Why does this happen?

[bsekf]

Same rifle (243), same range conditions, same powder charge (37 Varget). 80 gr. Nosler Ballistic Tip and 85 Gr Nosler Partition. At 200 yards the 80 BT groups 5 inches below the 85 gr Partition. At 100 yards the 80 gr groups an inch lower. I anticipated the heavier bullet would drop more and they would group about the same at 200. Where did I go wrong? What do you think will happen if I drop the powder charge to 36 varget under the 85 grain bullet?

Bill

 

[joshb]

In my experience, the 85 picks up more speed than the 80, after it leaves the barrel! Just kidding. I'm in a devilish mood.
Seriously, it could be barrel harmonics. The slight difference in bullets changes the timing of when they exit the barrel. Milliseconds, for sure, but it makes a difference.

 

[Clancy]

They do this side by side?

 

[SheepDog]

You have two different bullet weights and two very different constructed bullets. You are using the same powder charge. Why do you expect them to go to the same point of impact? Different weights with the same powder charge means that, all else being constant, the heavier bullet makes more pressure and accelerates slower. In your example the heavier bullet has a solid web through its middle. That takes even more pressure to squeeze into the rifling and may cause higher pressure and even slower acceleration. Even if you could get them to leave the muzzle at the same velocity one is making more pressure and spending more time in the barrel.

All of this without mentioning the ways a bullet can affect the harmonics of a barrel.

Have you chronographed the two loads?

 

[Zero333]

What joshb said... barrel harmonics. Muzzle whip sending the bullets at different angles.

 

[Barlow]

I have noticed over the years that if you have a 150 gr load for a 30-06 at 2900mv (example) and you load 180's at 2700mv and leave sights as is, the 180 will print higher at 100 than the 150's. It imo is recoil related. I wouldn't think a 5gr difference would make a lot of difference, but evidently it can. As stated above there are many factors that could and do enter in. Barlow

 

[bsekf]

OK, thanks...... Hadn't thought of the Partition staying in the barrel longer because of the construction and that might cause the higher group.

It just surprised me that from 100 to 200 the difference grew from 1 inch to 5 inches. When there was only a 1 inch difference at 100, I hoped I had 2 completely different bullets that were going to shoot the same at 200. This is my "porch" rifle for coyotes and deer, hence the different bullets.

Back to the drawing board. I'm going to back the Partition load off 1, 1.5 gr and see what happens.

Bill

 

[spclark]

When there was only a 1 inch difference at 100, I hoped I had 2 completely different bullets that were going to shoot the same at 200. This is my "porch" rifle for coyotes and deer, hence the different bullets.

I’d take each bullet on its own merits, then have ‘come-ups’ for each at distances I’d expect to be using them.

Either way, bullets with different physical characteristics that affect their relative ballistic coefficients will have different trajectories that cause what you’re asking about.

You can play with loads if you want to. Changing velocities may or may not help you achieve what your after, but even if you get close it’ll be for only one or maybe two distances, one as they’re on the rise (close-range) one farther out on their way down.

 

[Ggmac]

What's been previously said with this added , heavier bullets have more barrel time and more recoil , usually resulting in a higher poi out to a certain point .