Another fun discussion with a reader with some great questions and comments got me thinking again. As per usual, it made me realize that there are still many things for me to learn about when it comes to shooting sports. This time the question, in context of a cartridge comparison, was “would bullet weights/designs/manufactures alter these findings?”
Initially, I wanted to confidently answer with a “yes”. However, as I started to answer I realized I didn’t really know all the factors. I just learned some general rules of thumb that I’ve gathered through anecdotal experience or repeated phrases from various people. I’ve started to find that several of the repeated phrases tend to be old wife tales also based on anecdotal experiences that tend to be more myth than fact. So, here we are.
As I started writing this, I hold the following statements as generally true:
- Faster moving projectiles have flatter trajectories.
- Heavier projectiles are less prone to wind drift.
I’m going to start exploring these generalizations by looking at a single rifle cartridge from a single product line offered with the same projectile type in different weights. I expect the generalizations to hold true, but I also hope to be able to learn (and consequently explain) what the difference in bullet weight means. Moreover, I hope to determine whether or not the weight difference matters. You may have to bear with me for several posts as I have a feeling that the answers will vary as I explore different applications and platforms. For the purpose of this post, I’m looking at this through the perspective of long distance shooting applications.
As I’ve done in cartridge comparisons in the past, I’m going to look at Hornady’s Match product line. Yes, I like this product line. No, I’m not promoting it and don’t receive any compensation for promoting it. The main reason I’m using it is because they make the information easy to find. I’m going to specifically look at three 6.5mm Creedmoor (6.5 CM) product offerings from this line all using an ELD Match projectile.
|Advertised Ballistics||120 gr ELD Match||140 gr ELD Match||147 gr ELD Match|
|Ballistic coefficient||.486 (G1)|
|Muzzle velocity||2910 fps||2710 fps||2695 fps|
|Muzzle energy||2256 ft/lb||2283 ft/lb||2370 ft/lb|
Based on the advertised data alone without using a ballistic calculator, I assume that the 120 grain load will have the flattest trajectory and the 147 grain load will have the least wind drift. However, I’m unsure about guessing which load has the longest effective range for hitting paper or steel targets at extended distances. But I’ll place a bet on the 140 load for the longest effective distance as it seems to me like it’s the most common projectile weight for the 6.5 CM cartridge in the market today.
Did I place a winning bet? Let’s take a look at velocity over extended ranges.
|Velocity||120 gr ELD Match||140 gr ELD Match||147 gr ELD Match|
|500 yards||2028 fps||2059 fps||2090 fps|
|1000 yards||1347 fps||1516 fps||1577 fps|
|1100 yards||1245 fps||1424 fps||1487 fps|
|1150 yards||1200 fps||1380 fps||1445 fps|
|1200 yards||1159 fps||1339 fps||1404 fps|
|1300 yards||1090 fps||1262 fps||1326 fps|
|1375 yards||1048 fps||1209 fps||1273 fps|
|1400 yards||1036 fps||1193 fps||1256 fps|
|1475 yards||1001 fps||1149 fps||1208 fps|
|1500 yards||991 fps||1135 fps||1193 fps|
Well, looks like I lost that bet. I’ve highlighted the longest entry in the velocity table where the velocity was above 1200 fps. Dipping below that point means the projectile will start crossing the transonic barrier and will begin to destabilize. That’s essentially the effective range of the projectile where one can assume consistent accuracy.
I was surprised by these findings. I didn’t want to place my bet on the 120 grain load because it’s BC was significantly lower compared to the other two loads. I had thought the difference in BC between the 140 grain and 147 grain loads was small enough that the 140 grain load with its larger initial muzzle velocity may edge out the 147 grain in terms of distance. I was wrong.
The data highlights how the BC also comes into play in terms of how fast it slows down a projectile. Even at the 500 yard marker the 120 grain is already traveling slower than the other two even though it started out as the fastest. By the same token, the 147 grain which started out as the slowest has slowed less than the other two loads.
Pretty cool. But let’s look at the drop and see if my assumption about the 120 grain load being the flattest is true.
|Drop||120 gr ELD Match||140 gr ELD Match||147 gr ELD Match|
I’ll be a monkey’s uncle. I’m almost flabbergasted by the implications of this data.
Turns out the 120 grain started out with the flattest trajectory, but that only lasted a little past the 500 yard mark. At its maximum effective range of 1150 yards, the 120 grain load experienced more drop than the other two loads at the same distance. Remembering that drop is a function of the amount of time a projectile is exposed to gravity or how long it remains in free fall. This means that the 120 grain with its significantly larger initial muzzle velocity reached the 500 yard marker first, but due to its significantly lower BC it lost so much velocity that it was the last to reach the 1150 yard marker.
While I didn’t include any data under 500 yards in the table, the ballistic calculator shows the drop between the 140 grain and the 147 grain load to be identical all the way out to 350 yard mark. By the 375 yard mark, the 140 grain begins to experience more drop. This means two things. First, the initial muzzle velocity difference of 15 fps is insignificant and negligible. Second, the .031 BC (G1) difference is significant. At least significant enough to matter beyond 350 yards where the 147 grain load retains more velocity.
I don’t even want to look at the drift data now. But, here it is anyway.
|Drift||120 gr ELD Match||140 gr ELD Match||147 gr ELD Match|
Finally, one of my assumptions was correct. In terms of drift, it turned out that the heavier bullet experienced less drift. However, given that I almost struck out I’m wondering if this was really a result of the bullets weight or the BC. I suspect it’s the latter rather than the former.
Generally speaking, I have found that heavier projectiles of the same caliber and projectile design tend to have a larger BC. Again, I’m starting to wonder how much truth there is in that generalization and will probably start looking for exceptions. Does it hold true for all projectile designs of the same caliber? What about the same projectile design in different calibers? I’m pretty confident I can find examples of heavier projectiles with lower BC if I all projectile designs and caliber restrictions. But at that point, I’m not sure bullet weight matters.
At the beginning of this article I set out to learn what the difference in bullet weight means and whether or not it matters. In the context of the same cartridge with similar projectiles, I can now confidently say that different bullet weights mean different initial muzzle velocities and different ballistic coefficients. In terms of long distance target shooting, the difference in weight definitely matters.
Does this mean that for long distance target shooting the 147 grain load is the king of the loads I compared? On paper, it looks like the answer is yes. However, I really didn’t explore recoil impulse or perceived recoil. Then there is the “different rifles like different loads” canned advice that is commonly found in the community. So, I can’t definitively and confidently say yes, the 147 grain load is king.