How are cast bullet weights for 300 Blackout classified as supersonics and subsonics?

[HardColt]

Hello to everyone! Just been reading a lot regarding 300 Blackout but have not come across any information on how cast bullet weights are categorized as being supersonic or subsonic. I have a Lee 312 ,160 grains, 6 cavity mold. Cast lead bullets will be sized to 309 before PCing. Is 160 grains of lead cast bullet classified as super or sub? I hope somebody can give me some info. Thanks you in advance. God Bless.

[GhostHawk]

I think you will find that all depends on what you put behind the bullet.

Small charge of pistol powder will probably be sub. Larger charge of slow rifle powder is going to go over that 1050 fps supersonic barrier.

They do tend to use large for caliber bullets for sub-sonic as it gives more options for powder choices. Like the 230 grain in .300BO.

150 grain unless you go with very small loads is going to be over the 1050 supersonic edge. But it is possible.

Last I suggest you remember one old piece of advice.

Use Fast powder (fast burning, ie shotgun/pistol) to go slow.
Use Slow rifle powder to go fast.

There are a lot of threads on the .300BO both super and sub. I suggest you use the search and go read a few, get yourself an education.

[waco]

Another thing that will be a determining factor is are you shooting an AR-15 or a bolt gun? I have a bolt gun in 300blk.
I can shoot any weight boolit sub sonic with pretty good results. I do not have a gas system to contend with to run the rifle.

[gwpercle]

Subsonic, anything is subsonic if it's velocity is below the speed of sound, car, plane, baseball, boolit.
Regardless of the objects weight.

Sonic, if the objects speed is equal to, or above , the speed of sound, be it car, plane, baseball or boolit, then it is sonic. Regardless of it's weight.

Speed of Sound = 1,126 fps.

As long as you powder charge does not propel the boolit faster than 1125 , it can be slower, then the load will be subsonic regardless of the boolits weight.
It's not so much a weight of boolit factor but a speed of boolit factor, try whatever weight you have and see if the accuracy is there in the 1100 fps range . What Waco says about bolt and AR's is spot on. Bolt action are much easier to make happy loads with.
Gary

[HardColt]

Thanks for all the input. I was planning to cast the 160 grain for the 300BO powdercoated but will bear in mind what powder to load behind the bullet.

[sqlbullet]

I am gonna expand on this just a little.

As gwpercle states, these terms describe the speed of the projectile, not the weight. Light bullets with a light load may be in the subsonic range, and heavy bullets in cartridges with enough capacity can be pushed supersonic.

There are a couple of reasons to load for each range.

9mm subsonic ammo utilizes a heavy bullet to retain energy and momentum but loaded to avoid the sonic crack that comes from breaking the sound barrier. This ammo is really designed for suppressed guns, as we know that it is not the absolute best terminal performer from 9mm, that honor going to higher velocity ammo in the 115-125 grain range.

300 Whisper, the parent/twin of 300 blackout, was intended as a subsonic round both for noise mitigation and to enable long range accuracy. And, the two go together.

Gonna reference the movie The Right Stuff. If you have seen it, you will remember this, if not you should see it. Great movie.

As Chuck Yeager and the other test pilots approached the sound barrier (mach), flight control became increasingly problematic. One of the reasons for this is transonic instability. Compressible gasses behave differently on each side of the sound barrier, and at speeds very close to the sound barrier some of the air moving around the plane, or bullet, will be above mach, and others below mach. This creates some pretty nasty turbulence. This is known as the transonic zone. How far either side of mach it extends depends a great deal on the shape of the object around which the air is moving.

In the movie, Mr. Yeager reported that once he broke mach, the flight control issues disappeared. What had happened was he punched far enough above the sound barrier that all the air was hitting his craft at a supersonic velocity, resulting in far less turbulent forces.

Same is true for our bullets. They are much simpler shapes than an air craft that has wings, cockpit, windows and control surfaces, and so the speed range of turbulent flight is smaller. Sleeker bullets that adhere to the Whitcomb area rule will have the smallest turbulent zone.

All of these zones have names, of course, but before we get to that there is one last item of confusion. gwpercle left off one important item in his speed of sound number. That is the temperature, humidity and altitude, all of which influence the measured speed at which sound travels through air. His number is for air that is 68°, dry and at sea level. Higher humidity will raise the speed of sound, as does a higher temperature. Raising the altitude lowers it. So I can expect to reach the speed of sound in the rocky mountains on a dry summer day at a lower speed than someone at sea level in Florida just after the morning rain. So, knowing the speed of sound where you are right now involves some math and quite a few variables.

This matters to you when you use someone else subsonic load data. If I develop a subsonic load that works great in my blackout in the high rockies, and then send it to my friend in Florida, it may well be supersonic there.

So, finally we get to the terms this thread started with...super and sub sonic. There are actually six speed zones defined by NASA. Subsonic, transonic, supersonic, hypersonic, high-hypersonic and re-entry. The last three don't matter to us as they deal with speeds above 6,000 fps.

Subsonic is bullets traveling such that no air around them is pushed above local mach. Sleeker bullets will have a higher max subsonic number. Generally 950 fps is considered below subsonic for all projectiles, though good HPBT designs may reach 100-150 fps above that before encountering transonic instability.

Supersonic is far enough above mach that all the air acts supersonic. Generally we consider this boundary to be much closer to mach than the subsonic value is when considering projectiles from small arms. 1200 fps is pretty much supersonic for any bullet shape, though NASA defines this boundary at about 1450 fps because they are considering more complex shapes, like planes and rockets.

Transonic is the space between the two. 950 to 1275 fps is a safe bet for all projectiles, though for most the range is significantly smaller. Again, a good HPBT design may have a transonic range of only 50 fps on either side of local mach.

The reason you see such heavy bullets commonly in subsonic loads for the 300 blackout is they have great ballistics profiles for minimizing the size of the transonic zone where accuracy suffers at long range, as well as resisting crosswind better. Your 160 grain will do fine, but will have a larger transonic zone. When loading, if you get keyholing or erratic groups, back down the velocity to about 900 fps, and then work back up until accuracy degrades, or you hear the tell-tale sonic crack. These are the signs you are pushing into transonic.

[Reddirt62]

Ditto what Waco said. Subs use heavy bullets to work the action. I have a 110gr sub that is stupid quiet in a bolt fun but has no chance of cycling an auto action.

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[HardColt]

Thank you very much for taking the time to go into such great depth explaining ballistics,atmospheric variances etc. It is just a matter of how you plan to push that bullet into what speeds you want it to be to accomplish regardless of the profile and weight of the bullet. Referring to the right stuff I recall that his problem with the different air turbulances and pressure affecting the top and bottom and leading edge of the boundary layer of the flight control surfaces as he was breaking Mach. I cut and paste your response in my folder for future reference. I'm looking forward to casting that 160 gr. Lee mold and PCing it for my B/O when I get back stateside.

[Echo]

I don't mean to capture the thread, but I cast some Lee 230-grainers for son's BO, and they went all over @100 yds - couldn't keep them on the paper, whereas the factory spitzer 220-grainers made groups. And I have no idea why there was NO accuracy with my cast boolits. Three Oh nine diameter, tumblelubed, unsized, medium alloy, and looked good. Used 9-10 gr's Unique (as I remember) and was all over. Any ideas?

[Moonie]

I don't mean to capture the thread, but I cast some Lee 230-grainers for son's BO, and they went all over @100 yds - couldn't keep them on the paper, whereas the factory spitzer 220-grainers made groups. And I have no idea why there was NO accuracy with my cast boolits. Three Oh nine diameter, tumblelubed, unsized, medium alloy, and looked good. Used 9-10 gr's Unique (as I remember) and was all over. Any ideas?

Lots of people have had accuracy issues with that mold.

[sqlbullet]

As moonie says, that mold is very finicky. I have one.

I think alot of the issue is that cast boat tail bullets are just hard. I get much better results if I gas check them, but that is hard do as well as the bottom of the boat tail is a bit larger than a gas check shank.

Second issue is the length is right at the edge of what will work in a 1:10 barrel. Kinda like the 240 grain match king. Works in some barrels, not in others.

Back up to 25 yards, and then work hard on load development. Or snag one of NOE molds excellent 230 grain blackout molds.