Different Bullet Weights Having the Same point of Impact

[SwampWolf]

Why is it that, reportedly, some rifles will put bullets having different weights in virtually the same place at the same distance and others won't? In the latest issue of Rifle magazine, author Stan Trzoniec (whom I have a lot of respect for) tested a Cooper rifle, chambered in 30-06, and made the following observation: "...What really grabbed my attention, however, was how close the various bullet weights (150, 165 and 180 grains) were when it came to trajectory. For all practical purposes, all but one could be counted on to hit within one inch or less of each other even out to 400 yards..."

This isn't the first time I've heard of this apparently unusual phenomena but I've never understood how it happens. I understand that all rifles are dimensionally different from each other in one way or another, even rifles coming off the same assembly line on the same day. But it would seem that the ballistic properties of bullets having different weights are constants when it comes to downrange trajectories and points of impact, no matter what rifle they are shot from; all else being equal in terms of the type of ammunition, barrel length and twist, etc.

I've had a lot of different rifles in my time but never one that I could count on to put all of many different bullet weights in the same spot at the same distance. I've also posed this question in another forum but I wonder what ideas you folks might have. Thanks.

 

[Art Eatman]

I set up a round 22" steel plate at 500 yards from my house. Built a fairly sturdy shooting table. Meddled around one day with my '06, getting sighted in for 500. Came up 8" (32 clicks) from my usual 200-yard zero.

So I shot groups with Sierra handloads: 150-grain SPBT, 165-grain HPBT and 180-grain SPBT.

All groups were just under one MOA. All three of the group centers were within three or four inches of the POA. IMO, that's pretty close to the trajectories being the same.

 

[LoonWulf]

I think it might come back to something Barts pointed out before in terms of accuracy vs heat, barrel weight etc.
When properly assembled with mating surfaces square the way the gun moves is less effected by bullet weights than one that has a slightly untrue junction and or bedding.

This is pretty much a guess, as ive only got one gun that puts all its bullet weights into the same group basically, and thats been rebarreled and had its action trued up.

I also rarely shoot more than one bullet weight per caliber

 

[Llama Bob]

Why is it that, reportedly, some rifles will put bullets having different weights in virtually the same place at the same distance and others won't? In the latest issue of Rifle magazine, author Stan Trzoniec (whom I have a lot of respect for) tested a Cooper rifle, chambered in 30-06, and made the following observation: "...What really grabbed my attention, however, was how close the various bullet weights (150, 165 and 180 grains) were when it came to trajectory. For all practical purposes, all but one could be counted on to hit within one inch or less of each other even out to 400 yards..."

This isn't the first time I've heard of this apparently unusual phenomena but I've never understood how it happens.

There's two issues here - regulation and trajectory.

Regulation is the difference in angle of how the bullets come out of the barrel. It's unpredictable during manufacturing, because barrels are actually very flexible things and flex a lot before the bullet leaves the muzzle. However that flexing is ideally very repeatable as long as you don't change the load being shot so in single barrel guns, we just adjust for it by moving the sights. In doubles, it's a big problem because there's one set of sights but two barrels. You can read about the method to deal with it here:
https://www.theexplora.com/regulating-double-rifle-at-westley-richards/

Trajectory (for this discussion) is the drop of the bullet as a function of distance traveled. Slower projectiles (either due to lower muzzle velocity and/or more drag) drop farther. This is easy to understand from a physics POV - heavy, slow loads just drop more.

The only way that heavy, slow loads can shoot to the same point of impact at distance as light, fast loads is if the regulation and trajectory issues cancel out. If the slow load just happens to flex the barrel in such a way that it shoots say 1" higher at 100y, that will pull it up a 4" at 400y, which may be similar to a faster load.

This is however mostly an odd coincidence, and is not in any way indicative of a well made rifle or any special work put in by the manufacturer. If you changed loads (for example, going to a faster or slower burning powder or slightly different muzzle velocity) you'd see that it no longer worked as well. Switch to a different gun by the same manufacturer, and the slow load might shoot low or off to the right instead of high at 100y and the match would simply get worse the farther out you went.

 

[wally]

I think its fantastic when it happens and would be even better if you could expect it. But I often see significant (1+ MOA) POA/POI shifts with different lots of the same brand of ammo (I buy cases at a time) and sometimes even greater among different brands of the same bullet weight from different manufacturers.

When I first got my .308 RPR I did 10 shot groups of like 5 different ammo weights and brands using the same POA for the targets, and the POI was significantly different for all of them.

 

[jmr40]

It isn't unusual for rifles to shoot several bullet weights to the same POI at 100 yards or a bit farther. But the laws of gravity and physics are strictly enforced. At some point down range slower bullets have more time for gravity to work on them and they will drop more. There are a couple of factors that can make it appear that heavier bullets are dropping the same or even less. The difference in trajectory between a 150, 165, and 180 gr bullet fired from a 30-06 is simply closer than many think.

At 400 yards a typical 180 gr bullet will only drop about 4" more than a 150 gr bullet and about 2" more than a 165 gr bullet. If you, your load, and rifle were capable of 1" groups at 400 yards you'd see 3 distinct groups if you fired 5 shots of each bullet weight at the same target. In reality a 4-8" group at 400 yards is typical with most hunting rifles and shooters. If you were to do this you'd see all bullet weights overlapping within the 4-8" fifteen shot group. Most of the 150 gr bullets would be near the top, most of the 180's nearer the bottom with the 165's more in the middle. Group size is going to be larger than the difference in trajectory making it hard to differentiate between bullet weights. At farther ranges the differences are more apparent.

The 2nd factor is bullet BC's A 150 gr bullet with a poor BC in the .3's may start out at 2900-3000 fps from a 30-06. A high BC 180 gr bullet in the .5's may start out at only 2700-2800 fps. The 150 gr bullet will be moving much faster for the 1st 200 yards or so. But somewhere before 300 yards the much more aerodynamic 180 gr bullet will be moving faster and the difference will only become greater as range increases.

I suppose that if you're shooting far enough you'll reach a point where the 180 gr bullet drops less. But the 150 gr bullet took much less time to reach 300 yards even with poor BC's. Beyond 300 the 180 is faster, but you'd have to be shooting pretty far to make up for the head start the 150 got. I've played around with the theory a bit with ballistic tables and never found a point where the heavier bullet dropped less. Not saying it can't happen.

 

[Bart B.]

http://www.varmintal.com/amode.htm

If a 180 grain bullet drops 5" at some range but a 150 shoots to same point of aim but it only drops 4" at that range, it's really easy to determine the muzzle axis pointed higher when the 180 left. As the barrel vibrates at the same resonant and harmonic frequencies thereof for all rounds fired, the 180 moving slower left later on the muzzle axis upswing pointing an inch higher above its impact point down range.

This is Sherlock Holmes' reasoning; "Elementary, my dear Watson."

 

[sage5907]

I think you would have to use a common powder for all three bullet weights something like IMR 4064 to get it done. The same powder along with the same bullet like a Sierra GameKing. If you used IMR 4895 for one bullet weight and IMR 4064 for another bullet weight it wouldn't work because they don't shoot to the same point of impact.

 

[Art Eatman]

I used 4064 for the 150s and 165s. H414 for the 180s.

 

[Bart B.]

Considering the OP stated a 1 inch spread was acceptable, I see no reason why powder type matters. Having shot 150's and 168's with different types for each at 200 and 300 yards, same elevation zero for both at each range, respective.

 

[coloradokevin]

I'm not sure this is as true as the OP's post would suggest, and I'm not sure the author of the gun rag who claimed this was true provided sufficient facts. A lot of variables come into play here that determine whether or not this is true for any given load.

Different loads have different muzzle velocities, and different projectiles have different ballistic coefficients. Generally speaking, heavier bullets move slower, allowing gravity to work on them for longer. But, everything comes down to the bullet itself, and how fast we're pushing it. Unless these were very similar loads, the result on the target can be very different.

The two most similar loads I shoot are match loads I run through my .260 Remington. One load shoots a 139 grain Lapua Scenar bullet, and the other is built around a 140 grain Berger Hybrid Target bullet. These loads have been chronographed within 50 fps of each other, and the projectiles are only 1 grain apart in weight. You'll definitely see more variation than that in almost any different type of factory loads. Anyway, based on the relatively thin air we have out here, and an 8000 foot density altitude for my atmospherics, I show the following data from my ballistic calculator (data that has been verified on target):

139 grain Lapua Scenar bullet: 400 yards: drop of 25.7 inches. 800 yards: drop of 163.1 inches.
140 grain Berger Hybrid bullet: 400 yards: drop of 23.9 inches. 800 yards: drop of 150.8 inches.

Okay, so what do we see here? At 400 yards these bullets weren't too far off at 1.8" difference in trajectory. But, even that 1.8" is 80% more than the author of this gun rag claims you should see (with less than an inch being reported). And, remember that I'm talking about two remarkably similar bullets here. These are two of the most popular match bullets made for guns in the 6.5mm class! Even with those very similar bullets moving at reasonably close muzzle velocities, we're looking at 1.8" of difference on target by 400 yards.

Once you stretch this out to 800 yards the game really changes. At 800 yards you'll be looking at 12.3" of elevation difference on target, which is the difference between a hit or a miss in most cases.