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What the heck is goin on?

 
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  #1  
Old 08-29-2005, 07:40 PM
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What the heck is goin on?

I just got an Ohler 35 triple skyscreen chrony and then went out and shot through it. I shot 5 shots through it at 3 feet away from the middle skyscreen. I got an average of 2701 fps with my 7mm rem mag and 175gr. Sierra Matchking. I then moved it out to 53 feet away and shot through it 5 more times. It gave me an average of 2666. I ran these numbers through the JBM Calculations progam to get the actual BC. It gave me .466 It has a published BC of .608. I tried both of these numbers with the muzzle velocity of 2701 and it gives me TOTALLY wrong drop charts! I went out and shot the other day and the actual impact is WAY different than ANY drop chart I can make.

WHAT IS GOING WRONG? Any ideas are greatly appreciated.
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Old 08-29-2005, 08:13 PM
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Re: What the heck is goin on?

I have the 35P, and sometimes get real bogus numbers, shooting a big 300 close to a concrete block wall?

Are you using the two foot tube that comes with a 35P? Have you set all the dip switches accordingly? I'd get a four foot tube, regardless of any other problems you discover because I think it is a lot more accurate, which could be your problem?....just don't try to make one yourself, I had mucho problems until I bought the factory one. Besides that I am not familiar with your spacing, but maybe it is something new to me, because I would probably put the screens at ten feet and one hundred yards?

Good hunting. LB
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Old 08-29-2005, 08:22 PM
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Re: What the heck is goin on?

At those close ranges the bullet may still not be fully stable and that will cuase a low BC or high initial velocity loss. The 175 SMk is a the upper range of what you can expect to stablize. I assume you have a 9.25 twist barrel. After a couple of hunded yards you may regain the high BC from this bullet. Be careful not to shoot you chrony if you try to measure it out there. I recently tried the 175 SMK but the 160 AB with a long bearing surface and not so much boattail stabilizes better for me.


here is what Sierra says and I expect that it applies to your case because it is a match type bullet you are shooting


. Any of the firing test methods measures a ballistic coefficient of the bullet as it flies through the air, including effects imparted by the gun, the cartridge, and firing point environmental conditions, as well as imperfections in the bullet. Theoretically, the BC of a bullet depends only on its weight, caliber and shape. But in a practical sense, the measured BC of a bullet also depends on many other effects.

The gun can affect the measured BC value in two important ways: spin stabilization and tipoff moments. A bullet is gyroscopically stabilized by its spin, which is imparted by the rifling in the barrel. If a bullet is perfectly stabilized by its spin, then its longitudinal axis (which is also its spin axis) is almost perfectly aligned with its velocity vector. If a bullet is not perfectly stabilized (which is usually the case), the bullet may not be tumbling, but its point undergoes a precessional rotation as it flies. In previous editions of Sierra’s Reloading Manuals we have described this precessional rotation and have called it “coning” motion to aid in mental visualization of the motion. As the bullet flies, the point rotates in a circular arc around the direction of the velocity vector. <font color="blue"> Coning motion results in increased drag on the bullet, and any firing test method then yields an effective BC value for the bullet that is lower than the theoretical value.</font> The rifling twist rate in the gun barrel and the muzzle velocity together control the spin rate of the bullet, and therefore control its degree of stability.

When a bullet exits the barrel, it generally has a small angular misalignment, which ballisticians call “yaw.” Yaw is caused by tipoff moments of torque applied to the bullet by powder gases exiting the barrel nonsymmetrically around the bullet, or by barrel whip or vibrations. This angular misalignment will cause coning as the bullet begins to fly downrange. Coning can also be caused by an abrupt exit of the bullet from the barrel into a crosswind, although BC measurements should never be attempted when winds exist at the firing point.

The cartridge used in the firing tests affects the measured BC values mainly through the muzzle velocity it produces. As noted above, <font color="red"> muzzle velocity combines with the twist rate in the rifling to produce the bullet spin rate, which in turn controls stability</font>. In addition, BC values change with the instantaneous velocity of the bullet, and so the muzzle velocity directly affects the measured BC value of the bullet. For example, a 180 grain 30 caliber bullet can be fired at a much higher muzzle velocity in the 300 Winchester Magnum than in a 308 Winchester cartridge. The same is true for a 240 grain 44 caliber bullet from a 44 Magnum compared to a 44 Special. So, the measured BC values can be expected to be different just because of the different starting velocities.
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Old 08-29-2005, 08:22 PM
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Re: What the heck is goin on?

Suggest that you re-read your manual. Standard distance from start screen to muzzle is 10 feet. Get your screens farther apart as suggested, I had a machinist friend make me a setup for exactly five foot total, second screen is at 2.5 feet from start screen. The longer aluminum conduit is a great idea. Your switches have to match the distance your screens are apart, they are under your battery. I always check them by comparing with the chart in the manual. Used to use a ten foot spacing, that is more accurate but more diffictult to setup.
I also doubt that 53 feet is long enough to provide the velocity difference you are needing, the big 83 systems that I have seen in use all have the second set of screens at 100 yards. Tougher to shoot through the screens way out there but much better info. I routinely shoot through my Oehler screens all the way out to 500 yards, have two 35P's so I can get a muzzle velocity and down range velocity. Have a steel baffle setup to protect the tripods and screens and have killed several skyscreens over the years - even snipped a cable right were it entered the skyscreen body.

Good luck, you have a good machine, just get some experience with it and you will learn a lot.
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Old 08-29-2005, 08:51 PM
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Re: What the heck is goin on?

I second what Ian said.

3 feet is WAY to close to the muzzle to get an accurate reading. It states that in the destruction manual.
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Old 08-29-2005, 09:09 PM
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Re: What the heck is goin on?

First thanks to all! Bob, you (or sierra?) may be right because I was shooting in wind and yes it seems that 175 gr bullet is a little much to stabilize with a 9.5 twist barrel. So Yaw or wobble is quite possible. The velocities were as expected from the loads I was shooting but I will try to move the screens farthur apart on the next go around.
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Old 08-29-2005, 09:21 PM
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Re: What the heck is goin on?

Ian, I will indeed move the chrony out to 10 feet and then 100 yards and we'll see what happens I'll also order a longer rail and have it on it's way. I hope that it sorts something out. Although I have used several different BC's and none even come close to matching what I have for 200, 300, 400, 650? But first things first. Getting an accurate velocity and an accurate BC. Then I'll go from there.

THANKS
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