What the heck is goin on?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Kenster-Boy, Aug 29, 2005.

  1. Kenster-Boy

    Kenster-Boy Well-Known Member

    Messages:
    180
    Joined:
    Apr 1, 2004
    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.
     
  2. LB

    LB Well-Known Member

    Messages:
    423
    Joined:
    Jul 22, 2004
    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
     

  3. Buffalobob

    Buffalobob Writers Guild

    Messages:
    5,083
    Joined:
    Jun 12, 2001
    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.
     
  4. Ian M

    Ian M Well-Known Member

    Messages:
    2,410
    Joined:
    May 3, 2001
    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.
     
  5. goodgrouper

    goodgrouper Well-Known Member

    Messages:
    2,705
    Joined:
    Sep 3, 2004
    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.
     
  6. Kenster-Boy

    Kenster-Boy Well-Known Member

    Messages:
    180
    Joined:
    Apr 1, 2004
    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.
     
  7. Kenster-Boy

    Kenster-Boy Well-Known Member

    Messages:
    180
    Joined:
    Apr 1, 2004
    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
     
  8. Ian M

    Ian M Well-Known Member

    Messages:
    2,410
    Joined:
    May 3, 2001
    I would not worry about the numbers until you get some good data from your new machine. You are probably in a learning curve if this is your first Oehler and you might run a search for info on setting up Oehlers since we have covered that extensively. I use a laser to setup my screens, works great and saves time. Good luck with your new toy, you picked the best.

    By the way, black and yellow pieces flying through your scope picture is not good when you are shooting through Oehlers /ubbthreads/images/graemlins/smirk.gif I been there so many times I can rebuild skyscreens blindfolded.
     
  9. jb1000br

    jb1000br Well-Known Member

    Messages:
    1,307
    Joined:
    Jul 8, 2003
    [ QUOTE ]
    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.

    [/ QUOTE ]

    +1...no...+10!!

    with the bigger case, 15feet wont hurt either /ubbthreads/images/graemlins/wink.gif

    JB
     
  10. Mysticplayer

    Mysticplayer Writers Guild

    Messages:
    1,459
    Joined:
    Jul 27, 2001
    Let's see if recent experience agrees with yours. Using JBM ballistic software too and hornady bullets. With their listed BC or just mucking about, not even close. All scope adjustments retested and verified on a measured surface so I know that part is right.

    Solution...which G function are you using? G1? If so, the data the program spits out is completely wrong.

    With a 162gr Amax, I had to use the G7 profile and got within 2 clicks out to 650yds. Close enough for me.

    So the BC debate can continue to rage. A 0.65 G7 BC is way higher then a G1 0.65 BC. I now just enter what I field data then change the G function until I get something that is close.

    If three points are close, the rest of the data will also work out.

    Honestly, don't care which G function I end up with. I just want a drop table that works for my load.

    Jerry
     
  11. uncleB

    uncleB Well-Known Member

    Messages:
    558
    Joined:
    Aug 19, 2005
    T.O.,
    do you by chance play in eastern PA. if so quit jacking us around,and tell us what your agenda is.
    UB
     
  12. abinok

    abinok Writers Guild

    Messages:
    877
    Joined:
    Nov 25, 2004
    additionally, lets not forget that by using the same chronograph at both ranges, and consequently comparing average velocity of rounds fired at each range, you are effectively building the ES of your load into the accuracy of your BC measuring. Just something to think about.
     
  13. Mysticplayer

    Mysticplayer Writers Guild

    Messages:
    1,459
    Joined:
    Jul 27, 2001
    Nope don't live anywhere Eastern USA. About as far west in CANADA as can get without getting wet. So no, I do not BS and have no agenda except sharing experiences through burning powder. I think they refer to that as real world experience.

    I wonder what Eastern American agenda might be anyways? Want to elaborate????

    Speaking of which, I went out and reshot my drop chart as I do just to make sure everything is right for hunting season.

    Using the data generated by the JBM software, my drops at 5 distances were within 1 click (furthest one was off by 1 click) of predicted data. Ranges were verified using a Leica 800 rangefinder on a beautiful clear and windless day.

    Here are the come ups: 375yds 4min, 550yds 7.5min, 650yds 9.5min, 750yds 12.0min, 950yds 17min (predicted was 16.75min). Muzzle velocity is 3150fps. 7Rem Mag with moly 162gr Amax. Rifle Savage 110 w/brake (I like to spot my impacts). Scope my trusty Elite 4200 6X24 w/ mil dot.

    Just input my data G7 drag function. 0.625 BC and voila. That's the drop table that comes out. Pretty flat....

    Now change to a G1 drag function and change the BC value until you get something close. I bet you laugh at what that value has to be. Try with the other 5 G functions and have some fun. You will quickly come to the same conclusions I did.

    So when comparing BC's, ask which drag function was used in the calculation of that value. The number is not just the number after all.

    I hope that the bullet industry will someday standardize the BC values vs G drag function (all flat base bullets use one G function, BT/VLD designs another or whatever) so that we can continue to compare apples to apples as we have been lead to believe all these years.

    Jerry
     
  14. Kenster-Boy

    Kenster-Boy Well-Known Member

    Messages:
    180
    Joined:
    Apr 1, 2004
    Jerry, it's so funny that you say that now because I thought about that a long time ago and then for some reason I read something that said most bullets use the G1 drag function. So I figured that if it was standardized I just wouldn't mess with it but it could quite possibly be my culprit. Because it doesn't matter what BC I use it just NEVER comes anywhere close. I had one that was 4 minutes off at 300 yards /ubbthreads/images/graemlins/confused.gif. But I will give the other G functions a try, THANKS!