I need help understanding bullet drop between two bullets.

[Dennis]

Hello,

I recently picked up a Ruger Hawkeye FTW Predator .308 Win rifle. It has a 22" barrel and a Leupold VX-3 2.5-8 mounted on it. I sighted it in shooting Hornady Whitetail Interlocks at approximately 1.5" above the center of a target at 100 yards. I can put 3 rounds inside of 1.5" at 100 yards. OK, so far, so good.

Then I try some Nosler Accubonds and they group somewhat less than 1.5" at 100 yards, BUT the rounds fall approximately 2" below the center of the target. The Accubonds are somewhat faster and have a higher ballistic coefficient (B.C.) so I thought they would shoot a little flatter and thus land above where the Interlocks landed - not way below.

So I go back and light off a couple Hornady Interlocks and they land right where I sighted them in at so I know this rifle shoots the Accubonds way differently than the Interlocks. But lower rather than higher? This is what confuses me. All rounds were fired in a 100 yard enclosed rifle range, so wind is not a factor.

Both bullets were 150 grain and both are factory cartridges.
Nosler Accubond have a muzzle velocity of 2,875 fps and B.C. = .435
Hornday Interlocks have a muzzle velocity of 2,820 fps and B.C. = .338

Maybe I'm just not understanding something about basic ballistics and I would appreciate some input on this topic.

Thanks,
Dennis

 

[Sagebrush Burns]

It comes down to how the bullets vibrate the barrel rather than velocity differences.

 

[308longdistance]

Some barrels shoot different bullets to different points of impact. At 100 yards the stated MV and BC are of little consequence. At 600 yards the Accubond is still at 1700+ fps and 1000 ft-lbs of energy. The Interlock is far behind @ 1400+fps and 700 ft-lbs of energy.

 

[planetcat]

Maybe they are not shooting at their advertised velocities, unless you have chrono'd them. Then I am stumped. I've found the interlock Hornady round to be extremely accurate out of finicky rifles, but both are very good bullets. I'd pick one and stick with it. I'd imagine the Hornadys are much cheaper than the Noslers.

 

[Rumrunner]

Being faster they are in the barrel a shorter time then the Hornadys. The Hornadys shoot higher because the barrel is lifted higher in recoil for the slightly longer time they are in the tube. You are sighted in for the slower bullet. bullets. The sight in points will be different for each bullet. As far as shooting flatter shoot the Hornadys at 200 and 300 yards. then resight the noslers in at 1.5" high at 100 and shoot them at 200 and 300 yards and you will see they do shoot flatter.

 

[GunnyGene]

Dennis, as noted above there are many factors that account for the differences you experienced. Rumrunner mentioned primary recoil as one factor, and Sagebrush noted barrel harmonics as another. These and other seemingly minor differences in bullets/cartridge often act together to either amplify or cancel out each other. Planetcat suggested you pick one bullet/cartridge and stick with it. Good advice.

 

[Sugar River]

What you're seeing is the difference in POI due to barrel harmonics. Totally normal for 2 different loads to print to 2 locations.
100 yds isn't nearly enough distance to see the effect of BC. Try 600-1000.

 

[Dennis]

Wow, you all have given some great advice given about this topic! Thank you all for your kind contributions. I have a bit to think about and ponder on now for a while.

 

[GunnyGene]

Wow, you all have given some great advice given about this topic! Thank you all for your kind contributions. I have a bit to think about and ponder on now for a while.

Rex has some pretty good video tutorials on internal ballistics you might find helpful: https://www.youtube.com/playlist?list=PLJUaiRIEduNXoal2_PkBZi0vDCIcEPxUn

 

[Dennis]

GunnyGene,

Thanks for pointing me to Rex's videos - I will definitely check them out.

I come from a shotgun and 22lr background so this centerfire stuff is pretty new to me...even though l'm not so new.

Thanks again,
Dennis

 

[SweetWilliam]

Maybe since the Noslers are a higher velocity they are exiting the bbl. sooner, before the muzzle rises as high as it does with the slower Hornadys
It would be the same as if you used the faster Noslers to sight it in and then shoot the slower Hornadys & it shoots high. The muzzle has risen higher with the Hornadys by the time the bullet exits the bbl. it doesn't take much.

 

[Dennis]

SweetWilliam,

Thank you for your comment. It supports what Hunter and others said earlier in this thread and it certainly makes sense.

Thanks again,
Dennis

 

[Jimbo357mag]

Maybe since the Noslers are a higher velocity they are exiting the bbl. sooner, before the muzzle rises as high as it does with the slower Hornadys
It would be the same as if you used the faster Noslers to sight it in and then shoot the slower Hornadys & it shoots high. The muzzle has risen higher with the Hornadys by the time the bullet exits the bbl. it doesn't take much.

Different powders can do the same thing. If you change anything the POI can change. I have experienced side to side movement also with different bullets.

 

[Rick Courtright]

Hi,

Dennis, what a lot of the guys have said here comes under the umbrella term of "barrel harmonics." If you can ever see some high speed video of a barrel during firing, it's pretty interesting. The barrel has a "wave" that moves thru it from the ignition point to the exit point of the bullet, and each load combo affects it ever so slightly. To demonstrate at home what's happening, go outside, put a shut off nozzle on a garden hose, and turn on the water. See the hose wave about as it fills? The gun barrel does a similar "dance" every time we pull the trigger, much more subdued, but still there. That's one reason for using heavier barrels for certain target and varmint rifles: generally the more metal in the barrel, the less effect the wave will have.

When the gun's fired, any change in that wave can mean the barrel's pointed at a different place than we thought. We adjust the sights/scope so it's pointed where it needs to be with one load, and life's good. But change loads, and oops, we may be right back to square one! To give an idea how little change it takes in sighting, a 3 inch difference in point of impact at 100 yds translates into only about 0.018" movement at the muzzle of a 22 inch barrel.

So what you're seeing is perfectly normal! Now the trick is to decide which bullet you like, and work up a load that puts it where you want it. Some folks will use a particular factory round as their base, so their handloads shoot the same as it does. Good idea if you're likely to shoot both factory ammo and handloads once in a while. Sometimes, not always, you can get a couple of different bullets to shoot to the same POI without changing sights by using different powders and/or primers, adjusting the powder charge, and making other subtle changes to your recipe.

Have fun working all this out!

Rick C

 

[Dennis]

Thank you all again for your thoughtful answers. On one hand I have my work cut out for me figuring out which bullet, load, etc. On the other hand, I think this is going to be fun!

Thanks again!

 

[Kudu m77]

As some of the others said about barrel vibration is spot on.

Here is an illustration of what happens with your rifle barrel when the bullet leaves the barrel.



Pieter

 

[Dennis]

Pieter,

Thanks for the diagram. I understand recoil but this whole barrel harmonics thing is new to me.

Peace,
Dennis

 

[coach]

I've seen the same thing shooting a Super Blackhawk. The magnum rounds seem to shoot lower than the special rounds. I've always assumed it was the recoil and the time the bullet spent in the barrel. So by the time the special bullet left the barrel, the barrel had raised up just a little more than the faster magnum round.

 

[SweetWilliam]

I've seen the same thing shooting a Super Blackhawk. The magnum rounds seem to shoot lower than the special rounds. I've always assumed it was the recoil and the time the bullet spent in the barrel. So by the time the special bullet left the barrel, the barrel had raised up just a little more than the faster magnum round.

Your assumption is correct

 

[Sugar River]

I've seen the same thing shooting a Super Blackhawk. The magnum rounds seem to shoot lower than the special rounds. I've always assumed it was the recoil and the time the bullet spent in the barrel. So by the time the special bullet left the barrel, the barrel had raised up just a little more than the faster magnum round.

Your assumption is correct

Yes, that's is correct WRT handguns, the higher velocity load will print lower.
Rifles, however, are a different kettle of fish.