Sub-sonic 308 load - help

LAPUA markets factory loaded .308 WIN SUBsonic ammunition.

One of our Canadian Fisheries officers used to shoot those oh-so-cute Harbor Seals with it, here on the Wet Coast ... where, if a shot was overheard, he might become less than popular with some of the PETA leaning locals.

This guy eventually switched over to .300 WHISPER.

He and I have had some interesting conversations around the .30 CAL subsonics, and I got a couple of boxes of .308 Lapua SUBsoninic off of him to play with in my M14s.

Eventually,
I ended up agreeing with him, and scrapped the idea of .308 WIN sub-sonic as too inefficient, and went with a .300 Whisper AR15 instead.
LAZ 1
[;{)
 
H4831 said:
Some on here have been referring to the speed of sound as being about 1100 feet per second. Actually, speed of sound is about 1100 miles per hour, which is about 1633 feet per second.
I don't think speed of sound, or sub sonic, has anything to do with how much noise your firearm makes. ...
Sorry to rain on your picnic, with all the fun you guys are having, but lets talk about facts.
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Sorry H4831, but I dont believe your facts are accurate - Diemaco had it right, the speed of sound in air at standard conditions is significantly lower than your reference:
(from Wiki "...Sound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time. In dry air at 20 °C (68 °F), the speed of sound is 343.14 meters per second (1,125.8 ft/s). This equates to 1,235.3 kilometers per hour (767.58 mph) or about one mile in five seconds.")
​
To an observer downrange, there is quite a difference in perceived sound pressure between a supersonic and a subsonic round. As you note, both are created by an explosion, and so this is still a large factor in the sound heard, but the super sonic round (which begins to slow down the instant it leaves accelerating influence of the barrel ) creates a sonic boom which is often heard as a "crack". This sonic boom or crack is being created as long as the round remains above the speed of sound, it is not created only when the round passes through the sound barrier as many people incorrectly believe (we can blame Hollywood for that....)

Imagine standing on the shore of a quiet lake when a boat goes past. The boat creates a wake that fans out from the boat, and a period of time after the boat has past by, the wave comes past your position on the shore. This is akin to the sound waves hitting your ears after the round has gone past. As long as the round was supersonic when it went by, you will hear the "crack".

This is why even with a suppressor you need to keep the velocity subsonic, so that you dont give yourself away with the sonic crack of the round. Suppressors do not silence the explosion (thanks again Hollywood :rolleyes:) , rather they muffle it so that it is not as obvious what the noise source was.

Wiki is a great resource when you are looking for a quick refresher (I tossed my physics texts years ago :D)

Here are a couple of links that might help.

http://en.wikipedia.org/wiki/Speed_of_sound
http://en.wikipedia.org/wiki/Sonic_boom
 
Thanks Lazerus I went and checked out the LAPUA website. They have a load listed as 200 grs FMJBT and velocity of 1050 FPS at muzzle. Found some on a US site for $48.50 usd.

I'm working the load up so I can just carry some extra shells when I'm moose hunting instead of another gun for small game. $50 a box is a little much for me, but it would be nice to find out what powder they are using.
 
H4831 said:
Some on here have been referring to the speed of sound as being about 1100 feet per second. Actually, speed of sound is about 1100 miles per hour, which is about 1633 feet per second.

Sorry to rain on your picnic, with all the fun you guys are having, but lets talk about facts.
Click to expand...

Actually, to be factual...according to wikipedia, Physics Live Today (my high school textbook), Encyclopeadia Britannica, and theory of flight manuals used by the CF, the speed of sound at SEA LEVEL is 1125.8 feet per second, or 767.58 miles per hour. I also know, having personally broken the sound barrier behind the wheel of an F-18, that Mach 1 is DEFINITELY NOT 1100 miles per hour! Unless we are using the wrong formula the sound barrier is approx 667 knots. That works out to about 767 mph or about 1100 fps. Sorry H4831.

I used to load my SKS with a special short range load. 4 gr Unique, 220 gr .30 sierra RN. avg speed 762fps. Punched through 10 inches of wet newspaper at 35 yds, fell out of the air at 100. Counted for 2 deer in one year, could hardly hear the shot. Just my $.02.
 
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Sorry guys, for the wrong information I gave. 760, or so, statuate miles an hour is about right for speed of sound.
I'm still not convinced about the bullet breaking the sound barrier and making a noise. As I pointed out, it could only break the sound barrier in the air, when it was decelerating, somewhere down range. If the bullet is going faster than the speed of sound, it got that way in the barrel.
In shooting 44 mag revolver at 200 metres in silouette shooting, the bullet makes a very audible ding when it hits a metal ram. I was not using full up loads, so they must have dropped through the sound barrier before getting to the ram, but not a sound of it can be heard. The ones that missed the target certainly dopped to sub sonic in the air, but nothing can be head.
I have loaded many very low velocity loads with cast bullets in a rifle. I once was using my 30-06 with them in the basement. These were likely in the area of 400 to 500 fps. There is no noticeable difference in the noise as one lowers the velocity, except each reduction makes a bit less reort, just as one would imagine it should.
There is definetly no cut off area, where one could say, "There, that one was sub sonic."
 
When I was fireforming cartridge cases, I poured the charge from some pulled .30 carbine ammo into the case. Then I packed it down with cotton balls. The whole ball won't fit, but the idea is to hold the powder in the base of the case. I tried oatmeal, tapioca, rice and paper towels. The vegetable based fillers seemed to blend with the powder. Anything paper or fibrous coughed up a little cloud of fluff when fired. It was kind of funny to watch.
 
You will never get rid of the blast sound, the only thing with sub-sonic is that you will not get the sonic crack.

I highly advise you to use a chrony when working with sub loads.
I tried went down that rout in the past with my 308 lots of fun however there is much less latitude when working with sub sonic loads.
Meaning a small variation in load will have a major impact on velocity.
I used tight-group as a powder, worked very well; I found the results a bit more consistent than trail boss.
Didn’t need to use any Dacron or pillow stuffing for fillers.
The load was large enough in volume.
Didn’t expand the flash hole but I did use magnum primers to make sure I get a better burn to ignite the powder.
The bullet was flat base SP 170 grain
The loads I worked on started at 9 grains and went down to 7 grains.
I had to stop after going down to 8 grains since velocity started to become dangerously low to my liking.
Here are my results in my rem 700P 26’ barrel. I take no responsibility for these loads in any one elses rifle.
Use extreme caution and a chrony.

9grain 1134,1123,1135, 1120, 1118 group was about 2 ‘ at 100 yards and about 8 ‘ lower than my normal sonic loads.

8.5 grains 1005, 1011, 957, 966,1044 group was about 2.5” at 100 yards and about 9 “ lower

8 grain 901,929,878,935, 993 group opened up completely.
 
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Speed of sound.

A super sonic bullet trails a load wake that we hear as a "crack". It is continuous, and we hear it as the wave hits us. If I shoot on a range that has trees along it, I hear a bang, bang, bang... echo as the bullet passes each tree and the crack echos off each tree.

If the bullet is subsonic, that part of the sound is gone. In 308, the sound is very much reduced because there is no crack and a lot less muzzle blast.

Without a Chrony, it is difficult for the shooter to hear the difference.
 
Those who have worked in the target pits at a military range can tell you about the supersonic crack of a bullet passing overhead, followed by the distant boom of the shot a second or so later when atmospheric conditions are right.
 
I have a load for my 358 Win (same case) that is 800fps (subsonic). 158Gr HP pistol bullet over 6.5 grains of Red Dot shot gun powder, no filler. I have shot hundreds of these and they have all gone bang, very accurate out to 60-70 yards when they start to drop hard
 
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