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Old February 19, 2017, 12:23 PM   #1
dgludwig
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Different Bullet Weights But Same Impact Point?

I've had a question for a long time that maybe somebody here can clear up (I also posed the question in another forum). In the latest issue of the Rifle magazine, author Stan Trzoniec (whom I have a lot of respect for) was testing a Cooper rifle chambered in 30-06 and made this 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...".

Now I've heard this claim more than a few times in the past but my question is, if one rifle can do it, why not all of them? What is so different about one rifle that can place all of its bullets in one place, regardless of weight, and others can't? I understand that every rifle, even those made the same day off the same line, are going to be dimensionally different from each other in one way or another, but bullet weights and their attendant ballistic properties in terms of trajectory would seem to be a constant, no matter the rifle they're fired from (everything else being as equal as practical in terms of ammunition type, barrel length, twist and type, etc.).

I'd love to hear opinions on this seeming paradox (if it is).
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Old February 19, 2017, 01:02 PM   #2
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The author made an incorrect statement when he said the different bullets had the same trajectory.
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Old February 19, 2017, 01:09 PM   #3
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It would seem so, Dufus, but he's not the only person, "expert" or otherwise, that I've heard/seen make the same claim. Ergo, my question.

Thanks for the response. I think you're right.
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Old February 19, 2017, 01:22 PM   #4
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Every rifle is different, and every rifle is manufactured to different tolerances for example Savage vs. Cooper. Trajectories are very close to 400 yards with a lot of rifle cartridges, not just bullets. When you change bullets some rifles only require a little bit of tweaking on the scope to get it dialed in, some require a whole lot more, and very few not at all. Quality of materials used in the build and how it's put together make a huge difference.

Barrels also move when bullets are fired down them, the movement is called harmonics. If you have bullets exiting the muzzle regardless of weight at nearly the same point in the harmonic cycle, then those bullets should group very close together at short range. It should be easier to tune hand loads to achieve nearly same POI at short range than it will be to find factory ammunition to do the same.

However, you might sacrifice something to get all your different bullet weights to shoot same POI. For instance you might have a load that shoots Sub-MOA with 150, 165, and 180 grain .308 bullets, but to get them to shoot same POI you might have to settle for 1.5 MOA or larger groups with one or more of those bullet weights. You might also have to sacrifice some velocity as well to achieve same POI, which may or may not matter to some shooters.

For me I don't need a rifle to shoot multiple weight bullets to same POI. What I want is a rifle that'll shoot whatever bullet I choose to put down the barrel into small groups regardless of POI. I'll then adjust my point of aim to intersect with my point of impact at my desired range for what I'm hunting or shooting at. It's easier than chasing the holy grail of rifles that will shoot multiple weights in small groups with nearly the same POI/POA.
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Old February 19, 2017, 01:44 PM   #5
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You work up the load and sight in to do that. Group size and velocity aren't as important as where the bullet hits.
However, the whole exercise is kind of a time waster for a gun rag writer. A 150 or a 180 will not do anything a 165 will not.
Anyway, look at this, noting that each bullet weight was sighted in at a different distance. All three drop very close to the same amount at 400. Appears to be Hornady Superformance ammo with an 'SST' bullet. That's what used to be called their 'Light Magnum' line.
Any mention of what Trzoniec used? Not that it matters. Gun rag writers have been known to make data fit their theories before.
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Old February 19, 2017, 02:03 PM   #6
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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.
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Old February 19, 2017, 06:03 PM   #7
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FWIW I read somewhere (can't remember where so take this story with a grain of salt) that the .375 H&H Magnum was good at shooting different weight bullets to the same point.

Always wanted a rifle in that caliber but have absolutely no legitimate reason to have one so it's on the 'someday' list. If I got one it would be cool to have one of the British double guns in that caliber but for that I wouldn't have a good reason to own one OR the money to get one either. Oh well.
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Old February 19, 2017, 08:13 PM   #8
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I can't remember which Speer manual I read it in, but they remarked about the 416 RemMag, that it shoots different weight bullets to nearly same point of impact.

I have also heard that a quality hand lapped barrels have less friction than a typical consumer barrel. They also have superior crowns done by dedicated craftsmen. Could be that these barrels are more consistent in their harmonics. I don't know if these factors are really relevant, but it seems to make sense to me.
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Old February 19, 2017, 08:51 PM   #9
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My Tikka 260, with a slightly heavier aftermarket barrel shoots 100 and 120 gr Nosler BT's to the same POI at 100 yards. I can swap between the lighter (for varmints) and the heavier (deer) bullets and not worry about impact point. It's the only rifle I have or have had that'll do that.
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Old February 19, 2017, 11:20 PM   #10
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My Winchester M70 30-06 will shoot any accurate handload (for my rifle) of any weight bullet (that I've loaded) to within 1" POI at 100 yards. These loads include:

# 125 gr nosler B-tip @3400 fps

# 150 gr Hornady psp @3000 fps

# 150 gr Sierra pro-hunter psp @3000 fps

# 150 gr Sierra game king btsp @3000 fps

# 150 gr pro-hunter rn @3000 fps

# 165 gr Hornady sst @2900 fps

# 165 gr game king hpbt @2900 fps

# 180 gr pro-hunter psp @2700 fps

# 180 gr pro-hunter rn @2700 fps

Velocities have been measured by a chrono and rounded to nearest 100fps from my rifle with a 1-10 twist 24" barrel. Granted not all of these loads will shoot sub MOA but all will shoot sub 2" at 100 yards most within 1.5" at 100 and one will consistently shoot .5" 5 shot groups at 200 yards.
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Old February 19, 2017, 11:38 PM   #11
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If the ballistic coefficients are very similar and the muzzle velocities are close the impact points will be "close".

Change either BC or MV significantly and it's a measurably different impact point.
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Old February 20, 2017, 11:52 AM   #12
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Precision versus accuracy.

https://sciencenotes.org/what-is-the...and-precision/

In my head precision is more important, at first, then accuracy.

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Old February 20, 2017, 10:45 PM   #13
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Because I shoot a hefty powder charge of 4831 in my 243 that propels a 75 grain beyond 3500 fps. I've experimented and found the rifle can handle a couple different bullet weights with the same 75 gr. charge. 75-80-87. Not much difference seen in down range terminal accuracy but I know the little 243 is at its limit with those heavier bullets and potent powder charge. So I make a practice of not home reloading those heavier weights. I just don't have a need for em. 75 grain is the rifles preferred and mine too.
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Old February 21, 2017, 06:14 PM   #14
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I used to have a custom 270W that would fire my handloaded 110 Sierras @ 3300, 130 Sierras @ 3050, and 140 Hornadys @ 2900 into about a 2" group at 100 yds with no sight adjustment. Not sure what the trajectories were past that distance, but I was pretty pleased with it. For what it's worth, the rifle had a 24" bbl., a claro walnut stock and was full length glass bedded.
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Old February 21, 2017, 06:25 PM   #15
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My 7X57 will print 140, 145, 150 qnd 154 gr bullets very close to each other at 100 yds. At 200 they are still pretty close, so much so that I could use the same sight setting out to 200 yds and still hit an animal. By the time they get to 300 yds, however, the trajectories start to deviate quite a bit. And those bullet weights are all within 10% from lightest to heaviest. If I throw in 100 gr, 120 gr, 130 gr, 168 and 175 gr, there is no way I could use the same sight setting for the whole spectrum. It has a lot to do with the rifle, powder, etc, but it is rare enough to see a rifle print different bullet weights at the same POI at 100 yds, let alone 300 yds.
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Old February 21, 2017, 07:42 PM   #16
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For MVs within a couple of hundred feet per second or thereabouts at 100 yards? "It don't make no nevermind, nohow."

Likely more variation from different barrel harmonics than from trajectory.
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Old February 21, 2017, 10:30 PM   #17
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Point of impact is one animal.

Trajectory is an entirely different species.
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Old February 22, 2017, 10:58 AM   #18
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I've noticed that trajectory seems to have a direct effect on out-yonder-shot POI.
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Old February 22, 2017, 12:59 PM   #19
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Quote:
I've had a question for a long time that maybe somebody here can clear up (I also posed the question in another forum). In the latest issue of the Rifle magazine, author Stan Trzoniec (whom I have a lot of respect for) was testing a Cooper rifle chambered in 30-06 and made this 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...".
While I don't have an answer I do have an early 90s Remington 700 VSSF chambered in .308 Winchester which will consistently shoot sub MOA at 100 yards with Sierra 150 grain BTHP Match, 168 grain BTHP Match and 180 grain BTHP Match. The rifle did have work and has a Timney trigger. On average 5 shout groups will be within an inch of each other at 100 yards. Originally the scope was setup for the 168 grain BTHP Match bullets.

150 grain SMK
AA 2495 44 grains
2862 FPS

168 grain SMK
AA 2495 41 grains
2538 FPS

180 grain SMK
IMR 4064
2611 FPS

All brass was once fired LC 10 and all primers were CCI BR2 LR. The same ammunition loads did not fare as well in my AR 10 or my M1A. Why all three weights shot about the same point of impact I haven't a clue but in the two other rifles, while the groups were very good each load shot a different point of impact several inches away from each other at 100 yards.

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