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)

[Rikky Lee]

Check out the links such as this ==> http://www.sengpielaudio.com/calculator-speedsound.htm which gives you the speed of sound against a particular temperature. I drew a graph up of the temperature range in my area (plus and minus some degrees) and learnt a lot about why some bullet loads seem louder.

The other thing to watch out for is noise reflection off solid objects such as trees. Always make this seem noisier.

Finally, get someone else to shot your loads and walk around in back and sides (safely of course!!) at different distances from the firer and see what your perception of the bullet flight sound is like then.

Perception is subjective. Behind the rifle there is too much else happening to really be able to tell what noise the bullet is making at times.

[jthyttin]

I generally use the fastest powder available, and my target velocity is 300m/s which translates to 985fps.

Depending on my goal (reasonable pressure, min. ES, semiauto function, max. accuracy, min. fouling, min. noise, trajectory matching etc.) I may deviate some, either in powder choice or velocity-wise.

[Scalce]

I am just curious why I always see references to 300 Whisper rounds at 1050 fps but you guys seem to be wanting to get it around 1000 fps to keep it from going transonic.

Interesting

I think the 1000 fps makes more sense anyway unless there is some severe increase in drop or lack of accuracy but I am not sure 50 fps makes that much of a difference at that point.

Anybody have any real world drop charts for the various velocities?

[amafrank]

Originally Posted by Scalce

I am just curious why I always see references to 300 Whisper rounds at 1050 fps but you guys seem to be wanting to get it around 1000 fps to keep it from going transonic.

Interesting

I think the 1000 fps makes more sense anyway unless there is some severe increase in drop or lack of accuracy but I am not sure 50 fps makes that much of a difference at that point.

Anybody have any real world drop charts for the various velocities?

In my opinion 1050fps is an ideal speed that everyone would like to hit. It allows the max energy while still remaining subsonic. The reason everyone seems to be loading to 1000fps is to allow for the variation that occurs and the temperature changes as well. If I load up 500rds in the winter when I'm not shooting a lot I'd like that ammo to work out in the summer when I do. The increase in outside temp sometimes increases the velocity of the ammo into the transonic range. Some powders are pretty temp sensitive (H110-W296) while others are not (N110). I've seen H110 loaded at 1000fps during the winter spit out the same slug at 1150 during the hot summer. Needless to say we don't use H110 for anything other than load and shoot days. Others may have other reasons but that seems to be the consensus among those I've spoken with.

Hope that gives some helpful reasoning.

As for real world drop charts I'd suggest the JBM Ballistics website and their calculators. I've had great results using their trajectory calculator for figuring drops using my actual data from tests. http://www.jbmballistics.com/


frank