357 Mag Loads Flying Slow - Advice Appreciated

[bigedp51]

In conjunction with what diopter said in his above post. (he's almost as smart, good looking and modest as I am)

Cylinder Gap
http://www.ballisticsbytheinch.com/gaptests.html

"Barrel length: the longer the bullet is in the barrel, the more time there is for gas to escape through the gap."
"Bullet weight: A heavier bullet might mean that there is greater 'back pressure' on the expanding gas, forcing more out of the gap."


Mind the Gap: Cylinder Gaps on Revolvers and Their Effects on Power (and Fingers)
https://www.guns.com/news/2011/05/07/mind-the-gap-cylinder-gaps-in-revolvers-and-their-effect-on-power-and-fingers

 

[mactroneng]

I tried feeler gauges once but I can't remember what the result was, I'll get by buddy to bring his set to the range tonight and I'll measure them out.

 

[B]

Here is the link to the actual gap test data. My own testing showed even less variation with the gun I had tightened (almost zero difference). My .454 on the other hand showed a significant increase in velocity, with the lighter weight bullet loads showing the biggest increase.
http://www.ballisticsbytheinch.com/gaptests.html

 

[diopter]

My last family reunion:

Uncle Carlos in his gillie suit eating soylent green rations:

 

[mactroneng]

Update for all: I pulled 6 rounds, verified powder charges and then re-seated to 1.57" OAL, which is 0.02" shorter than before (not quite the 0.035" difference I've calculated though) and puts me roughly at the bottom of the cannelure. I estimate that I've gained 20-30 fps average from this, which I think makes sense.

I've also chrono'd by 500 mag loads (also with not-quite-max Longshot loads) and they are about the same differential from published as the 357 loads. So I'm not sure how they're they're testing their revolver rounds (as the 9mm seemed to be fine) but it's seem that maybe they're using a test rig with my fully enclosed action and not a revolver-type setup with an open gap between the cylinder and barrel.

(Actually, I just thought of this: I know a guy with a 929, maybe I'll shoot some of my 'good' 9mm loads through it and see what the difference is)

 

[jethunter]

1) I'm shooting from a S&W 686 with a 7" barrel vs the 10" test barrel Hodgdon uses. After doing a little research it looks like you lose maybe +/- 20 fps per inch of barrel length so I figure that knocks off +/- 60 fps (though maybe it's also non-linear and there's more of a reduction than that?)

20fps/inch is more of a rifle-length barrel rule of thumb. You'll lose more going from 10" to 7" barrel, you wouldn't be far off if you guesstimate roughly 100fps drop.

2) I found measurements of the Hornady bullet used in the Hodgdon data and measured the Campro bullets, and the Campro's are about 0.025" shorter. I also loaded to Campro's OAL which is 0.01" longer than Hodgdon data. With the two combined that's 0.035" more space in the case than accounted for from Hodgdon, which I imagine would drop pressure than therefore velocity, but I have no idea how that would quantify.

(As an FYI - further research showed that Speer's 158 gr JSP (so similar shape to the Campro) gets slightly lower velocities with Longshot as Hodgdon did, but loaded at 1.585" OAL and with a 5.64" test barrel length - but still 200 fps faster than what I'm getting)

Unfortunately I don't have another gun to try them out in (I might be able to meet someone at the range but I'm not sure). I was considering putting the few that I pulled back together with using the Hodgdon OAL of 1.58" instead of the Campro measurement at 1.59" but if this is only wasting my time, let me know. Or maybe even 1.57" given the difference in length between Campro and Hornady bullets, and the fact that nearly the entire cannelure is showing at 1.59"!

Overthinking.

EDIT: As I continued researching after posting this, it looks like keeping my OAL and increasing powder charge may actually be a safer bet (and be better for accuracy to boot). Thoughts

Now you're on the right track.

Also: In my experience there tends to be more spread in velocity with revolvers than in other types, might have something to do with the differences in barrel/cylinder gap, cylinder length, etc. I don't know why but have experienced a Dan Wesson 4" shot 150 fps faster than a Colt 4in using the exact same load over the same chronograph at the same range on the same day. Go figure.