extenal ballistics

[riverratt]

I have a question that has had me stumped for years, so i figured i would ask y'all if my theory is correct, or even close.

For years i have loaded 180gr rn pro-hunter in 30/06 for most of my deer hunting. It has always worked great for shots inside 150yrd. Several years ago my uncle bought some land that was mainly field and pasture, most shots are over 200yrd. The pro-hunter dont expand well at that range (near floor expantion velosity). After some searching i settled on the 165gr hpbt gameking bullet.

Now for whatever reason my POI at 100yrd is nearly identical (less than an inch differance) but the accuracy is not what i would expect. The 180gr rn out shoots the 165gr hpbt by at least a 1/2" but at 200yrd the 165gr out shoots the 180gr by about an inch. Why is this happening?

My theory is that the flatbaced bullet has more drag on the rear allowing it to stablize faster than the boat tail but im not sure.

[Brian Pfleuger]

Too small of a sample size under two (or too) different conditions.

What you describe is effectively impossible. In MOA terms, it is possible for a bullet to shoot a smaller MOA at a long distance than it did at a shorter one. It's rare but it can happen.

But as you describe it, too much difference over too little range.

More bullets would tell you the truth.

[Bart B.]

Besides too few shots per test group, there could be differences in reloading details. How many shots were in the test groups?

I doubt Sierra Bullets sees more than 1/4 inch difference in accuracy tests for both bullets in their 200 indoor test range. Their best 30 caliber boattail match bullets tested under 1/4 inch in their 100 yard range at the plant in California and now in their 200 yard range in Missouri, under 1/2 inch.

[Jimro]

It isn't "impossible" so much as the group size for the 180 gr round nose bullet will grow faster with range than the 165gr bt spitzer.

Bullets don't fly in straight lines, and groups generally get bigger in MOA terms as range increases.

180gr rn
0.5 MOA at 100 (0.5" group)
2.0 MOA at 200 (4" group)
Growth = 1.5 MOA

165gr btsp
0.75 MOA at 100 (0.75" group)
1.25 MOA at 200 (2.5" group)
Growth = 0.5 MOA

Since you said that the 180gr bullet was at the expansion velocity floor, what I think is happening is that the low velocity is letting conditions affect the bullet more over you 165gr load.

Conversely, a ballistician at Berger recommended 300 yards a minimum for evaluating accuracy potential for one of their bullets to an aquaintence of mine, he never could get sub-MOA performance from that bullet until after 300 yards, when 0.5 MOA is a 1.5" group.

You can get into stabilization theories for bullet flight, where bullets are initially unstable, then hit a "parallel node" of flight with other bullets fired from the same barrel, until they pass a "destabilization node" where they are scattered again. Many of these have been endlessly debated, but it isn't outside the realm of possibility that you will get a bullet that doesn't group as well at short range grouping better at long range.

Jimro

[Bart B.]

If muzzle velocity's got a large spread and the barrel's fairly whippy, it's possible that faster bullets leave sooner in the muzzle axis upward whip than slower ones. The slower one's greater drop down range is compensated for by them leaving at a greater angle above the line of sight. It's rare that this is observed at ranges less than 300 yards, but it could happen with the right combination of barrel whip and muzzle velocity.

The Brits discovered this happening with their SMLE's shooting ammo loaded with cordite over a century ago in rifle matches. Their accuracy at ranges 600 yard and greater was much better than at shorter ones from 300 to 500 yards. In contrast, their Mauser 98 actions used with the same barrel size and length shot the same ammo more accurate at shorter ranges.

[riverratt]

Bart my groups are based off of 5 shot groups 5 min. Between ea. Round. The rifle is a light conture 24" win 70. I have repeated this test several times and the outcome is always the same. 165gr hpbt 3/4" at 100yrd 1" at 200yrd. 180gr rn 1/2" or less at 100yrd 2" or more at 200yrd. The original measurments i gave you were the 2 greatest spreads.c

Call it OCD but it isnt enough to know that something happens i have to know why/how it happens. I am mostly currious if it is an effect of outside elements, wind, or another force at work that i dont understand yet. I am just now trying to understand the science behind everything.

[MarkCO]

It is not uncommon for flat based bullets to shoot a smaller MOA than Open tip match boat tail (OTMBT) bullets inside 200 yards but after that the MOA increases. OTMBT style bullets will typically have a smaller MOA past 200 than flat based spitzers.

You have discovered simply, what many shooters can not understand or believe. I have similar data as your for my .30-06. I shoot most of my final tune up load development work between 300 and 350 yards. Also 5 shots groups, but I am not as patient as you. 5 shots in about 2 minutes, but I put a shot of compressed down the bore between shots.

But why? Flat based bullets have a cone of fire (straight MOA) that is kind of shaped like a tranny fluid funnel. A MOA cone opening to about 200-300 yards, then the MOA starts to increase.

OTMBT bullets have a similar, but slightly larger cone MOA to start, but at 200 when the flat bases start to increase MOA, the OTMBTs have a zero increase in MOA, and with a really well developed load, the MOA can go negative, or a cylindrical (albeit it a curved downward one) group path. This varies by BC, velocity, twist rate and SD. This cylindrical group path is typically several hundred yards long. The lower the standard deviation of the round, and the less wind, the longer, and more likely to be negative MOA, the cylindrical path can be. There are of course extremes that are better or worse in certain environmental conditions.

Examples from a 24" Kreiger .223Rem.

50 grain flat based spitzer groups 0.4" at 100 yards, 1.1" at 200 yards, 2.9" at 300 yards and 5.3" at 400 yards. This load produced my smallest ever 100 yard group at 0.213" for 5 shots.

77 SMK groups .6" at 100 yards, 1.1" at 200 yards, 1.4" at 300 yards and 2.2" at 400 yards. This load produced my smallest ever 600 yard group at 2.33" for 5 shots. I was able to produce the same group sizes from about 320 yards out to about 450 yards with this load. From 330 to 420, the load drops right at 20". That made it pretty easy to set up an aiming point on a clear sheet of plastic and put another one 90 yards downrange to see the group of the same 5 bullets 90 yards further.

The MOA of the 77 is about half that of the 50s at 600 yards, but the 50s have a 33% smaller MOA at 100 yards than the 69s.

[Bart B.]

Sierra's ballistics tech I used to shoot matches with told me his tests with their 30 caliber hunting bullets in factory sporter barrels typically showed better accuracy with flat based bullets. Their loading manuals have mentioned this. Match grade custom barrels typically shot boattail bullets more accurate. But the difference wasn't more than about 1/4 to 1/3 MOA in their 100 yard range.

While it's possible that compensation for velocity spread happen with departure angle change from barrel whip, there's no way a bullet knows what trajectory direction to change to when it's at the outside edge of a midrange group to head back to the center of a further downrange group.

[Brian Pfleuger]

Originally Posted by riverratt

Bart my groups are based off of 5 shot groups 5 min. Between ea. Round. The rifle is a light conture 24" win 70. I have repeated this test several times and the outcome is always the same. 165gr hpbt 3/4" at 100yrd 1" at 200yrd. 180gr rn 1/2" or less at 100yrd 2" or more at 200yrd. The original measurments i gave you were the 2 greatest spreads.c



Call it OCD but it isnt enough to know that something happens i have to know why/how it happens. I am mostly currious if it is an effect of outside elements, wind, or another force at work that i dont understand yet. I am just now trying to understand the science behind everything.

What you describe is nearly impossible. All the bullets leave from the same point in space, if they spread out to 3/4" in 100 yards, they must spread out to very, very close to and almost always more than 2 times 3/4" at 200 yards.

Boattail bullets have been shown to "settle" and outshoot flat base bullets at long range that they did not outshoot at short range but your numbers are too dramatic and your ranges too short to see this phenomenon.

If you've done this several times, overlay your groups and make one conglomerate group for each bullet at each range.

[Jimro]

165gr hpbt 3/4" at 100yrd 1" at 200yrd. 180gr rn 1/2" or less at 100yrd 2" or more at 200yrd. The original measurments i gave you were the 2 greatest spreads.

Actually that doesn't sound too outrageous to me.

I had a 168gr HPBT load that would 1" at 100, and 1.5" at 200 repeatably.

Think about it this way, in the first 100 yards the bullet would drive +/- 0.5 inches into a 1" grop. In the second hundred yards it wold drive another 0.25" into a 1.5" group. All in no wind conditions. That was my introducting into "initial bullet instability" during flight.

Bullets are stabilized by two factors, spin and drag. The net drag effect on the bullet will keep the tip of the bullet pointed into the center of pressure of wind (according to Bryan Litz). If the bullet "wobbles" immediately after leaving the muzzle it will take the net drag a while to get the bullet into a "stable flight profile" which is generally 80 yards or so downrange (with a 1:10 twist) that is around 288 revolutions of the bullet.

If you test the bullets again at 300 yards, I bet you'll find they match closely to the 200 yard groups.

Jimro

[tangolima]

I agree with jimro. Boat tail bullets have better ballistic coefficient, compared to flat base, but they are longer. They have shorter flight time for longer distance, and thus better accuracy for long range. The price to pay is longer time to settle upon exiting the muzzle.

When a projectile exists a muzzle, it will have certain yaw. Gyroscopic stabilization corrects that, but the correction will over shoot. The projectile will oscillate back and forth with the magnitude of the yaw decreasing. BT bullets just take longer for this.

The range I frequent only has 150 yards. I always try to buy flat base bullets as I know the extra cost for fancy boat tail won't do me any good. But given the current shortage, I have to use whatever available.

-TL

[Bart B.]

I don't think the difference between equal quality bullets of either type and of the same weight and diameter will be very much. Not when the best of both types shoot under 1/4 MOA through 200 yards in bullet company indoor test ranges.

[Metal god]

I like Bart's idea of different barrel harmonics and Brains idea of over laying your targets to get more shots in your group sample . Using the exact same type of targets shoot a 5 shot group then place the same type target directly over the target you just shot and shoot another 5 shot group . Do this 4 times then remove the top 3 targets . This will now give you a 20 shot group on your first target and should tell you everything you need to know .

I'm sure it's possible but it does seem odd to me that a rifle/ammo combo that can shoot sub 1/2 moa at 100yds can barely manage 1 moa at 200yds . Could it be the BC of each of those bullets . The RN must have a much smaller BC and higher drag then the BTHP . Can that cause the bullet to stay less stabalized the whole time causing the group to open up faster yardage wise ?

[Bart B.]

Metal god, barrel harmonics are the same for every shot fired regardless of bullet weight, shape or muzzle velocity. All barrels (free floated) have the same fundamental (resonant) frequency and harmonic multiples thereof. Their shape and metalurgy stays the same for each shot.

There is a small spread of BC for a given bullet. The more a bullet's unbalanced, the lower its BC will be. Most decent hunting bullets have a BC spread of about 3 to 4 percent. Match bullets are down in the 1 and 2 percent range. A 4 percent spread equates to about 1/30th inch difference in bullet drop at 200 yards for .500 G7 BC 175 gr. 30 caliber bullets leaving at 2700 fps.

[Metal god]

hmm well I know what I was trying to say , just did not articulate it well . I meant those two very different bullets that I'm sure are loaded different as well , will react differently in the gun . I should have asked if these are hand loads and are they loaded to the best the rifle can shoot them ? Maybe he just needs to tinker a bit .

[riverratt]

Meal god the two bullets are indeed loaded different my 180gr rn is my original load 54.6 gr of IMR 4350. This was the only powder i tried worked it up for accuracy at 100yrd. After my uncle bought his property and i shot the first doe over 200yrd i was disapointed with the bullets preformance. I called sierra and the tech toled me that the bullet is reaching the floor expantion at just over 200yrd so it wouldnt be as reliable, he then sugjested the 165 HPBT gameking.

I worked the load up for the 165gr to match the POI at 100yrd not for accuracy. I liked the preformance of the 180 at under 100yrd and didnt want to change. The 165gr uses a separet die with no crimp i load 57.2gr of IMR 4350 in that one.

[Bart B.]

I'd load both bullet weights for best accuracy with maximum loads. Their trajectory's won't be more than about 1/2 MOA apart through 300 yards zeroed at about 200 yards.

And you shouldn't need to crimp bullets.

Metal god, what do you mean by those two very different bullets that you're sure are loaded different as well , will react differently in the gun. Please explain "react."