38 SC using SR 4756 strange chrono results

[dvc4you]

Soooo the only powder I could get to run my new open blaster is IMR SR 4756.

I am using MG 124 g out of Shuemann Tribrid II with winchester SR primers and armscor 38 supercomp brass.

The following are the results I got from my brand new Prochrono Digital:

8.2g 1272 fps 158PF

8.4g 1322 fps 164PF

8.6g 1480 fps 183PF

Does it make sense such a spike? All were chonoed on the same session several times using 5 bullets at a time....Powder is all from the same lot. Clear skies and 78 F.

[Steve RA]

I used to use 4756 in 9 Major and .38 Super and never had any spreads that large ( with that small a change in powder weight).

You might try using a different chrono just to check yours since you mentioned it being new.

[peterthefish]

I've heard that 4756 has a reputation for spiking near max fill / load, which may why Hogdgons has neutered their published load data for this powder.

[lee blackman]

Maybe the powder charge compressed at 8.6gr, that would cause it.

[Justsomeguy]

Or maybe the chrono's sensors come into direct sunlight as the day wore on. Since a chronograph generally reads the bullet's shadow to give the speed, different sunlight conditions can produce some weird readings. That is why on sunny days you should always place something over the skyscreens to shade the sensors.

[Hi-Power Jack]

How did the primers look?

[dvc4you]

The primers looked fine.

All of those are compressed charges

Sent from my iPhone using Tapatalk

[Hi-Power Jack]

The primers looked fine.

If primers look fine, I'd chrono 20 of the 8.4 and 8.5,

and see how that looks.

IMHO, five shots thru a chrono is a good start, but not

sufficiently large to be accurate - 20 of each should tell

you a lot more - and I'd shade the chrono.

[dvc4you]

I will go back to the range this afternoon to re-chrono my loads of SR-4756 of 8.6gr and 8.5gr.

It is a sunny day, almost no clouds, what do you suggest?

Induce some shade on the sensors, paint the bullets black with a marker?

[Hi-Power Jack]

Induce some shade on the sensors, paint the bullets black with a marker?

I haven't tried painting the bullets, but its not a bad idea.

But, yes, shade the sensors.

Can't wait to see the results:))

[Steve RA]

Put a target - USPSA - on top of the sunshades with the white side down.

[dvc4you]

So I was lucky enough that the sky was overcast. Results are more consistent.

Using MG 124 g out of Shuemann Tribrid II with winchester SR primers and armscor 38 supercomp brass. OAL 1.235. IMR SR-4756.

20 rounds through Prochrono Digital:

8.5g AVG 1306 HI 1333 STD 12 161.9 PF

8.6g AVG 1323 HI 1340 STD 14 164.1 PF

Primers show no sign of pressure.

Next step 8.8gr and 9.0gr

Edited June 6, 2014 by dvc4you

[Hi-Power Jack]

Sounds like you're on the right track. The PF 186 for 8.6 gr didn't sound right -

PF 164 makes much more sense.

[Justsomeguy]

Yes... cloudy days are much better for chrono accuracy, but can be done on sunny days when you properly shade the sensors. If you use a camera tripod to mount the chrono, you can also use the tripod's tilt feature to lay the chrono over 90degrees away from the sun and shoot through it like that. I always get good readings that way when the sun is directly overhead and I don't want to mess with taping something to the skyscreens which generally has to be larger than even a USPSA target is to properly shade both sensors completely.

[Hi-Power Jack]

...lay the chrono over 90 degrees away from the sun...

Great idea!!! Never would have thought of it ...

That's the kind of thinking I love here at BE.

Thanks for the tip. :bow:

[dvc4you]

Yes... cloudy days are much better for chrono accuracy, but can be done on sunny days when you properly shade the sensors. If you use a camera tripod to mount the chrono, you can also use the tripod's tilt feature to lay the chrono over 90degrees away from the sun and shoot through it like that. I always get good readings that way when the sun is directly overhead and I don't want to mess with taping something to the skyscreens which generally has to be larger than even a USPSA target is to properly shade both sensors completely.

Thanks for the tip! I used the technique and it worked perfectly:

8.8gr avg 1350 hi 1375 lo 1322 PF 167

9.0gr avg 1365 hi 1384 lo 1346 PF 169

[Neomet]

This is why I made my uber-high tech enclosed chrono. I took the box I got my RF100 in and cut a couple holes in what became the top that were slightly smaller than my sun screens directly in line with the chrono sensors. I racer taped the sun screens over the holes. Cut one hole in the front to insert the chrono and read the display and another hole above it front and back for the bullets to pass through.

Gives me very reliable readings. It is a little clunky to carry and I need to set it on a table but with the AZ sun it is definitely worthwhile.

[GARD72977]

We were using my crony to get velocity for a friends reloads. I had a spike of about 130 fps for a change of .2gr of powder. I thought he had made a mistake since he has not been reloading very long. I let another friend use my crony and he found that if you favored the right side you would get 100 fps difference than the left. He was shooting a paper target (with a air gun) and happened to change to another target without moving the gun. I would have never found this because I usually try to line up the middle of the crony with the target.

[RH45]

Yes... cloudy days are much better for chrono accuracy, but can be done on sunny days when you properly shade the sensors. If you use a camera tripod to mount the chrono, you can also use the tripod's tilt feature to lay the chrono over 90degrees away from the sun and shoot through it like that. I always get good readings that way when the sun is directly overhead and I don't want to mess with taping something to the skyscreens which generally has to be larger than even a USPSA target is to properly shade both sensors completely.

Thanks for the tip! I used the technique and it worked perfectly:

8.8gr avg 1350 hi 1375 lo 1322 PF 167

9.0gr avg 1365 hi 1384 lo 1346 PF 169

I hope you bought a 12# can. Every time I change lot numbers, I have to re-work my load.

[RH45]

I used to use 4756 in 9 Major and .38 Super and never had any spreads that large ( with that small a change in powder weight).

You might try using a different chrono just to check yours since you mentioned it being new.

Steve

Eventhough I can make major with 4756 in .38 Supercomp, I can't come close with either of my 9mms, and the case was FULL.

[Steve RA]

I was using 130 grain WW .38 super bullets and a case full of powder with the cartridges loaded as far out as the mags would allow. Went 1442 at a couple of matches, I always just let the brass stay where it fell. Was a totally stock 17L, one of the first in the country and never had a problem with it.