How does one develop a sub-sonic load?

P

Pond James Pond

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I have a suppressor for my .308 now and when I am not developing all new loads to run suppressed I am trying to tweak my existing loads to do so!!

Naturally, a subsonic load would be even better.

Now, I have no need of a subsonic load, so this would be more of a case of seeing if I can and how it performs rather than filling a requirement on my part.

Now my guess is a heavy, bullet and a powder charge that has relatively decent case fill, but is quite fast burning and forgiving of lower pressures.

However, the rifle has a 1:12 twist and a 26" barrel (CZ 550 Varmint), so the twist may not suit a heavy bullet and the barrel length makes for easily built velocity. Seeing as typical .308s run at about 2600fps and this would need to closer to 1100fps, it seems a tall-ish order.

So, what bullet weight/type would you choose and which powder would you recommend?
Powders I can get are VV and some Hodgdons.

On a final note, how would such a cartridge perform?
Would it barely split paper at 300m?
Would it have the flight trajectory of a 600gn 45-70?

Curious to read the replies.
 
Some folks make up subsonic loads by turning a heavy (like, 180-200 grains) boattail bullet backwards. That causes its its nose to take up some of the extra powder space, and also orients the bullet so its static stability is working with you along with the spin. I'm not sure how much difference that really makes; haven't calculated it.

You can forget about getting good case fill. The .308 has too much powder space for any powder that will produce subsonic velocity and still burn reasonably consistently and cleanly.

About 8 or 9 grains of Trail Boss or Tin Star with a tuft of polyester pillow ticking over it to keep it against the flash hole would be where I would start. You want the tuft because the case fill is so low with these loads that you are in a range that produces erratic pressures and spikes if you don't.

That load range should make a good flat meplat small game load with 9 mm pistol level of muzzle energy and it should shoot to 50 meters/yards with adequate accuracy, about like .22 rimfire, but with more punch from the heavier bullet. Longer ranges will produce a lot of drop to dial the sights up for, but maybe you could get to 75 meters/yards with a little holdover.
 
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The speed of sound is 343.2 metres per second (1,126 ft/s). Any load, using any bullet, traveling less than that will be subsonic. A 150 with Trail Boss is a tick over that out of a 1 in 12, 24" barrel. Your 26" barrel will not because of the 2 extra inches.
"...how would such a cartridge perform?..." Loss of velocity reduces range and in the case of an HP the bullet may or may not expand upon impact.
 
You both mention Trail Boss. Trail Boss is IMR. Are IMR and Hodgdon the same brand?

I also see N320 (which I have) has almost the same (slightly slower) burn rate as Trail Boss. Is there potential?

If so how would one start trying to find a load that will not leave a bullet stuck in the bore and or cause a pressure spike through some other mechanism, whilst at the same time not causing a Kaboom by being too high?

Seems like a bit of a tight-rope walk and 300m is certainly out of the question...
 
I load 308 subs to shoot suppressed. I use a 175 gn Nosler HPBT over Trailboss. Im on vacation now so i cant give you a powder charge off the top of my head.

My 308's all have fairly short barrels 16-18 inches. I have no idea how much of a difference your 26" tube will make on velocity.

If you are still looking for load data after thr 15th of this month, drop me a PM and i'll look it up
 
The speed of sound varies with atmospheric conditions. At ICAO standard temperature and pressure it is closer to 1116 fps, but that's 59°F, which is cooler than most of us shoot in at this time of year.

This is a good calculator for finding it in your conditions.

As a general rule of thumb, though, given the normal variability of shot-to-shot velocity, just stay below 1050 fps and you will be subsonic in most conditions and your odd higher-than-average velocity round won't produce an unexpected sonic crack.


James,

IMR Trail Boss is rebranded ADI AS25BP, an Australian powder. The two numbers are interchangeable. It has an energy density of about 3000 joules per gram, which matches black powder. The N320 you mentioned has an energy density of about 4100 joules per gram, which means you would need inversely proportionally less charge weight even if the burning characteristic curve had the same shape (which it doesn't). The second difference is that Trail Boss is exceptionally bulky, having a bulk density of just 0.3 grams/cc, where N320 is at more like 0.54 grams/cc. So, given the lighter charge weight and higher bulk density, N320 will be a smaller charge and take up about four tenths of the space the Trail Boss would do.

I am leery of using N320 for another reason. The Finnish Gunwriter's site reports a subsonic .308 load using about 3 grains of N320 detonated and destroyed the rifle of one gun writer who didn't believe that could happen. You would be getting dangerously close to that load level if you tried the substitution. I just flat out would not risk it.

A big plus with Trail Boss is that it is almost astonishingly clean burning. I have fired it in new cases in .45 Auto that still looked like new, clean brass on the inside afterward.
 
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Only ones I have done were 10-11 gn of trailboss with a cast an coated 150gn bullet. Pretty accurate but you will need a bolt gun, they won't cycle a semiauto.

A 147 out of a 9mm is just as practical and makes even less noise suppressed.

Rule of thumb doing SS rifle rounds is to start high and work down, don't even bother if you don't own a chronograph.
 
Seems like a bit of a tight-rope walk and 300m is certainly out of the question...
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Why would 300m be out of the question? Is just a matter of a different drop (ok a lot different:)) than the regular load. The adjustment built into your scope might not be enough for the drop of a subsonic load at 300m, but with suitable sights, its quite do-able.

Subsonic means below 1100fps, generally.

There is a way to ensure proper placement of very small powder charges without putting a tuft of fiber in place to hold the powder against the case head. Simply load the chamber, and then point the muzzle vertical, then lower the rifle to shooting position (without pointing the muzzle below level), aim, and fire. Its not totally positive, but it usually works.

I have also heard of filling the void space in the case with oatmeal / cream of wheat. Never tried it myself, but some old time books mention it. At the very low pressures involved, it seems to do no harm, keeps the powder in place, and simply blows out the barrel when fired.

If you have access to some, I would try cast bullets. A bullet in the 170gr range with a charge of pistol powder works, but you must be EXTREMELY careful about the powder type and charge.
 
Well, this is all very interesting and I'm not saying no, but right now, I am a tad nervous about embarking on this particular journey.

In terms of rounds and hours I am still a novice reloader and so I am somewhat conscious of what constitutes too big a step for my skill-set...

OK, I've read between the lines (;)) and it seems N320 is out.

Trail Boss sounds like a great option except that I've found no stockist of IMR.

All shops sell here is VV: Finland is a neighbour and part of certain trade associations so it is the cheapest way to get powder in, not to mention it has an excellent selection of powders anyway.
Only one shop here has decided to branch out to Hodgdon's powders, hence why I was sticking to those to manufacturers.
 
While I have seen some load data for .308 using N310, I have only used TB. I would just wait on it and avoid any potential issues (real or imagined) 10 grains of TB will fill a .308 case over half full.

Like I said earlier your not really missing anything unless you don't already have a pistol caliber carbine. One other thing TB only comes in 9 oz containers so of your a "number of rounds per pound" guy your numbers will be way off.
 
One other thing TB only comes in 9 oz containers
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I thought I was getting a good deal on my first "pound" of Trail Boss, it was the cheapest can on the shelf.

It was only after I got home that I realized that since Trial Boss is so "light and fluffy" the 1 pound can size only held 9oz weight.

James, since VV is the easiest for you to get, why not ask VV if they have a recommendation?
 
James, since VV is the easiest for you to get, why not ask VV if they have a recommendation?
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When I have previously asked for advice they have been less than forthoming. The standard answer was: all our data is published on the reloading pages. Basically, if what you want to know is not there, tough!

I can see their point if I'm to start playing with loads that could make my gun go bang in a bad way, they'd rather not get involved. Still some guidance, unofficially, would be fine by me...
 
I did a quick google search and came up with this article. It seems to have lots of good advice, but what do I know?!

The good news is it mentions VV powders and other tips.
Anyone care to glance at it and tell me if the advice is sound or not?
 
A shooting buddy at my club is a seasoned sub shooter. I believe his pet load .308 load is the 200gr smk with about 9gr of TB. He likes the heavies. It's pretty cool watching him ring steel , all's you hear is the mellow snap , then a healthy "ding" off the steel.
 
I did a quick google search and came up with this article. It seems to have lots of good advice, but what do I know?!
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That was what I was referring to in #10. FWIW I don't modify cases using TB.
 
I've looked at the sole Hodgdon supplier that I know of and TB isn't listed so I may be stuck with trying N310.

I'll wait and see for a bit and look around. It may be there is a stockist I don't know of yet.
 
180gr bullet and 6-7gr Red Dot should get you about 1100fps in a .308...

If all you can get is VV, then I'd suggest N-312, but I have no idea how much...
 
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