Velocity goal

[Glocksecure]

What is your velocity goal? 1050 fps is below the speed of sound but is transonic. Do you aim for 1000fps and just deal with the higher ones making more noise, or do you try to go 975 so all of them are really quiet? Right now I am launching 220gr SMK 1029-1061fps over 20 rounds with 8.5 grains of Lil'gun. Most of them are transonic and making noise. I want quiet, but I want as much velocity as possible. Thanks for the help.

[Expatriot]

I try to shoot for about 975 fps to stay under the transonic range. Anything between 950 and 980 fps should be good. A few extra fps isn't going to make a lot of difference in the trajectory or terminal ballistics.

[Scalce]

What kind of sounds are you hearing when the bullets are in the transonic stage?

Is it kind of a whirring noise as they fly downrange?

I am shooting a semi so I may need to turn it to single shot to listen for this.

[amafrank]

Transonic speed means that some part of the bullet will be creating supersonic airflow and it will sound just like a supersonic shot. You will hear a crack. Its not as loud and may be a very short crack if you are shooting at longer ranges as the bullet slows dropping out of the speed range. The speed of sound varies depending on temperature so what makes noise at one temp may not at another. We've been trying to keep loads at 1000fps as we've never noted any transonic cracking at that speed, it is a nice round number which works well in calculations and keeps about the max velocity you can really use. 975fps sounds like a pretty small drop but its 5% slower. In the end you mainly need to work up a load that is consistant as all your range tables can be done for any load but the tables are only good if you have a consistant load. The drop difference at 300yds is pretty substantial between 5% + or - so be consistant.

Frank

[Scalce]

I was wondering why I was getting a crack occasionally with speeds around 1060+.

I thought maybe my chrono was off.

Thanks for the info

[LouBoyd]

This is a useful graph from Finland: A noisy bullet has more drag, produces more turbulance, and requires a faster spin rate. Optimum subsonic speed for energy vs noise is at the lower knee of the curve.



The speed of sound is a function of temperature and is slower when colder.
You can shoot a littel faster in hot air. Atmospheric pressure has little effect on the optimum speed, but higher pressure increases drag and noise. (raises the entire graph).

http://en.wikipedia.org/wiki/Speed_o...la_for_dry_air

Moist air raises the speed of sound a bit since H20 molecules are lighter than N2 or O2 molecules. Moisture (not condensed) also makes air less dense (less drag)