Sub sonic in 300 blackout with 150gr cast / jacketed

OK, not strictly a cast boolit question, but there is more knowledge on this sight than anywhere I know.

My BIL is playing with a 300 BO 7" barrel pistol. He has a lot of old 147 gr military pull downs along with a Lee 150 gr mold. His goal is to find an extremely quiet load for both bullets. Not really concerned if it works the action or not, just looking for a quiet and accurate back yard load (very rural- back yard about 300yds long). I suggested 5.0 grs of Unique, thinking 38 special, 150 gr lswc , approx 900 fps. Didn't work. Got an obvious more than sonic 'crack' when fired with the 147 gr fmj. Why would the 30 BO with 7" barrel produce that much more velocity than my 38 special with 6.5" barrel? I understand that pressures / velocities between the 35 vs 30 caliber will be different, but that much difference surprised me. Also, any suggestions on a load that would acheive what he is trying to do?

Just so you know, we have both been reloading and casting for over 45 years. Probably 50 or more calibers between the 2 of us. Everything from 22 Jet to 45-70. He was a competition shooter for years - long range rifle to bullseye.

"Why would the 30 BO with 7" barrel produce that much more velocity than my 38 special with 6.5" barrel?"

Several reasons for the differenc but the primary reason is the different expansion ratio between the different firearms. The 300 BO has a lower expansion ratio hence a higher velocity and, most likely, higher chamber pressure.

Thanks, Larry. I appreciate your reply.

Exactly what I was thinking, but still surprised me at how MUCH difference it was.

Larry, could you explain what "expansion ratio" means?

The 300 BO is the 300 Whisper just by another name. JD Jones owner of SSK developer of the 300 Whisper published his load development data on line on the old SSK site. He used H-110/296 with heavier bullets than the 150s

My load with 230 grain plain base cast is 9 grains of H-110 and came right from JD's data.. An adjustable gas block usually helps with the AR platform when changing powders and bullet weights.
In order to get full function you may need a bit of buffer weight and recoil spring adjustment depending on the AR setup. However, the original Whisper design was for full function, with only a barrel change, using the same existing lower, upper and magazine.
Anyone new to subtonics, should keep in mind that the speed of sound, limits the velocity, so an increase in impact energy is only attainable by an increase in bullet weight, at or near the same velocity. The idea and the cartridge design intent is to shoot heavy for diameter bullets, under the speed of sound, in an AR platform. However, in an AR platform, the timing is a very real complication for full function at subsonic velocity. No one with an AR intends to shoot it single shot if they can get full function.

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Originally Posted by dg31872

Larry, could you explain what "expansion ratio" means?

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It is the ratio of the effective volume of the bore [base of the bullet to muzzle] to the volume of the cartridge case to the base of the bullet. An expansion ratio (ER) of 8 to 1 means the bore volume is 8 times greater than the case volume. To put that into a bit of perspective, given equal barrel lengths a 243W will have a smaller ER than a 308W. The 308W will have a smaller ER than a 358W. Again, given equal barrel lengths, equal powder charges and equal bullet weights, the 243W and the 358W will give higher pressure and higher velocity than the 308W because the expanding gas has a smaller volume of bore to expand into.

A 38 SPL in a 6" barrel will, many times, have a larger ER than some cartridges in 20" barrels.

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Originally Posted by curiousgeorge

OK, not strictly a cast boolit question, but there is more knowledge on this sight than anywhere I know.

My BIL is playing with a 300 BO 7" barrel pistol. He has a lot of old 147 gr military pull downs along with a Lee 150 gr mold. His goal is to find an extremely quiet load for both bullets. Not really concerned if it works the action or not, just looking for a quiet and accurate back yard load (very rural- back yard about 300yds long). I suggested 5.0 grs of Unique, thinking 38 special, 150 gr lswc , approx 900 fps. Didn't work. Got an obvious more than sonic 'crack' when fired with the 147 gr fmj. Why would the 30 BO with 7" barrel produce that much more velocity than my 38 special with 6.5" barrel? I understand that pressures / velocities between the 35 vs 30 caliber will be different, but that much difference surprised me. Also, any suggestions on a load that would acheive what he is trying to do?

Just so you know, we have both been reloading and casting for over 45 years. Probably 50 or more calibers between the 2 of us. Everything from 22 Jet to 45-70. He was a competition shooter for years - long range rifle to bullseye.

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Another member here ran some loads through quick load when I had a similar question.

Basically you need a tiny case. The projectile touching or seated deep as possible on a small charge of fast powder and a LONG barrel to achieve a low muzzle pressure. Think along the lines of 22 short with a 24” or longer barrel.

Personally I don’t think you will get there with the 7” barrel unless you go with a primer and wax bullet load.

Sierra lists sub sonic data. They like RL7, H or IMR 4198 and Trail Boss.

https://sierrabullets.files.wordpres...t-03052018.pdf

Try 4.5 grains if 5.0 was too much.

I use 5.5 grains Unique for 190 to 200 grain bullets and they work great for subsonic loads.

I know this might be a bit off topic, but I was thinking about picking up a BO in a Ruger American and trying to play with this for a short-range (<100 yards) pig and deer rifle with cast or jacketed bullets, but I just keep thinking the BO is a solution in search of a problem?? In other words, why the BO over a 30-30? And no, not talking about suppressor. The BO reminds me of a 30 Herrett.

The Blackout is popular because it is a cheap and easy conversion from .223 in the AR platform.

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Originally Posted by imashooter2

The Blackout is popular because it is a cheap and easy conversion from .223 in the AR platform.

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It also works great subsonic and suppressed.

4.5gr of Unique pushes 150gr boolits out of my 16" bbl .300blk to 1100fps.

It sits between the .30 Carbine and 7.62x39 with supersonic loads, making it a great home defense and short range hunting choice.


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Have an AR-15 in 300 Blackout and a big suppressor I built and filed a Federal Form 2 on. The gun shoots excellent with 200 gr Sierra 30 cal boolits that I drive just under 1100 fps. The heavier boolits allow for more gas pressure that operates the bolt carrier. No sonic crack. The gun is good out to 150 yards because of the slow boolit. Don't think I would want to try a 7" barrel. Not enough dwell time on the pressure rise to work the action and keep it sub-sonic.

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Originally Posted by Rockindaddy

Have an AR-15 in 300 Blackout and a big suppressor I built and filed a Federal Form 2 on. The gun shoots excellent with 200 gr Sierra 30 cal boolits that I drive just under 1100 fps. The heavier boolits allow for more gas pressure that operates the bolt carrier. No sonic crack. The gun is good out to 150 yards because of the slow boolit. Don't think I would want to try a 7" barrel. Not enough dwell time on the pressure rise to work the action and keep it sub-sonic.

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I’d love to hear about your suppressor. JD Jones used to build them for 300 Whisper that went back towards the action over the barrel.

You’re right on the dwell time with 7” and subsonic. I drilled the gas port all the way to 1/8” and it still wouldn’t cycle subsonic. I might try slower powders one day to increase the pressure at the port, but the barrel is currently in the unused parts box.

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Originally Posted by imashooter2

Sierra lists sub sonic data. They like RL7, H or IMR 4198 and Trail Boss.

https://sierrabullets.files.wordpres...t-03052018.pdf

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Thank you for the link. I had my wife help me get it printed out. I wonder if there is a similar page for 277 Wolverine?

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Originally Posted by frkelly74

Thank you for the link. I had my wife help me get it printed out. I wonder if there is a similar page for 277 Wolverine?

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Google MDWS forum or look for .277 WLV

I run a 7.5 inch barrel on an AR with a can. The load is 230 grain boolet, 9.8 grains AA1680. It runs, cycles the gun well and locks the bolt back with no issue. It is very quiet. Makes more noise hitting the berm than firing.

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Originally Posted by Finster101

It also works great subsonic and suppressed.

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Yes, I get that it’s a conversion from 223 and therefore a cheap steady supply of brass, but other than suppressed, has it been a popular short range deer round for anyone, when compared to say, 7.62x39?