[zman1813]

I've been loading 300 blackout rounds for a little while now. I measured the fps of the rounds when I shot them and they were just under the speed of sound. Once I saw how close they were to not being sub sonic, I lowered the powder by a few tenths of a grain to make sure I don't ever have any strays that go over the limit. How close do y'all ride that line or is it harder to break that sound barrier than I'm thinking?

 

[K75RT]

Several factors can affect what the speed of sound is, such as elevation, temperature and humidity. https://physics.stackexchange.com/q...s-affect-the-speed-of-sound-in-a-given-medium

 

[Mindy up North]

I've been loading 300 blackout rounds for a little while now. I measured the fps of the rounds when I shot them and they were just under the speed of sound. Once I saw how close they were to not being sub sonic, I lowered the powder by a few tenths of a grain to make sure I don't ever have any strays that go over the limit. How close do y'all ride that line or is it harder to break that sound barrier than I'm thinking?

You should really follow published load data specific to subsonic loads. What components are you using? Some powders can be inconsistent at best when loaded light, and at worst, create a dangerous condition. Reason would dictate that reducing powder charges is similar to gradually stepping up, and this generally holds true while working up most "normal velocity" loads, but I would proceed with caution working outside of published data for subsonic.

 

[BigEd63]

On a different note or caliber 9mm and 147gr bullets. AA#5 used to be great on the low end for subsonic loads in the UZI Carbine I had owned.
And I never went below published data.

 

[Dutch]

It really depends on the cartridge and bullet mass. For 124 grain and 148 grain 9mm I load it as low as it will go and still cycle the sub guns. I don't mess with 115 grain subsonics in the 9mm. For rifle rounds it can be dangerous to go too low. You may find that your powder doesn't burn properly and your case has a large chunk of unburned powder in it. Generally I tend to stick with about 100fps under supersonic with a powder that ignites properly. My 7.62x39 loads and 30-30 are about 100 to 150 fps under supersonic.
I don't have any experience with 300 AAC, so can't help with that round specifically.

 

[K75RT]

There is a site out of Finland that has lots of info on subsonic and light loads....Gunwriters' Handloading Subsonic Cartridges, Part 1 and Gunwriters' Handloading Subsonic Cartridges, Part 2

 

[zman1813]

Thank you everyone for the help, I will definitely not be going any lower than what I have been safely firing. Oddly enough the published data I looked at didn't include a range. It was just one number so I just have been staying close to it. Thanks again everyone!

 

[Dutch]

If it would help getting data from a different loading manual, I’d be happy to post a picture. Let us know which one might be useful.