300 Blackout Subsonics. How Low can you go.! I haven't found out yet...

[John Gault]

So I've been trying to find the bottom end on this 300 blackout subsonic load. Using published data for starting point (high end) which is different than my standard Low End rule..

Have loaded the following components:
208 grain Hornady HPBT
CCI Military small rifle primer
Accurate 1680
Reprocessed 300 Brass from multiple headstamps

9" SBR with Saker 762 Mounted ON (except typically the first round to make sure I'm not keyholing)

Started with somewhere above 12 grains
Made 3 of each in .3 to .4 grain increments
So.... And I'm not going to list all the numbers below that I chrono'd. Just the last few for effect.
12.0
11.7
11.4
11.1
10.8
10.4 940-960
10.1 = 910-940

9.6gr. Not Chrono'd yet but cycles and Bolt stays open on last round..

Am I missing something important? I have not seen anybody online indicate such low powder charges for this weight and powder... Possible that people only go as low as they can load with the suppressor BEING OFF the firearm so that they know they can fire reliably Minus the suppressor?

And it's still louder that I was hoping for

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[v0lcom13sn0w]

this is info i need to know! i will be loading subs very soon for my 300blk and suppressor(when i get the stamp)

a buddy of mine loded up some 220gr SMK over 10.5gr IMR4227. never chrono'd but w/o suppressor(dont have it yet) it sounds subsonic like a 45 and cycles my bolt.

 

[wired]

Thats about right. I dont ever go over 10.8 grains of 160 for anything over 200 grains of bullet weight. 208's typically run about 10 grains of 1680 for 950 FPS with a 7.5" barrel.

 

[AMProducts]

I guess I don't really understand why you would do this. There is a correlation between velocity and sound, but it's not necessarily causation.

If you really want super-quiet, get an adjustable gas system and turn it off, or get another can. You've already lost most of the punching advantage of a .30 cal by shooting subsonic with a short barrel. Dropping velocity further just means less hitting power on the target.

Which brings up another issue. In my experimentation, most of the time, when shooting suppressed even with really quiet bolt actions, most of the "noise" from shooting was actually the bullet striking the target, whether steel, or even wood, or a rubber backstop. It sounds like banging on a 2x4 with a hammer nearly universally.

In a lot of ways, this is kinda why I lost interest in the .300BLK, and all of the other "big bore subsonics" there just isn't enough performance left to really justify using it over a pistol-caliber sub gun. 9mm-147gr is what? $15/bx of 50, .300BLK you're probably at $35/100 just for the big bullets.

 

[wired]

You dont drop any velocity with a short barrel because you arent trying to gain or keep velocity. 950-1050 is your range. Anything above that gets into transsonic territory and you get an occasional pop. I run 220's - 240's and Ive dropped a few Nutria out to 100 yards dead in their tracks. With hardcast bullets its a lot cheaper than $35 per 100.

Face it. Its an exercise in having fun and its not the most practical thing in the world. Thats good enough.

 

[AMProducts]

You dont drop any velocity with a short barrel because you arent trying to gain or keep velocity. 950-1050 is your range. Anything above that gets into transsonic territory and you get an occasional pop. I run 220's - 240's and Ive dropped a few Nutria out to 100 yards dead in their tracks. With hardcast bullets its a lot cheaper than $35 per 100.

Face it. Its an exercise in having fun and its not the most practical thing in the world. Thats good enough.

Three things happen with a longer barrel, first you can get parasitic drag when there isn't enough pressure to sustain the velocity and friction takes over, the second when this happens you're beyond the thermodynamic curve of the gas and as the bullet rushes out the muzzle you're pulling the pressure down, which means you're cooling the gasses, and finally, you're moving the rapport of the gun further away from the shooter. The inverse square law applies well to sound, however in the PNW with it's cooler, wetter, and hence denser atmosphere sound carries better, and also because of the higher density the speed of sound is lower.

There are two local ranges near me that go out to 1000 yards, one is at about 2500 feet, the other is at 4900ft. At the higher range, it's easy to make 1000 yard shots with pretty marginal .308 and .223 loadings (168's and 69's) as the bullets stay supersonic past that range. Go down to the other range, and accuracy falls apart at about 800 yards.

I totally get the "for fun" aspect of it. However, "for fun" I'm looking at a lot of other variables

Is it possible your occasional "pop" is just FRP?

One reason why I'm leery of doing this is because when putting long bullets into a barrel like this, you start getting into territory where it's really easy to stick a bullet in the barrel. Your chances of a KB are fairly low, but bullets stuck in the barrel is kinda've a day ruining problem, and it's worse if you don't notice it.

 

[wired]

Inverse squares have nothing to do with the conversation. Its about getting the quietest working semi auto feeding AR with the most punch on target. You load to your barrels length to maintain that 950-1050 fps. Much over 1050 and the occasional transsonic makes a healthy crack that ruins the whole point of shooting with a silencer. IMHO no 300 BO needs to go supersonic. It wasnt designed to be a supersonic cartridge. Use something else for that.

Its OK to have more than one gun and its OK to shoot more than one caliber. 240 grains of 300 BO going 1000 FPS hits quite a little harder, shoots flatter and sounds a LOT better out of an AR than 147 grains of 9mm out of an AR.

 

[Member 34319]

Good infos thanks.

Picked up a Handi in 300blk.

Likely won't ever own a can so I'm going the opposite direction. Working to push the heavy, and lighter stuff as fast as possible.

Kind of want this tiny arse rifle to be my new brush deer rifle. Figure a 110-150g projectile should do well at up to 100.

I also have some 220s from an older rifle I have been playing with.

So far I've loaded up to 19.0 grains of H110 in front of a 150 spitzer. Still some space in there too seating at 2.26".

Worked from 10g to 12g with the 220 SMK.

Only tried H110 so far. I've got an older jug of N110 to try as well.

I did find the 220s on 9.5g H110 to be subsonic. Not too sure if they'll cycle a semi auto though.

I've seen cans fix a low charged round though. So may be completely capable of going lower and still cycling with a semi and a suppressor.

Sorry this is kind of the opposite of what the OP was looking to talk about! Just thought I'd share.

 

[wired]

FWIW 8.5 grains of H110 will cycle a 7.5 to 8.5" 300 BO with 208 Amax's just fine and keep in the low to mid 900 fps's.

The 9mm vs 300BO argument doesnt really fly either. Most 147 grain 9mm is trans sonic in 8-10 inch and above rifle length barrels and accuracy is terrible past 50 yards or so.

 

[Member 5605]

How low?

Until the bullet sticks in the barrel...

 

[AMProducts]

Its OK to have more than one gun and its OK to shoot more than one caliber. 240 grains of 300 BO going 1000 FPS hits quite a little harder, shoots flatter and sounds a LOT better out of an AR than 147 grains of 9mm out of an AR.

I get that it may hit harder, but how does it shoot flatter if they're doing about the same velocity?

 

[John Gault]

How low?

Until the bullet sticks in the barrel...

Where's the unlike button again

 

[John Gault]

Thanks everyone for your input so far.
I'll chrono the 9.6gr and I've got one round loaded at 9.1gr and I'll be looking for an actual hole in a target , Or a wooden dowel ...(thanks Dyjital)

 

[CamoDeafie]

I get that it may hit harder, but how does it shoot flatter if they're doing about the same velocity?

EDIT. Aerodynamics. Most 9mm bullets are NOT spitzer pointed or boattails... whereas the .300 bullets already have an optimized aero profile compared to the pistol bullets.

 

[John Gault]

Good infos thanks.

Picked up a Handi in 300blk.

Likely won't ever own a can so I'm going the opposite direction. Working to push the heavy, and lighter stuff as fast as possible.

Kind of want this tiny arse rifle to be my new brush deer rifle. Figure a 110-150g projectile should do well at up to 100.

I also have some 220s from an older rifle I have been playing with.

So far I've loaded up to 19.0 grains of H110 in front of a 150 spitzer. Still some space in there too seating at 2.26".

Worked from 10g to 12g with the 220 SMK.

Only tried H110 so far. I've got an older jug of N110 to try as well.

I did find the 220s on 9.5g H110 to be subsonic. Not too sure if they'll cycle a semi auto though.

I've seen cans fix a low charged round though. So may be completely capable of going lower and still cycling with a semi and a suppressor.

Sorry this is kind of the opposite of what the OP was looking to talk about! Just thought I'd share.

I'd be interested in seeing a pic of a dab of H110 next to N110 for comparison on grain type/size if you can make yourself do it.. I'm leaning that direction but haven't found any N110 locally. I'm not huge fan of metering on the H110.

 

[John Gault]

I guess I don't really understand why you would do this. There is a correlation between velocity and sound, but it's not necessarily causation.

If you really want super-quiet, get an adjustable gas system and turn it off, or get another can. You've already lost most of the punching advantage of a .30 cal by shooting subsonic with a short barrel. Dropping velocity further just means less hitting power on the target.

Which brings up another issue. In my experimentation, most of the time, when shooting suppressed even with really quiet bolt actions, most of the "noise" from shooting was actually the bullet striking the target, whether steel, or even wood, or a rubber backstop. It sounds like banging on a 2x4 with a hammer nearly universally.

In a lot of ways, this is kinda why I lost interest in the .300BLK, and all of the other "big bore subsonics" there just isn't enough performance left to really justify using it over a pistol-caliber sub gun. 9mm-147gr is what? $15/bx of 50, .300BLK you're probably at $35/100 just for the big bullets.

I'm working on the premise that this could be a close quarters home defense rifle that doesn't need to travel far but hits pretty hard while being easy on the ears for me. 500 pcs of Hornady 208 HPBT on sale delivered 26.5c ea. Overall cost reloaded by me not much more than factory 223.
I also load 110gr pills for supersonic fun time....

 

[AMProducts]

EDIT. Aerodynamics. Most 9mm bullets are NOT spitzer pointed or boattails... whereas the .300 bullets already have an optimized aero profile compared to the pistol bullets.

Oddly enough, this isn't actually true. When you're dealing with subsonic bullets, you're not dealing with any nose pressure drag (shock-wave) as such your drag function becomes a function of the surface area that's exposed to the air stream. On a .30 cal long ogive bullet.


Here's something the germans did in WW2 to work with the L31 suppressor for the MP40, it's a heavier weight bullet with a similar form factor, but a rather noticeable tail sticking out the back.

In a design similar to this, the whetted surface area would be lower, due to a transition from laminar to turbulent flow where there is a transition from the skin to the back of the bullet. However these were 9-gram bullets since they were "eisenkern" (iron core) which was cheaper than lead, which gives it a weight of about 140 grains.

The other thing that's interesting to this conversation, a 9mm has an approximate parabolic ogive, which is actually almost ideal for a sub-sonic projectile. (there's a table on the "nose cone" wikipedia page) Nose cone design - Wikipedia, the free encyclopedia

By comparison, the tangent or secant ogive rates as terrible just about anywhere it's tried. If you look at some of the modern missile designs (SLBM's especially) they use aerospikes as a way of moving the shock and friction drag forward of the main cone. But this kind of technology isn't really practical in small arms designs.

 

[CamoDeafie]

Oddly enough, this isn't actually true. When you're dealing with subsonic bullets, you're not dealing with any nose pressure drag (shock-wave) as such your drag function becomes a function of the surface area that's exposed to the air stream. On a .30 cal long ogive bullet.

View attachment 312307
Here's something the germans did in WW2 to work with the L31 suppressor for the MP40, it's a heavier weight bullet with a similar form factor, but a rather noticeable tail sticking out the back.

In a design similar to this, the whetted surface area would be lower, due to a transition from laminar to turbulent flow where there is a transition from the skin to the back of the bullet. However these were 9-gram bullets since they were "eisenkern" (iron core) which was cheaper than lead, which gives it a weight of about 140 grains.

The other thing that's interesting to this conversation, a 9mm has an approximate parabolic ogive, which is actually almost ideal for a sub-sonic projectile. (there's a table on the "nose cone" wikipedia page) Nose cone design - Wikipedia, the free encyclopedia

By comparison, the tangent or secant ogive rates as terrible just about anywhere it's tried. If you look at some of the modern missile designs (SLBM's especially) they use aerospikes as a way of moving the shock and friction drag forward of the main cone. But this kind of technology isn't really practical in small arms designs.

So its back to physics? I have a very rudimentary knowledge of aerodynamics....
For a 300 subsonic bullet the ideal shape is nearly same? So why does it seem shoot flatter than a 9mm? Weight and speed?

 

[AMProducts]

if speed is equal, time to target is equal, and the distance the bullet falls is given by the formula:

d = 1/2gt^2

And thus for an equal time to target, the drop will also be equal.

If you take two bullets of equal weight, with one being of larger diameter it will have less surface area until the length is equal to the width, at which point it essentially becomes a sphere (sphere being the most efficient way to enclose volume with the least amount of surface).

"shoots flatter" is kinda've a misnomer, all bullets obey classical physics, you could use the orbital mechanics equations but that's way to involved for this discussion (but it works well for long range ballistics and artillery). What can change from bullet to bullet is it's drag function, and more drag means reduced velocity to target, which means more time, and more drop.

Generally speaking, my inclination is to say if you're going from a pistol to a rifle and comparing "drop" between those, it's likely a result of perception of the sight base. I know I can arc a 9mm projectile out of my handgun into a 150m ram by aligning the top of my rear sight with the bottom of my front sight and then covering the ram with my front sight post.

So according to the chart posted here: 9mm Ballistics Chart & Coefficient (Luger) GunData.org an "american eagle" 9mm FMJ round has about 253 (units, I'm guessing ft-lbs) of energy at 100 yards, moving at about 950FPS. Now a Hornady ELD 208gr with a starting velocity of 950, has an energy of 417ft-lbs, and a 100 yard energy of 395 ft-lbs.

Granted the 9mm isn't a sub-sonic loading. But it's in a pistol, which is a lot more convenient to march around your house with in the dark, you know, unless you get attacked by tactical ninjas.

Now you've got me interested. but I'm up past my bedtime.

 

[wired]

The whole point of the 300 BO is to remain subsonic and carry enough force to do damage on the target. A 9 MM cartridge is limited to about 158 grains as its top loading for weight but the 147 grain is the standard heavy load for a number of reasons. Its very difficult to get 147 grain 9mm to stay subsonic out of a carbine even an SBR and the 9mm carbine does not fire from a locked breach so there is considerable noise coming from the ejection port. The 300 is a locked breach gun so it is much quieter. A 208 Amax from a 300 BO travelling at 1000 FPS is going to hit the target at 100 yards with around 450 Ft/Lbs of force while a 9mm 147 grain is going to hit at around 290 ft/lbs. A 45 ACP would be a better comparison to the 300 BO energy wise but the overall package does not lend itself well to suppressing the way the smaller/tighter bore of the 30 cal silencer does. That and the 45 ACP is also not going to be a locked breach design. They are very loud at the shooters ear out of a AR. If I step up into the 240 grain category I can easily throw a 240 grain HPBT bullet into the 530 Ft/LB at the muzzle range wit almost 500 Ft/Lbs at 100 yards.

I can easily maintain 1 1/2" groups out to 100 yards with a 300 BO with a 7 inch difference at 50 yards and be remarkably quiet doing it. 9mm or 45 arent going to do that.

BTW... They dont get stuck in the barrel...