Oehler Velocity readings?

[Nicholas]

Hi, I have an Oehler 35p with the 4' bar.
I have a few questions:
As fellow owners know the Chronograph prints out two separate velocities for each shot. I am assuming the first reading is for the first set of screens and the second for the second? The difference between the two readings averages about 14 fps.

Which reading is the most accurate to use?


Also I was reading the manual and it tells you to add in the velocity that is lost in the 10' gap between the muzzle and the chronograph.

Do you guys add in the lost velocity? What about when putting velocities in ballistic programs, for a ballistics program would you use the actual reading on the chrono or the actual velocity at the muzzle????


If the two readings are for each set of screens then I can assume that the difference is the velocity loss for the two feet between the middle of the screens. So I would have approx 7 fps loss per foot, mulitplied by 10 feet gives me 70 fps loss by the time the bullet gets to the chronograph.

I may be getting into too much detail but I want to know the answers to these questions.

 

[Brent]

The screens use two stop screens and one start screen to get the two chrono readings. Here's how they work;

The first screen the bullet passes over starts both chronographs, the second screen stops the PROOF chrono and the third screen stops the MAIN chrono.

The MAIN crono reading is the column to the right on the printer which all the other data is derived from, the PROOF chrono reading is on the left side and stands alone... no other data is calculated from this chrono reading.

The MAIN chrono's accuracy is better than the PROOF because the screen spacing is 4' rather than that of the PROOFS 2'. The proof will detect large errors and help you to keep your sanity in check.

I use the Oehler 43 mainly and it converts it to MV with the distance to the screen I type in. The 35 needs to have the velocity loss added back in for the distance the MIDDLE screen is set away from the muzzle. If I'm nailing down an average MV for a load to use at LR in a ballistic program, I'll get that specific. The 43 is always MV though, so it's not an issue.

Here's what to do;
Velocity loss in 100 yards with your load divided by 300' will give you the velocity loss per foot you need to figure what the MV is for the muzzle to screen distance in feet you use. A ballistic program will tell you the loss at 100 yards.

At 3000 fps the 4' screens will be within 4 fps of actual, the 2' screen spacing will be within 8 fps. 1' would be 16 fps and likewise 8' would be 2 fps.

Good luck and enjoy!

 

[Nicholas]

Brent

Thank you very much! You ansered every question. Makes much more sense now.

I will run the numbers.

 

[Ian M]

90,
I find that the truer my screens are to the actual bullet path the lower the spread between the two velocities, I can change the height of each end of my rod that holds the screens and find that sometimes a 1/2 inch or so will make a difference. Nice to have the two velocities as close as possible as there can't be much loss in a couple of feet.

I just go with the second reading for all my charts. On the other hand, maybe velocity variations in that first 10' feet can be used as an excuse for some of my misses...

 

[Guest]

I'm a perfectionist myself but I can't see why you would even add the 10' back in. It can't be off by more than 6 or 7 fps.

 

[Nicholas]

It appears to be off 3.8 fps, not very much but you got to ring out all the velocity you can.

 

[Holmes]

I admit to being one of those nitpickers that factors in the 12' additional velocity.

With my cartridge it amounts to around 11 fps.

I need every little bit of help I can get and the fewer variables the better

 

[Guest]

SR90, the 300GR SMK is off by about 3.8 with the 338, but 156 Cautertio is off between 6 and 7 in my 7mm STW.

Either way....good enought for me!