Puzzling chronograph readings

[eljefe]

My friend and I shot some 357 magnum reloads across
the chronograph today. The load is 8.9 grains of Longshot,
a cci magnum small pistol primer and Sierra 125 jhp.
The revolvers are a 4" 686 and a 6" model 19.

What is perplexing is that the 686 produced velocities
that were about 100 fps higher than the model 19.
We even tried a 6" gp100 and the 686 produced higher
velocity than the gp100. I fully expected the 6" barrel to
produce higher velocity readings.


Is this a function of a defective chronograph? I have not
had it for very long. I shot a handful of 44 special loaded
with standard cci lp primer and 7.5 grains of Unique under
a 240 lswc and got readings of around 975 fps.

Any input is appreciated.

[Rick Hodges]

I remember reading in one of the old Speer manuals. They used the same ammo in 6 consecutively serial numbered 4" model 19's and got velocity readings from 900+fps to 1250+fps.
The point was: each gun was a rule unto itself. The 4" gun might have a tighter cyl. gap or slightly different barrel specs. I don't think it is your chrono.

[marshall623]

Difference in cylinder gap is first thing that comes to mind .

[NoZombies]

I suspect a tighter cylinder gap, and possibly a slightly looser bore.

[HeavyMetal]

I was at a IPSC match back in the late 80's and witnessed a similar deal.

ammo was being tested to be sure it made major, you provided 6 of your rounds to test on a pendulum.

new shooter showed up with a 6 inch model 19 and complained loudly because they planned to test his ammo with a 4 inch model 19.

the test was made with the 4 inch and made major however barely, it still made major. not content the new shooter whined until they gave in and tested 6 rounds in his 6 inch model 19.

it failed miserably and was told he was going to be scored minor PERIOD!

so some 4 inch guns do indeed shoot faster than 6 inch guns of same make and model making the statement that each gun is a rule unto itself a complete truth for us to always remember!

HM

[shooting on a shoestring]

Yes it’s real. This is why you should work up loads for your gun and not just grab a recipe from someone, some place or some manual. Not only are the velocities different, the pressures are also.

On a similar note, a longer barrel doesn’t necessarily mean better groups either.

[eljefe]

Thank you for the replies. I have very little experience with
chronographs. I really enjoy this site, everyone is so helpful.

[Forrest r]

While cylinder gap plays a role in the velocity, the cylinder hole size itself and the bbl are the main players.

Bought a nib 586 back in 1987. Put 100,000 rounds thru it and sent it back to s&w to have the timing rebuilt/bbl set back/new forcing cone. 200,000 round mark same thing, back to s&w. At 275,000+ rounds it was shot out, was getting +/- 100fps from the same loads it has shot/tested over a chronograph since the 1st chronograph I bought back in 1990.
https://i.imgur.com/H6iZxrO.jpg

While not the greatest picture you can see the rings that were flame cut into the cylinder holes making that area of the cylinder larger/loose pressure/gas escaping until sealed.
https://i.imgur.com/IZlwpUY.jpg

A lot of hot loads went down the tube of that 586, what the forcing cone looked like along with the lands at the 275,000+ round make. Note that the forcing cone only had 75,000+ rounds on it. The drive side of the lands are round and the other sides of the lands are still sharp.
https://i.imgur.com/Sg4LjIe.jpg

When buying a new revolver/new to me revolver I always mic the spent brass, it will show if the cylinder holes/chamber are oversized. I've owned 4 different 586/686's now and still own a 586 & 686, they've always performed extremely well.

I've always tested revolvers/pistols over a chronograph, if the firearm was extremely low compared to other firearms. It went down the road. I own/use/shoot dw's and bbl's are easy/cheap enough. I test every bbl for velocity, it's a real eye opener when it's the same firearm/cylinders & the same cylinder gap is set. It takes awhile to come up with some keepers, 4"/6"/8".
https://i.imgur.com/DiDmnFO.jpg

The big thing is to test 4 or 5 different loads before you form an opinion.

[Silver Jack Hammer]

I had chronograph velocities decrease with an increase of powder charge during testing one time. What I concluded was the relatively slow burning powder was not burning efficiency at lower pressures.

Longshot is a slow burning powder, the 125 gr. boolit is light for caliber. Where is the 8.9 gr. charge on the load data? Is it high, low or middle of the road. I’ve got the Speer #15 and Lyman 50th in front of me and I don’t see Longshot in either manual.

[eljefe]

I had chronograph velocities decrease with an increase of powder charge during testing one time. What I concluded was the relatively slow burning powder was not burning efficiency at lower pressures.

Longshot is a slow burning powder, the 125 gr. boolit is light for caliber. Where is the 8.9 gr. charge on the load data? Is it high, low or middle of the road. I’ve got the Speer #15 and Lyman 50th in front of me and I don’t see Longshot in either manual.

The starting load is 8.7 grains as stated on Hodgdon's website. The load produced an accurate group from the 686. It produced no pressure signs. We are going to experiment with different loads. I am also going to take a much closer look at the forcing cone and chambers of the 19. Thank you again to all who replied.

[str8wal]

There are many variables between different guns; cylinder gap, groove diameter, chamber size, throats. All play a part in the velocity equation. Barrel length is only part of it.

[reddog81]

How many shots were fired over the chronograph? Too few shots and your numbers aren't reliable. You can have 100 FPS spreads in the same gun with the same ammo.

[Mal Paso]

I think it was 3g of Clays under a 250g 44 boolit ran 50 fps slower in the 6" than the 4. I thought at the time it was running out of gas and slowing the last 2 inches.