Subsonic SKS - A few initial results

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Northman999

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So, I wanted to develop a subsonic 7.62x39 load for use in my SKS.

This would be used to dispatch small game or coyotes at ranges of 50 yards or less. After some reading on the subject I boought some Trail Boss powder and grabbed some components already on hand. My range was snowed in, so I could not really do any accuracry testing, but I was able to set up my chrony in the snow and at least measure velocity and listen for noise differences. Here's what I got...

#1
7.62x39 (SKS)
Win brass
CCI #200 LRP primers
180gr Win PP bullet
6.0gr TB
710 fps (3 round average)
ME: 201 ft/lbs (equivalent to .38 special, 158gr RN)

#2
7.62x39 (SKS)
Win brass
CCI #200 LRP primers
180gr Win PP bullet
7.0gr TB
760 fps (3 round average)
ME: 231 ft/lbs (equivalent to .38 special +P, 125gr HP, or .45 auto 185gr FMJ)

#3
7.62x39 (SKS)
Win brass
CCI #200 LRP primers
220gr Hornady RN
5.0gr TB
380 fps (3 round average)
ME: 71 ft/lbs

I found that load #1 was quieter than load #2 - even that 50fps makes a difference. Load #3 was the quietest (basically like a loud springer pellet gun), but probably too slow to be effective. I could likely boost #3 by about one grain of TB, but probably no more than that, due to case capacity, so I'll probably drop the 220gr RN from further testing. All these loads seemed very low pressure, none even came close to ejecting the spent casing, recoil was almost non-existant. Noise from all these loads were less than a normal 22 rimfire; they were all more similar to noise from a .22 short subsonic.

When I get to the range and try accuracy testing, I'll post what I get. Right now I'm leaning towards load #1, 6.0gr of TB and a 180gr PP as a very nice subsonic load to fine tune.

I will likely be experimenting with TB in .308 win and 303 British as the weather warms up. I was pretty happy with my first experience with it.

PS - I did not drill out my flash holes, or use magnum primers or anything like that. Same brass and primers as I always use with this caliber.
 
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I have sometimes played around with center fire rifles, trying for accurate low velocity loads, also.
This thing about sub sonic and super sonic is a new term in shooting. It was one of those topics thought about when professional writers couldn't think of anything else to write about.
In other words, complete hogwash!
Those loads you were making were well below the speed of sound, and as you noted, even one grain difference made a difference in the noise. Quite true. You could keep going up a grain at a time and the noise would gradually increase. You could do this increasing until you were past the speed of sound, but the same would apply. More powder, more speed, more noise.
The speed of sound, or the sound barrier, has absolutely nothing to do with it. A sound wave, or sonic "boom," is heard as an object rises through, or falls back through, the speed of sound. A bullet is so small that nothing can be heard with the human ear from this miniscule sound wave.
People here will argue with me for saying this, but think of this.
I was once a regular competitor in silhouette shooting with 44 magnum revolvers. The bullets left the gun a hundred fps, of so, above the speed of sound, but fell through the sound barrier well before reaching the 100 metre target.
No one ever heard any noise, whatsoever, of the large bullet falling through the sound barrier, or dropping below the speed of sound.
As a point of interest, with a bottle neck center fire cartridge, the bullet will reach the speed of sound before it has travelled 2 inches down the bore. Do yu think this has anything to do with the very large muzzle report from the rifle?
 
H4831 - My experience with reloading has suggested that a gradual increase in noise as velocity increases, as you describe, is much more true to life than a mouse-fart load suddenly cracking like a whip once a magical number is reached. As for your question about the large muzzle report from rifles (I'm assuming you mean at normal velocities), I can't say for sure, but it would just seem intuitive for me that all that energy has to come out somewhere, and the more engery you have, the more violently it will come out (all other things being the same)...so I would say it has nothing to do with the bullet breaking the sound barrier inside the rifle.

Juster - My MV's were a little lower than I would have guessed, but I was okay with that. I'm looking for the best blend of quiet and powerful. I know I can add more powder and likely get the 180's to around 925 fps or so with TB, and I may try this, but I still want to keep the noise level down. Right now the 180's are producing noise that probably could not be heard by another hunter 100 yards away, if we were separated by timber and brush a fellow would find in a normal hunting situation. That suits me pretty well...
 
Gatehouse - I looked at your subsonic threads before and found they were encouraging. You certainly were getting great accuracy in your 300 WSM.

The whole idea of using TB (or similar) to create subsonic, or just even reduced velocity loads is quite appealing I think. I will likely put this sort of thing to work with a few calibers over time, both to create quiet, short range hunting rounds, and also to build some very low recoil rounds for my wife and son to shoot. My wife loves the look and history of milsurp rifles for example, but shooting full power 303 Brit or 8MM mauser in rifles with steel butt-plates is more than she is willing to do. I'm confident I could work up some extremely mild loads for these calibers that would get her having a great time at the range however...
 
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