When does my 168 go subsonic? - M14 Forum

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When does my 168 go subsonic?

This is a discussion on When does my 168 go subsonic? within the Bolt Action forums, part of the Gun Forum category; Something popped into my head last night and caused me to lose sleep. I have a r700 with a 26" barrel and my hand loads ...


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Old October 28th, 2016, 08:26 AM   #1
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When does my 168 go subsonic?

Something popped into my head last night and caused me to lose sleep. I have a r700 with a 26" barrel and my hand loads have a MV of 2665 shooting 168smk's. I could look up the info on my night force program on my PDA but can't seem to remember how to do it. Now my question is about altitude. Will 5000' to 10k asl extend the distance before going subsonic? I know around where it is at 300asl for my load, which is around 750 to 800 yds. Will it change with thinner air or am i thinking too much? I know I had take off exactly .1 mil when I was at 4500' asl shooting out to 700 or so.

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Old October 28th, 2016, 10:12 AM   #2
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Yes, more altitude will add more range. The subsonic transition isn't a precise event, there is a transonic range where the bullet starts to get wobbly even while it is still technically supersonic. So that bullet might start to go wobbly a bit at 700, be more wobbly at 800, and by 900 be as wobbly as it is going to get. If you know your comeups, that's what you really need.

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Last edited by 2336USMC; October 29th, 2016 at 12:06 PM. Reason: So people don't waste time reading my bad guess...
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Old October 28th, 2016, 10:43 AM   #3
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Something you have to remember is the BC is based on velocity, it changes as you go down range. So, ballistic calculators get progressively less accurate the further you get from the velocity the BC was derived from.

That said, Hornady's calculator shows an increase of approximately 75-100 yards for each increase of 2200 feet to the start of the transonic region.

JBL's shows at 5000 feet ASL, a 168 gr SMK, launched at 2665 fps, hits Mach 1.1 (the start of the transonic region), about 160 yards further down range....


Last edited by lysander; October 28th, 2016 at 11:34 AM.
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Old October 28th, 2016, 11:43 AM   #4
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Something you have to remember is the BC is based on velocity, it changes as you go down range. So, ballistic calculators get progressively less accurate the further you get from the velocity the BC was derived from.

That said, Hornady's calculator shows an increase of approximately 75-100 yards for each increase of 2200 feet to the start of the transonic region.

JBL's shows at 5000 feet ASL, a 168 gr SMK, launched at 2665 fps, hits Mach 1.1 (the start of the transonic region), about 160 yards further down range....
Yeah Sierra puts out what their BC changes to at a given velocity. The night force software Thales that into account as well as having a validation feature. It's the most accurate program I've ever used. Which is why I still use it an my antique Dell axim PDA.

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Old October 28th, 2016, 01:17 PM   #5
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At 2650 fps 168 SMK goes subsonic at 915 yards at 1000 feet
At 2650 fps 168 SMK goes subsonic at 1025 yards at 1000 feet

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Old October 28th, 2016, 05:40 PM   #6
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At 2650 fps 168 SMK goes subsonic at 915 yards at 1000 feet
At 2650 fps 168 SMK goes subsonic at 1025 yards at 1000 feet
Really depends on temperature . . .

And, the drag function changes dramatically in the transonic region (slightly higher at first, then almost immediately much lower when the shockwave detatches), and I would not trust any calculator to provide exact numbers.

The point at which is hits the transonic region, which is the important point, as mentioned earlier, is about Mach 1.1-1.2, and the velocity at that speed is temperature dependent.

I would calculate the 70 degree speed of sound, multiply that by 1.3 and not go for accuracy below that speed as calculated by your ballistic application using your local average barometric pressure.

EDIT: Here is a graph of the drag function:

A 40 degree increase in temperature gains you about a tenth of a Mach, so your initial drag will be less, with an increase in range before you hit the transonic region...


Last edited by lysander; October 28th, 2016 at 06:16 PM.
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Old October 28th, 2016, 06:12 PM   #7
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no idea at all here?

I am new to this and the long range stuff but would sure like to learn... Can someone point me in the right direction?

This winter hope to try my M1A at 800 to a 1000 meters am at or near Sea Level here


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Old October 28th, 2016, 07:12 PM   #8
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I am new to this and the long range stuff but would sure like to learn... Can someone point me in the right direction?

This winter hope to try my M1A at 800 to a 1000 meters am at or near Sea Level here
You could just go out and try it but I would suggest learning how everything works together. There's a program for PC called shooter ready. It teaches pretty much everything accept position and trigger pull. Overall nothing beats trigger time

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Old October 28th, 2016, 09:13 PM   #9
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You could just go out and try it but I would suggest learning how everything works together. There's a program for PC called shooter ready. It teaches pretty much everything accept position and trigger pull. Overall nothing beats trigger time

Where do you find 'shooter ready' dantheman?

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Old October 29th, 2016, 05:33 AM   #10
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A 175 grain Sierra Match King, loaded to 2620 fps at the muzzle, will keep you comfortably above the transonic range all the way out to 1000 yards.


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Old October 29th, 2016, 06:39 AM   #11
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What's your density altitude? Altitude above seal level is nearly useless for long range. Also, try using a G7 ballistic curve instead of G1, velocity downrange doesn't matter nearly as much using it. You could also try using Hornady's 4DOF ballistic calculator on their website. Most 168gr bullets do not transition from supersonic to subsonic very well. You might be better off trying 155 Palma, or 175's.

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Old October 29th, 2016, 06:58 AM   #12
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Where do you find 'shooter ready' dantheman?
http://www.shooterready.com

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Old October 29th, 2016, 07:02 AM   #13
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What's your density altitude? Altitude above seal level is nearly useless for long range. Also, try using a G7 ballistic curve instead of G1, velocity downrange doesn't matter nearly as much using it. You could also try using Hornady's 4DOF ballistic calculator on their website. Most 168gr bullets do not transition from supersonic to subsonic very well. You might be better off trying 155 Palma, or 175's.
I have around 3k 168gr smk's and same in amax. So I won't be shooting anything else for some time. When I get more they will be 175gr smk.

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Old October 29th, 2016, 12:20 PM   #14
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So my first SWAG in my first post was low, by quite a bit. I edited that out so people don't waste their time reading it.

According to the Sierra Infinity ballistics software which uses all of Sierra's own measured BCs for their bullets, moving from 300 ASL to 5000 ASL will gain about 150 yards before the bullet goes transonic, if everything else stays the same. But, since it's typically colder at higher alt, the air is denser so you give up some of that. If the air temp at 5000 is 30 degrees colder, you gain about 125 yards range.

Sierra's BC numbers are neither G1 nor G7, they are actual numbers measured by firing at various velocities. However, in this case, the lowest number Sierra measured for their 168 SMK was tested at a boundary of 1600fps, so their last BC is just a single number that applies at 1600 and below. That doesn't tell us what happens to the BC of the 168 at transonic/subsonic velocities.

There are smart guys at Sierra who will answer ballistics questions over the phone and I bet they know what the BC of the 168 SMK is at 1200 FPS, they just haven't published it. Also, my copy of Infinity is pretty old now and I bet they may have done more low-velocity testing of some of their .30 cal bullets specifically for guys loading .300 AAC Blackout, where a 168 might only launch at 900-1000 fps for subsonic loads.

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Old October 29th, 2016, 01:11 PM   #15
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question?

Quote:
Originally Posted by lysander View Post
A 175 grain Sierra Match King, loaded to 2620 fps at the muzzle, will keep you comfortably above the transonic range all the way out to 1000 yards.


lysander/dantheman,

either of you ever try the 155 Amaxes? how long are they good for?

500/600?

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