Speed of sound ?

[Bull 228]

I got out and ran a few test loads through my recently built 300-221 Fireball 10.5" Noveske barreled AR today. I loaded 200 gr. Sierra Game King bullets with Rem 7 1/2 primers over H110 powder. I started by loading 5 rds. each of 9.5, 9.6, 9.7, & 9.8 grains of powder. Temperature today was approx 70 Degs. and elevation is approx 5900 ft. I had the 9.5 grn loads down to 995.8 fps and I swear that it still sounded super sonic. (This is through a AAC 762SD). I haven't loaded any down at lower powder charges yet, but I figured that at approximately 1000 fps I should be nice and quiet. Any input? Thanks
NT

[alorton]

The speed of sound can vary with temp but anything under 1000fps should be safely under. Perhaps you have an incorrectly calibrated chronograph or you have unrealistic expectations if how quiet a suppressed weapon should be. Have you shot suppressed guns before for comparison?

[Bull 228]

Morning,
As a Class 3 dealer I feel like I have had a better than average exposure to suppressors. (no intent on being a smart ass, just qualifying) I have always figured that between roughly 950 to 1000 fps one should expect to be safely sub sonic. I had not had any prior experience with the "Whisper" and it is quite possible that I had exaggerated expectations. But it sure seemed to me that I was still getting the sonic crack. In hind sight I should have video taped the shoot so that I could listen to the shots from a different perspective. I think I will mix up another batch of ammo at slightly lower fps and see what the results are. Thanks for the input, I am always happy to hear from others with more experience than I. I am definitely still inexperienced in the Whisper department.
NT

[TCCrewchief76]

were you shooting indoors or under the cover of a roof? That always makes things WAY louder...

[Bull 228]

I was outdoors and in the wide open. Anyone have any more load data for their "favorite" pet sub loads? Any input and or advice is always welcomed and appreciated.
NT

[TCCrewchief76]

A few 10.5" AR's I've built are running 1000 FPS with 8.5 grains of Ramshot Enforcer with 220 MKs.

Kevin

[Bull 228]

After speaking with a source yesterday he advises me that if I am getting 995 ish fpa that I am for sure sub sonic. I think that I figured that it was going to be much more quiet. Thanks for your load info also though.
NT

[alorton]

Morning,
As a Class 3 dealer I feel like I have had a better than average exposure to suppressors. (no intent on being a smart ass, just qualifying) I have always figured that between roughly 950 to 1000 fps one should expect to be safely sub sonic. I had not had any prior experience with the "Whisper" and it is quite possible that I had exaggerated expectations. But it sure seemed to me that I was still getting the sonic crack. In hind sight I should have video taped the shoot so that I could listen to the shots from a different perspective. I think I will mix up another batch of ammo at slightly lower fps and see what the results are. Thanks for the input, I am always happy to hear from others with more experience than I. I am definitely still inexperienced in the Whisper department.
NT

You are correct that if you are under 1000fps you should be subsonic. That's why I was curious as to the possibility of having a chrono that reads a little low. I've heard of shooting across two chronographs side by side and having them read 100fps different than each other.

I asked about experience with suppressors because some folks think what they see in the movies is how their gun should sound. A Whisper is quiet as far as suppressed guns go but it isn't "movie quiet" so to speak. I would do exactly what you plan, lower the velocity just a bit and see if you get better results. You might also try different powder combos.

My rifle likes 2400. I use 8.5gr for a 240SMK or 8.0gr for a 208 A-Max. They both come in at just over 1000fps.

[Bull 228]

I think the chrono is good, but have never thought of that being the problem. Will try to find a way to test it. Who manufactures the 2400 powder you mentioned, I am always open to try out womething new. I have a box of the 240gr. SMK in route as we speak.
NT

[alorton]

Alliant 2400, listed in most reloading manuals as just "2400".

A few other popular choices are Lil Gun, Viht N110, or AA1680.

[Expatriot]

At near 6000 ft elevation you may well be running up against the transonic range. The speed of sound is lower at higher elevation. Get your velocities down below 950 fps and you should know for sure.

[Bull 228]

thanks for the help. always appreciated.
NT

[robrob]

The speed of sound in the air is inversely proportional to temperature and not related to altitude (except that at higher altitudes is is usually colder). The speed of sound is the same at 78 degrees at sea level or 78 degrees at 5000 feet. The basic rule of thumb is the colder it is the slower the speed of sound, the easier it is to get noisy (transonic) lead downrange.

[TCCrewchief76]

OK, you ABSOLUTELY HAVE TO TRUST YOUR PARTNER ON THIS...stand at the 100 yard berm and have him shoot over the 100 yard berm into the 200 yard berm. You will understand just how quiet the Whisper is at 1000 FPS with a suppressor, and just how hard that damn round hits. The impact into soft dirt at 200 yards (100 yards away from you) will be much (maybe 3 or 4 times) louder than the sound at the firing point...ask me how I know.

Kevin

PS: I need to take some video of this and post it here. It's a real eye opener. Imagine someone whistling, without the tone, making just the sound of the air coming out. This is what it sounds like as the bullet goes by. Incredible.

[Bull 228]

Hopefully going to get some time to get out this weekend and tinker with some new loads. I'll let you all know if I get any better results.
NT

[John@JCDLESales]

A Couple Handy Calculators For SOS. And Don't Overlook IMR 4759. I have been working with it for several years with the 7.5X55 Swiss ie. read .30 Cal. But you have to work this up or down for your particular application.

Links To SOS Calculators:

http://www.sengpielaudio.com/calculator-speedsound.htm
http://www.srh.noaa.gov/epz/?n=wxcalc_speedofsound

[Alleycat]

mmmmm K31 7.5x55. The things I've done with that gun.....:wink:

[John@JCDLESales]

You just have to love an Old Mil Weapon that will shoot MOA with a 160 Gr. Cast GC Bullet atop 19.0 Gr IMR 4759. And has the recoil of a .22 Hornet.:smile:

[BWE Firearms]

John,

Thanks for the links. I was just wondering how much the speed of sound would change with temperature. I am getting ready to send our 50 THUMPER up to Ohio for a show and tell and want to make sure I get the load right.