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TX, USA
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Posted: 4/19/2024 11:07:53 PM EDT
I'm working on a new 300blk load. Have run a full charge range with chrono and have accuracy results too.
Turns out there are two sweet spots where accuracy is great and the spread/StdDev results are beautiful. One is more of a "traditional" subsonic speed and the other, while still subsonic (both speed-wise and by my ears), it's closer to the top end of what would be defined as subsonic. What's the highest you'd push a sub round speed-wise? Assume for this exercise you have no concerns with temp sensitivity and that on a 100 degree day the round would still be subsonic. How would you make the decision? |
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Posted: 4/19/2024 11:48:18 PM EDT
[Last Edit: Sandlewood_3]
[#1]
Speed of sound is temperature dependent. Max of 1050 fps if you are normally at 32 degrees F or higher. 1000 fps if you would be below that. Also your load Velocity will vary so high deviations will sometimes get you a sonic boom. My observation is the sonic boom needs the bullet to travel a certain distance for you to notice it.
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TX, USA
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Posted: 4/19/2024 11:57:28 PM EDT
[Last Edit: Metonymy]
[#2]
Isn't subsonic around 1120 at sea level though?
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USA
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Posted: 4/20/2024 12:13:44 AM EDT
[Last Edit: Trollslayer]
[#3]
Link to National Weather Service Speed of Sound Calculator
At 20 °C (68 °F), the speed of sound in air is about 343 m/s (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn), or one km in 2.91 s or one mile in 4.69 s. Effect of temperature on properties of air Temp Speed of sound [°C] [m/s] 35 351.88 30 349.02 25 346.13 20 343.21 15 340.27 10 337.31 5 334.32 0 331.30 −5 328.25 −10 325.18 −15 322.07 −20 318.94 −25 315.77 1 m/s = 3.281 fps |
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MN, USA
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Posted: 4/20/2024 1:55:55 AM EDT
[#4]
Quote HistoryOriginally Posted By Metonymy: Isn't subsonic around 1120 at sea level though? Give or take a few fps for atmospherics, yep. Most guys target 1050ish area to have some cushion tho |
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TX, USA
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Posted: 4/20/2024 9:17:52 AM EDT
[#5]
Quote HistoryOriginally Posted By s4s4u: Give or take a few fps for atmospherics, yep. Most guys target 1050ish area to have some cushion tho And there's the root of my question. The two loads I found are as follows: [Velocity / SD / ES] 1088 / 4.2 / 10.1 1110 / 3.6 / 10.3 The faster one is so close to the top of "subsonic" that there's not much wiggle room, but it's nice having the extra ft/lbs of energy (only +8 though compared to the other load). I feel like I'm playing "The Price is Right" here. How close can I get without going over? But it is reasonable to keep some wiggle room. 300blk is such a a super fun round. I have a 208gr load that shoots a single hole at 50 yards. This new (to me) bullet isn't far off. I'll probably go with the slightly slower round to keep that wiggle room and ensure it stays sub. |
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MI, USA
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Posted: 4/20/2024 12:29:42 PM EDT
[#6]
+8 is basically nothing. Go with the more conservative load if you want to stay subsonic all the time.
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USA
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Posted: 4/20/2024 4:08:52 PM EDT
[#7]
This is adapted from what's on Wikipedia:
* Transonic flow is seen at flight speeds close to the speed of sound, typically between Mach 0.8 and 1.2. * Transonic airspeeds see a rapid increase in drag from about Mach 0.8. * It was found that shock waves can cause unsteadiness to the air flow. What it means to you: Using that low value as a target would place you at 0.8 * 343 m/s = 274 m/s (900 fps) <-- seems low, even 22LR goes out the bore at about 1020 fps. Above that value, you may find transonic effects, such as reduced accuracy (unstable aerodynamics). |
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AZ, USA
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Posted: 4/20/2024 9:50:33 PM EDT
[#8]
So the higher the temperature, the faster the speed? I was under the impression it was the other way around.
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TX, USA
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Posted: 4/21/2024 12:43:06 AM EDT
[#9]
Quote HistoryOriginally Posted By ArizonaAR: So the higher the temperature, the faster the speed? I was under the impression it was the other way around. You are correct. Higher temperature equals faster speed of sound. https://www.engineeringtoolbox.com/air-speed-sound-d_603.html |
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USA
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Posted: 4/21/2024 12:57:44 AM EDT
[#10]
Quote HistoryOriginally Posted By ArizonaAR: So the higher the temperature, the faster the speed? I was under the impression it was the other way around. The gas molecules are moving faster at higher temperatures, so higher speed of sound. |
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AZ, USA
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Posted: 4/21/2024 9:33:46 AM EDT
[#11]
Thank you both
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SC, USA
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Posted: 4/21/2024 10:02:05 AM EDT
[#12]
I used to target 1050, but had a few slip super depending on barrel and conditions. So now I target just over 1k. This has been 100% successful for me so far.
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Posted: 4/21/2024 10:16:06 AM EDT
[#13]
I intentionally loaded Hornady HAP 121 grain .356" (.38 Super - now discontinued) bullets with 5.0 grains of Power Pistol in 9mm to achieve 1065 fps to keep them sub-sonic. Shooting them from a longer barrel (load development was conducted with a Sig P226) breaks the sound barrier.
1065 fps from a 121 to 125 grain 9mm bullet is sub-sonic and at the bottom velocity for store bough ammo weighing the same. It won't remain sub-sonic running them through a carbine. The problem is you need enough pressure and velocity to run the action reliably. Going to heavier bullets (147 or heavier) in 9mm eliminates the problem. |
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Posted: 4/21/2024 11:28:49 AM EDT
[#14]
My 300blk load is 1090fps I havent had any problems yet.
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TX, USA
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Posted: 4/21/2024 11:30:19 PM EDT
[#15]
Quote HistoryOriginally Posted By bigtex84: My 300blk load is 1090fps I havent had any problems yet. And that's just about where my selected load is going to end up. Chrono data at 80° ambient temperature showed me right at about 1085. Loaded a couple hundred today which should make for a fun range trip in the near future. |
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Posted: 4/22/2024 10:01:34 PM EDT
[#16]
Why worry about energy unless you are using one of the few expanding subsonic bullets anyhow- most likely you will punch a hole straight thru at either velocity.
I would stick to 1050 or even 1000 FPS personally. Anything faster than 1050 will occasionally have one go super in my SBR depending on the day. That gives you plenty of leeway for not worrying too much about temp and pressure when testing as compared to when using it. And like I mentioned, unless you are pushing an expanding bullet to impart that energy into the target, it really won’t matter. I also like 208 gr bullets in .300 subs, but they sure don’t do anything except punch very nice holes thru the target in my testing. |
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TX, USA
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Posted: 4/22/2024 10:32:29 PM EDT
[#17]
Quote HistoryOriginally Posted By tac556: Why worry about energy unless you are using one of the few expanding subsonic bullets anyhow- most likely you will punch a hole straight thru at either velocity. I would stick to 1050 or even 1000 FPS personally. Anything faster than 1050 will occasionally have one go super in my SBR depending on the day. That gives you plenty of leeway for not worrying too much about temp and pressure when testing as compared to when using it. And like I mentioned, unless you are pushing an expanding bullet to impart that energy into the target, it really won’t matter. I also like 208 gr bullets in .300 subs, but they sure don’t do anything except punch very nice holes thru the target in my testing. That's precisely this load. Working on the Hornady 190gr Sub-X bullet. 
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USA
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Posted: 4/22/2024 10:46:09 PM EDT
[Last Edit: Trollslayer]
[#18]
When you stay subsonic, go for large diameter, heavy bullets. <-- I'm channeling 45 ACP here.
If your caliber is fixed, go as massive as possible. |
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OH, USA
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Posted: 4/22/2024 11:04:53 PM EDT
[Last Edit: Millennial]
[#19]
It’s not a distinct barrier, as far as sound is concerned. Like 10fps under Mach 1 it’s whisper quiet and 10fps over it’s loud AF… that’s not quite how it works.
Also, the speed of sound can vary notably with atmospheric conditions. In the transonic region, as a bullet approaches the speed of sound, the dB of the bullet flight also increases. For example, If the speed of sound is 1050fps a bullet travelling 850fps will be noticeably quieter than a bullet going 1025fps. |
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MO, USA
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Posted: 4/23/2024 7:43:26 AM EDT
[#20]
My 9mms run right at 1050fps and was noticing more pop in the colder temps vs shooting in warmer temps. Finally got to chrono them and thought they were going too fast when I load developed them in the cooler days.
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USA
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Posted: 4/23/2024 12:19:59 PM EDT
[#21]
The propellant gases are supersonic, even if the bullet isn't. So, there is a loud crack (noise) from that.
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TX, USA
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Posted: 4/23/2024 9:14:42 PM EDT
[#22]
Quote HistoryOriginally Posted By Trollslayer: The propellant gases are supersonic, even if the bullet isn't. So, there is a loud crack (noise) from that. These rounds will be shot 100% suppressed, so I'm good in my scenario, but good point in general. |
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