powder goes up velocity goes down??

[ivan7382]

I made some .357 Mag rounds using 7.5 gr Herco and 158 JHP bullets. I shot these and got average velocity of somewhere around 1050 fps. I also made rounds with 8.1 gr Herco and 158 JHP (max load listed is 8.2 gr) and the crimp on these was a little too much but they fit the cylinder fine so I tried them anyways. These shot an average of 680 fps. These last ones had mabye slight signs of high pressure but nothing serious. But this huge difference in velocity is weird, I wanted to know if anything has experienced anything like this?

 

[John4me05]

Did you target them.. were the holes normal... Seems weird to me for FPS to fall off with mroe powder

 

[ivan7382]

Yea they shot straight and accuracy wasnt bad. I was shooting very close to the chrony and my girlfriend said she saw more muzzle flash than usual that day, so the only thing I can think of is that the muzzle blast was screwing up the chrony but I would have figured it would have done that on the lighter load too so I dont know. It was 30 degrees F outside and I shot those rounds back to back.

 

[cmidkiff]

I have seen a crony give low false readings when it's getting too close to the muzzle blast. I was shooting some mid range .204 rugers, at around 3600fps, all was fine. Switched to a faster load and started getting weird results (900fps, 1200fps, 6200fps) Pushed the crony out another 5 feet and everything settled back down.

Wasn't my crony... Guy that was letting me use it managed to put a .460 smith through the electronics a few minutes later Now I've got to get myself one.

 

[Ol` Joe]

Muzzle blast, not flash, plays with chrony screens something terrible. Move back a few feet and try again. I`ve had people with a muzzle brake on their rifle tripping and upsetting my croney from 2-3 benches over. The suns direction also can influance chronies. The reading can change from the light source moving from overhead to lower on the horizon.

 

[Steve C]

Your chronograph was giving you a false reading. Set the chrono at 10' from the muzzle to avoid those kinds of problems. Heavier load sends out more gas, smoke and flame. Other things like low batteries, flashes of light like lightning can also throw off the readings. There's no way you will get slower velocities with more powder when using the same components. 8.1grs of Herco should have been pushing a 158gr JHP at over 1,200 fps from a 4" barrel.

 

[Walkalong]

I walk off 5 easy steps to set my Chrono. Real scientific like.

 

[ivan7382]

thanks for the input

8.1grs of Herco should have been pushing a 158gr JHP at over 1,200 fps from a 4" barrel.

Thats the number I was expecting. I am shooting a 4" barrel Taurus Tracker

 

[pinkymingeo]

I'm sure that was a chrony problem, but it's not unusual to see velocity drop as charge increases, when you reach the max for a gun/bullet combo. You might also see increased fps spreads and SD's.