Subsonic 308 moderators

[Strike]

• Nov 23, 2024

I posted this on a thread ".308win subsonic loads with bullseye".
I use a Wildcat Evolution Moderator.
My advice is to do your homework and as with all monolith rounds, beware of a serious risk of ricochet.
After reading extensively on loading Sub Sonic, I did as most suggested and worked down rather than working up in powder weight.
I have been playing with sub sonic .308 for a while now.
I read the .308 Sub Sonic article by Barton Gun Works. It is a very interesting read and useful.
I looked at pistol powders. Burn rate and grains per Litre, looking for the max volume to weight. Trying to maximise case fill.
I wanted N320, however settled on N330 as it was available.
All loads were tested (checking for signs of tumbling) without a mod fitted in the first instance.
.308 M12 Mauser
20 inch barrel
1/11 twist
No modifications were carried out on the PPU case.
CCI large primer
Hornady 190grn Sub X
Started at 10.5 grn N330
worked down, too 8grn N330 In .5grn decrease.
All loads proved safe.
Loads above 9.5grns were super sonic
Loads 9grn and under were Sub Sonic.
8grns loads were very quiet at 940fps
8grns Zeroed at 50m were accurate <3/4 inch, when used with my Hik HD LRF Balistic holdover, Howerver, I had to play with the the MV in the balistic calculator. Once sorted, the round proved accurate using the holdover provided, out to 134m, 134m being the maximum distance available at the time.
I'm not quite sure what use I could make of this round for hunting, however it has been interesting.

 

[jthyttin]

Any mod works for Subsonic loads
They don't suddenly change the laws of physics
If it has baffles and worked with supersonic loads it will work with subsonic loads
...
A round that produces 1150fps through a decent mod can be remarkably quiet and very effective.
...
More people **** up this choosing to use inappropriate pistol powders instead of researching a sensible burn rate that matches cartridge to barrel length
...
308 is a lot easier as it already allows for 180-220gr weight class bullets

While any mod "works", there's substantial difference between them, and stellar performance with supersonics does not guarantee even decent performance with subs.

From your post, it's clear that you're not striving for "hollywood quiet" and maybe never seen (heard) that kind of performance.

For no compromise performance, you want

- least amount of
- lowest pressure gas
- and good number of efficient design baffles (basically K baffle)
- in a design that guarantees, as far as possible, that bullet seals the gas inside the mod
- and especially does not let the gas in contact with the air inside the mod in a waythe resulting burn can escape before bullet

Btw 1150fps is not quiet at all, you get much better performance when you get below the transsonic region (900fps or so). This is magnified with larger calibers.

And standard 1-12" twist 308 is not going to stabilize 220gr. I'm not going to guess where the limit is, but when we shot more moderated AKs (1-9.5" twist) Hornady 220gr RN was the upper limit of stabilization. Maybe you meant that standard 308 will always stabilize 180gr (probably true with RN bullets) and some of them will stabilize heavier and/or longer bullets.

Oh, if you don't want have a special mod made, the most efficient design is one that has most number of baffles and since the jump between muzzle and first baffle is usually the greatest -> pay attention to the design and try to select one where bullet has the best chances of sealing the first baffle before escaping gas. If you're not concerned about first round pop (or mitigate it with other means) then concentrate only to the number of baffles (and their clearance with the bullet).

 

[CBH Australia]

While any mod "works", there's substantial difference between them, and stellar performance with supersonics does not guarantee even decent performance with subs.

From your post, it's clear that you're not striving for "hollywood quiet" and maybe never seen (heard) that kind of performance.

For no compromise performance, you want

- least amount of
- lowest pressure gas
- and good number of efficient design baffles (basically K baffle)
- in a design that guarantees, as far as possible, that bullet seals the gas inside the mod
- and especially does not let the gas in contact with the air inside the mod in a waythe resulting burn can escape before bullet

Btw 1150fps is not quiet at all, you get much better performance when you get below the transsonic region (900fps or so). This is magnified with larger calibers.

And standard 1-12" twist 308 is not going to stabilize 220gr. I'm not going to guess where the limit is, but when we shot more moderated AKs (1-9.5" twist) Hornady 220gr RN was the upper limit of stabilization. Maybe you meant that standard 308 will always stabilize 180gr (probably true with RN bullets) and some of them will stabilize heavier and/or longer bullets.

Oh, if you don't want have a special mod made, the most efficient design is one that has most number of baffles and since the jump between muzzle and first baffle is usually the greatest -> pay attention to the design and try to select one where bullet has the best chances of sealing the first baffle before escaping gas. If you're not concerned about first round pop (or mitigate it with other means) then concentrate only to the number of baffles (and their clearance with the bullet).

Interesting comments.
I don’t understand the science behind it or the design differences as it’s not an option to have them for hunting here so I haven’t researched these.
I used one in South Africa and I know they are not restricted but there are very limited reasons these are approved in Australia.
Government agencies get them under pest management approvals and pest contractors also get them under approved pest contract shooting business licenses.
It’s fairly restrictive for landholders and more so to a private individual.
Hunting doesn’t meet the “Genuine Reason “ for “permit” approvals.

I assumed, incorrectly, that a good suppressor that shows a good result with supersonic ammo would be as effective proportionally or more so very effective on subsonic ammo in the same firearm .

 

[Edinburgh Rifles]

Interesting comments.
I don’t understand the science behind it or the design differences

I assumed, incorrectly, that a good suppressor that shows a good result with supersonic ammo would be as effective proportionally or more so very effective on subsonic ammo in the same firearm .

I do
You are not incorrect, efficiency is efficiency

While any mod "works", there's substantial difference between them, and stellar performance with supersonics does not guarantee even decent performance with subs.

From your post, it's clear that you're not striving for "hollywood quiet" and maybe never seen (heard) that kind of performance.

For no compromise performance, you want

- least amount of
- lowest pressure gas
- and good number of efficient design baffles (basically K baffle)
- in a design that guarantees, as far as possible, that bullet seals the gas inside the mod
- and especially does not let the gas in contact with the air inside the mod in a waythe resulting burn can escape before bullet

Btw 1150fps is not quiet at all, you get much better performance when you get below the transsonic region (900fps or so). This is magnified with larger calibers.

And standard 1-12" twist 308 is not going to stabilize 220gr. I'm not going to guess where the limit is, but when we shot more moderated AKs (1-9.5" twist) Hornady 220gr RN was the upper limit of stabilization. Maybe you meant that standard 308 will always stabilize 180gr (probably true with RN bullets) and some of them will stabilize heavier and/or longer bullets.

Oh, if you don't want have a special mod made, the most efficient design is one that has most number of baffles and since the jump between muzzle and first baffle is usually the greatest -> pay attention to the design and try to select one where bullet has the best chances of sealing the first baffle before escaping gas. If you're not concerned about first round pop (or mitigate it with other means) then concentrate only to the number of baffles (and their clearance with the bullet).

On this rare occasion I am not making statements based on the direction of the wind, colour of my socks or personal opinion.

To my knowledge there are only a handful of people in the UK (or even globally) who have spent the time and money to evaluate and publish the unbiased performance of multiple moderator brands and designs under controlled test environments.

The US and UK military have, but their selection of mods is not the commercially available ones and their interest and focus is not on sound attenuation.
Peter Jackson of Jackson Rifles has
I am one of the others.

Reviews and Tests – Edinburgh Rifles

Peak dB is Peak dB whether in the sub 100dB range or above it.
Running close to subsonic velocities makes no difference whatsoever to the efficiency of the mod.
What the moderator is meant to be doing is reducing the blast of high pressure gas venting to atmosphere, NOT impacting the supersonic crack associated with supersonic bullets.
That gas pressure is just that. pressure.
The mod either restricts the gas leaving the muzzle, slowing it down and cooling it effectively .....or it doesn't!

By far the most significant aspect of moderator design for efficiency of sound attenuation is bore to calibre clearance. Smaller the better.
The next most significant on larger cartridges is chamber size. Allowing for larger volumes of gas to expand, cool and be redirected.
Baffle design is largely irrelevant. a smaller bore to calibre tolerance can make even the most inefficient mod significantly more efficient.
K Baffles are an outdated design and were largely used as they could be stamped from thin steel.
Some of the most efficient moderators we tested had the lowest number of baffles.


I have built several systems of mod, cartridge and ammo that produce the firing pin strike and a thud at the target, nothing else of note,
You do not require sub 900fps to achieve that
The comment on my 1150fps Grendel loads was made on the significance of chasing the last vestiges of whatever the **** "Hollywood quiet" is...
In the same sense that a supersonic round unmoderated vs moderated impacts animal behaviour markedly in spite of being clearly audible, chasing that last bit of sound attenuation is largely irrelevant to my particular application.


No self respecting rifle builder is putting a 1:12" on a 308 unless there is a very very good reason
vast majority are 1:10 which will stabilise a 220gr just fine
Any 30 cal we built of late has a 1:8" twist as there is no detrimental effect on any bullet design running a faster twist and huge upside when running monol;ithics
But you don't need a 220 gr to create subsonic 308

 

[jthyttin]

I am one of the others.
Reviews and Tests – Edinburgh Rifles
Peak dB is Peak dB whether in the sub 100dB range or above it.
Running close to subsonic velocities makes no difference whatsoever to the efficiency of the mod.
...
K Baffles are an outdated design and were largely used as they could be stamped from thin steel.

You really don't know what you're talking about. Start by searching the 'net for K baffle. They cannot be stamped, they have to be milled from bar stock or cast. And then they have to be clipped and maybe "worm holed" to get the best attenuation. This is usually manual work for hobbyists, of course automated for serial production (after you find out optimal configuration).

You mix supersonic to this discussion of subsonic. Supersonic is one thing, subsonic another and then there are airguns that also are different beast (larger volume of lower temperature air for given MV).

By your thinking, most effective mod for airgun is rimfire mod or even centerfire mod. Just about anybody knows that specifically designed airgun mods are better.

It's just that there are few if any, centerfire rifle mods designed specifically for subsonic use. Go on and trawl e.g. silencertalk.com if you're interested, although it's been quiet (sic) for some years.