A YouTube channel recently reported the differences in points of impact when firing 20 different 308 Winchester loads with the same rifle under the same conditions.

That piqued my interest because although I, like virtually all other shooters, knew that changing ammunition could cause POI shifts, it never occurred to me to explore how much difference there could be among various factory loads at a moderate distance like 100 yards. That is, when precise accuracy isn’t required, as when hunting a deer-sized game animal or for certain other purposes, does changing ammo types matter that much?

The following was a very limited experiment with tiny sample sizes, so it’s not intended to be an absolute definitive answer to the question. It’s also important to note that the shooting distance was 100 yards; insignificant differences at that relatively close distance can become huge at longer ranges. Anyway, as limited as it was I conducted the experiment for my own knowledge and is presented here for anyone else who might be interested.

The targets, top row left to right are loads 1 through 3, second row L-R are loads 4 - 6:





The rifle was a Tikka T3x TAC A1 chambered for 308 Winchester; TBAC can; the scope magnification setting was 25×; shooting from a firm bench with a heavy rest at front and a wedge-shaped bag at the rear.

Target 1: Hornady 155 grain ELD Match TAP; three shots. That’s the load the rifle is more or less zeroed with, and it was fired to establish a baseline. Although it was only a three-shot group that doesn’t prove anything regarding precision of load and rifle, I commonly get groups that size and was gratified this time that it measured 0.466 inch center to center, or about 0.45 MOA. Oh, yes: Those three shots were fired from a cold, clean bore.

Target 2: Hornady 165 grain GMX TAP Heavy Barrier, two shots.

Target 3: Hornady 110 grain TAP Urban, three shots.
I fired an extra shot with this load to confirm what the chronograph was showing. The extreme velocity spread was huge with just the three shots: over 190 fps. But despite that variation, the “group” still measured about 3/4 inch CTC, and would have easily satisfied three-shot group fans.

Target 4: Federal 175 grain Sierra MatchKing, Gold Medal Match, two shots.

Target 5: Speer 168 grain Gold Dot Soft Point, two shots.

Target 6: Winchester 180 grain Power-Point, two shots.
That load was from a box I’d had in my stash for decades, and I thought it might have performed much worse.

Those were the results and are worth whatever one thinks they are.

The bullet weights are what make the difference, do they not? Also, it is not specified - if even known - what the bullet shapes are (boattail vs. flat based), with the boattail being more streamlined. It also seems counter-intuitive that the lightest weight (110 grains on target #3) gave the highest POI. I thought light/fast bullets were supposed to hit lower.

The 110 grain bullet in the Hornady TAP Urban load has a flat base, as does the Winchester 180 grain Power-Point. The other bullets have boattail bases.

Two of the 110 grain shots hit about the same elevation as the Heavy Barrier load and not too much different than the Speer 168 grain Gold Dot. A test like this can be misleading because of the tiny two-shot sample sizes. The average velocity of the 110 load was over 200 feet per second than that of the midweight bullets’, but that average was skewed from what it might have been with a meaningful sample size because of one reading that was well over 3000 fps.

In general, fast (light) bullets with their lower ballistic coefficients lose velocity faster than the midweight bullets with their higher BCs in this test, so they would hit lower at long distances, but 100 yards isn’t much of a challenge. Another factor might be that the elevation of the range where I was shooting today is over 10,000 feet and the air density is about 69% of sea level. That means light, low BC bullets aren’t penalized as much by drag as at sea level.

My purpose for the experiment was to determine whether the various loads would all hit close enough to the point of aim to have the practical accuracy necessary for certain targets at a moderate distance.

I may do more extensive testing of the 110 grain Urban load just out of curiosity. I don’t have a lot of it, but I have even less use for the ammunition at this stage of my life, so I may do some more testing in the future. It would be interesting to know if the large velocity spread shows up in larger samples; probably, but as Admiral Grace Hopper observed, “One accurate measurement is worth a thousand expert opinions” [or guesses].

quote:

Originally posted by egregore:
The bullet weights are what make the difference, do they not? Also, it is not specified - if even known - what the bullet shapes are (boattail vs. flat based), with the boattail being more streamlined. It also seems counter-intuitive that the lightest weight (110 grains on target #3) gave the highest POI. I thought light/fast bullets were supposed to hit lower.

308 Win POI at such a short distance is a primarily from muzzle velocity, with a bit of ballistics coefficient thrown in. BC has minimal effect at 100 yards, as the time of flight is the ballpark of .10 to .12 seconds. MV comes into play slightly, as the higher MV bullet that impacts in .10 seconds is less affected by gravity than the lower MV bullet that impacts in .12 seconds. But in theory it's still a pretty small difference. In the field the POI differences can be more pronounced.

In practice I noticed somewhat similar differences to what Sigfreund shows here. BTW, my 308 is zeroed at 100 with 168/175 grain factory loads. Federal & Hornady exhibit essentially the same POI.
- The highest POI was with Hornady 110 TAP. My notes show that the POI was close to an inch high. However, I fired only one box and I didn't find accuracy to be all that great in my rifle.
- The higher velocity 155 grain match rounds impacted a little higher. Fiocchi 150 SST was about 1/4" high. Hornady 155 HPBT Match was 1/4" to 1/2" high, and the same for Corbon Scenar 155, and for Southwest Scenar 155. Applied Ballistics 155 Berger was 3/8" high.
- ASYM 175 was a fairly hot load, with a POI 1/4" high. Same with Black Hills 175 HPBT and Fiocchi 175 SMK.
- Hornady's Superformance ammo was generally 1/2" high. It regularly popped primers on hot days.
- CorBon's 190 SMK load POI was 1/4" to 1/2" low. Inconsistent accuracy, but it did produce one really tight group.
- Lapua Scenar 167 and 185 were both about 1/4" low -- they both are mild loads.

As I see it, the concept higher impacts with heavier loads comes from pistol shooters with less developed recoil management skills. The heavier bullet causes more muzzle flip. Conversely, muzzle flip is better controlled with lighter loads, and therefore muzzle flip is less and POI is lower.

I’ll often use oddball ammo, when I have some, for initial sight in. This is mostly when putting a scope on a rifle. After I get things close I switch to the ammo I plan to hunt with.

I realize this isn’t rocket science, doing so. I also usually sight in at modest ranges, 75-100 yards. I can do up to 75 or so in my back yard. The way & location I hunt, seldom shoot beyond 80 yards.

I also know as one gets to the longer range target shooting, tiny things have a big impact.

Today’s update (not pictured).

I fired an older Hornady offering the 155 grain A-MAX Black, whose velocity for eight rounds averaged 2780 fps, which was significantly higher than the baseline 155 grain TAP load. The point of impact was right at the point of aim.

The surprise was the Hornady 178 grain HPBT (hollow point boattail) Superformance Match. The point of impact was about 1 3/4" below the 155 TAP load’s. That would still have been close enough to zero for most practical purposes at 100 yards, but I expected the POI to be closer.

What this does tell us is that the small divergence is still OK to put a bullet in an animal's (two- or four-legged) center of mass, but could be problematic with a smaller target at a longer range, say, a head shot on a hostage taker. Also that when you find your "'pet' load" you should zero for and use at least mostly that load. Do I have this right?

quote:

Originally posted by egregore:
Do I have this right?

Yep, you do.
Without benchrest-type support, many shooters don't have the fundamentals to differentiate POI variations across most factory loads. The "noise" of shot dispersion masks POI differences at 100 yards or so.