I do, but AFAIK, it DOES have to do with twist rate of the rifle. We'll use an AR for example:

1/12 or more is good for 55 or lighter bullets, while we are now down to 1/7 for heavier up to 75 grain bullets. A twist rate has a certain "range" which it likes certain weight bullets - the science behind it, that's for others or the Internet.

My guess is that the lighter the bullet and the slower the twist, it doesn't have enough time to stabilize the bullet with enough spin, causing it to deviate off path easier, but not too bad to tumble the bullet???

I'm either totally wrong, or partially right...we'll see what others have to say.
One thing I do know is right: Like I said, a certain twist rate will work right within a certain range of bullet weights. My Rem 700 in .308 works best with 150-168g bullets. Heavier or lighter, I start getting bad drift.

 

[gamestalker]

The term I probably should have used is spin drift. I'm obviously experiencing either too high of an ROT, or not enough ROT,
( rate of twist). What ever it is, it has always forced me to make radical adjustments to both windage and elevation when shooting varmit bullets. And for obvious reasons I understood the difference in elevation with lighter bullets, but the drifting to the right thing has always baffled me, and still does.

 

[GLOOB]

Light bullets travel faster. Hence they drop less than a heavier bullet at the same distance. As the bullet drops, the air is pressure is slightly higher on the bottom of the bullet, compared to the top. With a left hand twist, this means the bullet will slightly drift to the left, on average. If you zero in this heavier bullet, a lighter bullet will go high right at the same distance.

Now I'm not sure that this phenomenon actually results in noticeable lateral drift in a rifle. But it is for real. Same thing happens to a football. A ball thrown by a righty tails off slightly to the right. Lefty, the other way.

Out of the average rifle at typical distances, I'd think that barrel harmonics are going to affect POI way more than spin drift.

 

[Ridgerunner665]

At 200 yards what you're seeing isn't spin drift...most likely its the effect of barrel timing and harmonics.

Sent from my DROID RAZR MAXX!

 

[Lost Sheep]

Barrel harmonics would be my guess, but it occurs to me that no one has mentioned Coriolis effect.

Just a wild guess.

Lost Sheep

 

[JamieC]

Makes me think of pitching, put a certain spin on the ball when you throw it, it curves.

 

[Jasper1573]

Coriolis doesn't noticeably affect a bullet at such short ranges. I don't know the exact distance, but you have to get way out past 500 yds to even consider coriolis.

 

[kingmt]

Coriolis effect is more noticeable when your trying to walk from the bar stool to the urinal & seems to get worse the later it gets.

My guess is barrel harmonics for the bullet path.

 

[Lost Sheep]

Coriolis doesn't noticeably affect a bullet at such short ranges. I don't know the exact distance, but you have to get way out past 500 yds to even consider coriolis.

Thanks, Jasper1573.

I thought that Coriolis Effect would be too slight to make differences as large as the OP cited, but the question got me stumped, so I grasped at a straw.

No cigar for me.

Lost Sheep

 

[tryshoot]

I have a 270 that shoots 11in high and 2in right with 110 gr bullitts behind imr 4032. 130,140,and 150 hit right.(forgot to say 200yards).