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375-408 CheyTac (Kirby Alert!!!!!!) question

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Unread 09-10-2006, 07:08 AM
Bronze Member
Join Date: Oct 2005
Posts: 50
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

Morning guys,

Dave, you said it perfectly. When we were doing our Doppler radar testing at Yuma Proving Grounds, we had access to the brains and observations of all those testers there. They noticed this with every high velocity round that was shot. They have one of the 2 remaining HARP guns there, we got to see it. High velocity to them is over 5000 fps, but the effects of velocity is the same, faster you push it, the faster she slows down. It's all in the SHAPE of the trajectory that is affected. Higher velocities push out the rising branch farther, that "flatter trajectory" part of the trajectory that hunters talk about. The falling branch IS farther downrange, but is steeper. Not all trajectories are of the same shape, UNLESS they are fired into a vacume. Then there is an even parabolic curve because gravity affects the projectile the same from beginning to the end. The air resistance is greater at high velocities and less at lesser ones. The RATE of deceleration will not stay the same of course as the projo slows down. BUT, overall, on the average, it will be decelerating faster than a bullet that is launched at a slower speed.

Then there is the darn issue of transitional speeds, when she goes from supersonic to sub-sonic flight. That's where they loose their minds. If you've ever shot 168 gr. Sierra MK at 1000 yards in a standard load, you'll see the effects, they nearly always go through the target sideways, tumbling after destabilizing from the transition to sub-sonic flight. That supersonic shockwave collapsing over the rear of the bullet while it is yawing and pitching causes havoc on them.

Sorry, read through these all, haven't had a chance to stay in the discussion. Have been on the mountain doing some work. Later guys, this is an awesome discussion!!! Sniping Operations Executive
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Unread 09-10-2006, 09:08 AM
Silver Member
Join Date: Apr 2006
Location: Chickasaw Point, SC
Posts: 206
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

Good morning,
This is really getting to be a very entertaining and informative discussion. While I was in college, I always wondered why we were required to study this stuff as I saw no practical application for it; now I know. It appears that triggerfifty has his facts in order concerning the physics of energy depletion and aerodynamic resistance. I’ve been trying to formulate an axiom that would illustrate the maximum efficient velocity of a 300 grn., .338 dia. bullet with an average b.c. of .8; but due to all the possible variables and/or my inept mathematical skills, I haven’t found a formula that will work. One thing that has been proven to be a self-evident truth is: Those of you who can consistently make accurate shots at 1000+ yards are using components built by very skilled and precision craftsmen and your hand/eye coordination and situational awareness is equivalent to that of any professional athlete. Of all the millions of people who participate in the shooting sports you guys are in a tiny minority. You defiantly have my respect, and you deserve the highest rewards for your efforts.
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Unread 09-10-2006, 02:01 PM
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Join Date: Jun 2004
Location: Fort Shaw, Montana
Posts: 6,841
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

Alright, lets see the numbers now. I want to see the down range numbers to compare head to head just for comparision sake.

Lets say a 300 gr SMK loaded to two different velocities, say 3500 fps and 3200 fps.

WHat are the numbers at say 1500, 2000, 2500 and 3000 yards?

Drop, wind drift, flight time, retained energy.

I will admit I am also a pretty simple guy. I know how to build a rifle to shoot relatively well. All this indepth ballistic information is over my head most of the time.

That said, I work on what I see in the actual field. What I see when I compare a 36" barreled 338 Kahn loaded with a 300 gr SMK to a velocity of 3100 fps is that this rifle is EXTREMELY consistant to about 2100 yards at out 3500 ft altitude.

Past that, everything goes to hell consistancy wise and pretty much to the exact yard. Shoot at 2200 yards and you will be hard pressed to keep the bullets in a 10 yard radius whereas just 100 yards closer it will easily shoot in the 1 moa range consistantly.

On the same day, with my 338 AM loaded with the same bullet and fired out of the same 1-10 twist Lilja barrel but at 40" of length, with a muzzle velocity of 3400 fps, the results are totally different. Out to 2400 yard, the rifle shoots easily to 1 moa with very consistant, predictable bullet placement for that range on every shot.

I have tested a bit to 2500 yards and still consistancy is in the 1 moa range easily in good conditions if I do my part.

But you tell me that in actual real life tests, the slower round will outperform the faster one at long range. I am not seeing that from actual shooting and seeing where the bullets land. No dopler, just watching bullet impact at various ranges.

I do not understant what would be different with my shooting then with your testing but maybe elevation is the issue here. My air is thinner up here, less resistance to the bullet so maybe your results are only valid at sea level and not at any real elevation.

Luckily, most of the best shooting is well above sea level [img]/ubbthreads/images/graemlins/grin.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]!!!

Anyway, just confused a bit here, not hard to do at times!!!

Maybe at higher altitudes, the faster bullet does not slow down as quickly as it does at sea level?

Kirby Allen(50)
Kirby Allen(50)

Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

Farther, Faster and Flatter then ever before.

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Unread 09-10-2006, 06:44 PM
Platinum Member
Join Date: Jul 2005
Posts: 1,651
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

Fifty, I am confused at what triggerfifty is trying to explain. I have never heard this type of explination before, but like I said, I am not a Mathmetician, so it is confusing. Anyhow, I am trying to learn, but this is confusing.

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Unread 09-10-2006, 06:55 PM
Silver Member
Join Date: Apr 2006
Location: Chickasaw Point, SC
Posts: 206
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

I will do my absolute best to meet your request for the math calculations.
Elevation and wind are not at issue in my figures, but I think I see where the obvious discrepancies come into play. My calculations are strictly based on physics and aerodynamic resistance. And to be honest my aerodynamic skills are very rusty. My calculations do not consider a parabola shaped path of trajectory or the collapse of the vortex behind the bullet when it goes sub-sonic, I’m not a ballistics expert, but I do spend several hours a month calculating energy depletion, so my numbers are restricted to proving that a projectile decelerating from a greater speed loses energy and speed at a greater rate than a similar projectile with less total deceleration. I’m not doing this to agree or disagree with anyone’s knowledge and experience, there just seems to be a great deal of interest and question in the various theories of velocity and resistance.

Here are my calc’s:
.338, 300 grn. SMK, b.c.= .768 > 2300 fps, .760 < 2300, .750 fps < 1800 fps; meplat dia. = .015" (estimate)
resistance factors: absolute barometer = 26.82”; absolute atmospheric pressure = 12.93 psia; relative humidity = 57.8%; Mean Free Path = .000001”; Frequency of Collision @ 3500 fps beginning velocity = .98525;
Frequency of Collision @ 3200 fps beginning velocity = .91405

Starting velocity = 3500 fps
Range (yds) Velocity (fps) Energy (ft/lbs)
0 3500 8158.8
1500 1953.2 2541.3
2000 1557.4 1615.7
2500 1244.5 1031.7
3000 1047.3 730.6
3500 936.3 584

This round should go sub-sonic @ 2785 yds.

Starting velocity = 3200 fps
Range (yds) Velocity (fps) Energy (ft/lbs)
0 3200 6820.1
1500 1883.2 2450.2
2000 1453.2 1507.6
2500 1183.4 908.1
3000 1031.1 719.3
3500 937.6 584.8

This round should go sub-sonic @ 2526 yds.

This is just my interpretation of the issue, I’m sure Kirby’s practical experience and triggerfifty’s Doppler testing are far more accurate and applicable. Any comments or corrections would be most welcome.

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Unread 09-10-2006, 07:36 PM
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Join Date: Dec 2001
Location: Mukilteo, WA
Posts: 1,092
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

A couple notes:

Some are wondering how the faster bullet slows down faster with the same BC. That's just the way it works. With the same cd the force of drag is proportional to the square of the velocity. If you look at any chart assuming a constant BC, you'll see the bullet loses more velocity in the first 100 yds than it does in the second, more in the second than in the third, etc. That will happen all with the same BC. It's just the way things work. This is why a 200 fps advantage at the muzzle does not equate to a 200 fps advantage downrange--the slower bullet will catch the faster bullet on the velocity vs. distance curve. Keep in mind, that's all with the bullet's drag curve not changing at all.

That said, I would like to see some more detailed numbers showing what triggerfifty described. In fact, I'd like to take back the comparison I made earlier. I should have though about that a bit more before posting.

With a 200 fps advantage at the muzzle, the slower bullet will catch up velocity vs. distance wise (less than 200 fps downrange). But if the muzzle advantage equates to 200 yds, I don't see how the slower bullet will catch up distance-wise as much as I stated without a serious change to its drag curve. Sure, it could happen. With a 200 yd head start you shouldn't expect exactly a 200 yd supersonic range advantage...but it should be pretty close unless something odd is happening to the bullet which changes its drag characteristics (overstabilization, not "nosing over" at long ranges, etc).

Like I said, it could happen, but I think it would take a serious change to the bullet's drag curve to do it. If triggerfifty has an explanation of how it happens without, I would be very interested to hear it.
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Unread 09-10-2006, 10:04 PM
Join Date: Nov 2004
Location: El Reno, OK
Posts: 1,497
Re: 375-408 CheyTac (Kirby Alert!!!!!!) question

Okay, I guess I am still just not seeing how the faster of the 2 identical bullets would go sub sonic first. I understand what people are saying here but I guess. I just dont see it. This is no ones fault but my own. When numbers show up on here my mind drifts off to hunting, fishing, naked chicks, you know the usual stuff [img]/ubbthreads/images/graemlins/grin.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]

Everything in my poluted mind tells me that two identical bullets launched at differing speeds would have the faster bullet retain super sonic ability for a longer time.

Again not argueing here, numbers just scare me [img]/ubbthreads/images/graemlins/grin.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]

Thank God my wife does the check book!!!!!!!

Steve Elmenhorst
Third Generation Shooting Supply
"Products for shooters, by shooters"
monday-friday 8:30-5:30 CST
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