Range Report 338 LM Chronograph Readings

[.375Mojave]

Did some load development with my Savage 110 in 338 LM in the desert North of Vegas yesterday and am pretty sure my chronograph is not calibrated correctly.

I ran through a series of loads using Retumbo and Reloader 22 with 250gr SMK's. All charges were in groups of 3 with all recordings made then averaged. Velocity deviations in any given load ranged from as little as 2 or 3 fps to about 22 fps.

This seems appropriate. Where things seem out of whack are my average velocities. My Speer manual is telling me I should see 2900 fps from max loads but my 90 gr RL22 averaged 3033 fps and 98 gr Retumbo loads averaged 3193 fps.

The chronograph was as near perfectly aligned and levelled as I could make it and was 10' from the muzzle per the manual. I had a friend sitting behind me taking and recording readings so I was following instructions to the letter.

Two possible answers, one my readings are correct and I am getting higher than book velocities. This seems doubtful but I would like to hear other people weigh in here. The other possibility is calibration and I would like to know if there are any suggestions on this. It is a "Chrony" unit, nearly new.

Thanks

 

[bohem]

Re: 338 LM Chronograph Readings

Was the unit clearly folder all the way open?

If it's not buy just a little bit you will shorten the distance between the sensors and get abnormally high readings.

Some may point to muzzle blast but that will give you huge swings in velocity, if it was as consistent as you're saying then I suspect you didn't quite get it open all the way.

I've also found mine to be very fussy with lighting and needing fresh batteries.

 

[.375Mojave]

Re: 338 LM Chronograph Readings

Now that you mention it the unit was still not perfectly flat. I tried to force it all the way down but it seemed to be against a a hard stop.

I will take a look at that again and see if there was something keeping it from completely opening.

thanks,

 

[oneshot.onehit]

Re: 338 LM Chronograph Readings

They defiantly like fresh batteries, mine taking a 9 volt.

Another thing that seems to help with variable readings is that if it’s a sunny day, an umbrella shading the chronograph really tends to help with getting good numbers. A few of us have come across this with different models used.

 

[BGE541]

Re: 338 LM Chronograph Readings

You leave your ammo in the sun? What powder you running? Same seating depths on all of them? How many rounds did you fire before cleaning? Just saying theres alot of things that could contribute to swings like that.

 

[Aimsmall55]

Re: 338 LM Chronograph Readings

Reloader 22 is the most temp sensitive powder I have ever shot. Summer I shoot 210 bergers 2907. Winter it's about 2750. Probably the powder I would say unless you hadn't mention retumbo. What kind of scales you using? What was temp?

 

[.375Mojave]

Re: 338 LM Chronograph Readings

Thanks for the questions. Some answers as follows,
1) the loads were all seated .010 off the lands.
2) The temps started in the 40's and got into the mid 50's.
3) the rounds were in the sun but the brass never got to skin temp. I would guess never warmer than 55 or 60 degrees.

All this said, I am going to focus on the fact that the chronograph was not perfectly flat. There was interference from the tripod which prevented it from opening fully. This would have effectively moved the beams closer together.

The other thing I may do is shade the chronograph. It was a sunny day and I was using the recommended plastic canopies for the condition, but it might be an issue.

I hope to get out this weekend and compare more data to what the manual says I should be getting. I will post the results.

Thanks for the input.

 

[AHorn]

Re: 338 LM Chronograph Readings

I get more velocity than Sierra says I will with my 338 Lapua. If it was consistant, just faster than your manual said it should be I would say your chronograph is fine.

 

[esromvre]

Re: 338 LM Chronograph Readings

Also look at your barrel lingth compaired to theirs.

 

[.375Mojave]

Re: 338 LM Chronograph Readings

Good points. I am getting my data out of a Speer manual and now that you mention it I think they are all using a 24" test barrel.

 

[Hoverp]

Re: 338 LM Chronograph Readings

Shoot it through two different chronographs. That way you can check yours and have some confidence ! That's why Oehler 35p use3 screens to check itself and compare, since that's a pretty expensive unit, borrow someone else's and see.

 

[X-fan]

Re: 338 LM Chronograph Readings

There is no way you are getting 3200fps from a 250 without sticking your bolt shut...

You shot in direct sunlight and or shook your chrono screens with muzzle blast. Move the chrono back to 20 feet and create shade for the sky screen ports.

 

[SVG308]

Re: 338 LM Chronograph Readings

My impacts at 1,020 yards show that I'm pushing a 300g SMK at 2875fps with a Rock 30" barrel in my GAP 338. I think 3150-3200 would be doable with a hot 250g load.

 

[TimResin]

Re: 338 LM Chronograph Readings

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SVG308</div><div class="ubbcode-body">My impacts at 1,020 yards show that I'm pushing a 300g SMK at 2875fps with a Rock 30" barrel in my GAP 338. I think 3150-3200 would be doable with a hot 250g load.</div></div>

whats your load? I have a GAP M2008 30in" .338lm

 

[steve123]

Re: 338 LM Chronograph Readings

Those Chrony's are a waist of time and money.

If you don't have access to a real chrono (Oehler 35P), the best thing to do is derive velocity from actual comeups "providing you have accurate distances" using Litz G7 BC and DA. That'l get you within 20fps.

Ditch the R22.

 

[X-fan]

Re: 338 LM Chronograph Readings

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: .375Mojave</div><div class="ubbcode-body">
All this said, I am going to focus on the fact that the chronograph was not perfectly flat. There was interference from the tripod which prevented it from opening fully. This would have effectively moved the beams closer together.

The other thing I may do is shade the chronograph. It was a sunny day and I was using the recommended plastic canopies for the condition, but it might be an issue.

I hope to get out this weekend and compare more data to what the manual says I should be getting. I will post the results.

Thanks for the input. </div></div>

Sounds like you are on the right track.
Shooting any chronograph in the direct sun can create problems...the better ones have taller sun shades built around the sensor is all...Compare all the models and you will see what I mean..the Oehler has the tallest....and the Chrony has the shortest.

Keep in mind the chrono reads the shadow of the bullet...Nothing more or less and is a dumbass simple device.

Shade the openings for the sensors evenly and all will be well.

 

[turbo54]

Re: 338 LM Chronograph Readings

Just for a data point:

85.5gr RE22 + 250 Scenar + 26" TRG42 = 2800fps @ 65°F on an Oehler 35

 

[.375Mojave]

Re: 338 LM Chronograph Readings

I have gotten my unit properly set up now. The unit was not folding completely flat and definitely giving me slightly skewed readings. As to it's accuracy compared to the Oehler I can't really say without having a side by side comparison.

I don't have my notes with me and will double check but I believe my Chrony was giving me 2800+fps with 86gr of RE22 and 250 SMK's so that looks like it lines up pretty well with your load and speeds.

I will check my notes this weekend and post them for comparison.