Optimal length for 300 BLK

MMH

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I will be cutting down my 16” barrel on my 300BLK Sig MPX and permanently installing a shroud to keep the ‘barrel length’ over 16 inches. I will be shooing mostly suppressed & mostly subsonic. I will however shoot some supersonic, where I will want maximum energy/velocity.

I will shoot everything from 125 gr. super to 225 subsonic. I know that a 9” barrel is very common for a 300 SBR. Is this the optimal size? What would a 11” barrel gain?
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C_R_Slacker

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If you're going to have a 16" "barrel" anyway, why cut it down? As long as subs are still subsonic out of your current barrel, you gain absolutely nothing by cutting it down. You actually lose supersonic performance which you say you want to maximize. If i were you i would either keep what i already have or get a pistol so i could actually have a short barrel.
 

MMH

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If you're going to have a 16" "barrel" anyway, why cut it down? As long as subs are still subsonic out of your current barrel, you gain absolutely nothing by cutting it down. You actually lose supersonic performance which you say you want to maximize. If i were you i would either keep what i already have or get a pistol so i could actually have a short barrel.
So you can shoot your pistol (without a stock to keep it legal) as accurately as a rifle? How much more velocity would a 16" barrel have over a shorter barrel (9" or 11") especially w/ the heavier (higher BC) bullets?

The reason I want to shorten the barrel is that I plan on shooting mostly suppressed. An AR is a fairly small/light platform. Adding a 9" suppressor to a 16" barrel would make it cumbersome and muzzle heavy. If I was really fixated on supersonic long range performance, I would not really be considering a 300BLK as a good starting point. The only reason a 300BLK exists with a 16" barrel is because the ATF says that rifles must have 16" barrels, not because it makes any sense.
 
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TxWelder35

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So you can shoot your pistol (without a stock to keep it legal) as accurately as a rifle? How much more velocity would a 16" barrel have over a shorter barrel (9" or 11") especially w/ the heavier (higher BC) bullets?

The reason I want to shorten the barrel is that I plan on shooting mostly suppressed. An AR is a fairly small/light platform. Adding a 9" suppressor to a 16" barrel would make it cumbersome and muzzle heavy. If I was really fixated on supersonic long range performance, I would not really be considering a 300BLK as a good starting point. The only reason a 300BLK exists with a 16" barrel is because the ATF says that rifles must have 16" barrels, not because it makes any sense.
Still trying to figure out why you would put a barrel shroud on it.
 

C_R_Slacker

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I really don't follow what you're trying to say...you said you were going to permanently affix a shroud to keep it legal. Then you said you were going to add a suppressor, but you don't want to do that to a 16" barrel even though that is effectively what you would have with a shroud. Also, there are no "heavy, high bc bullets" for 300 blackout unless you're shooting subsonics, which defeats the purpose of having a longer barrel for more velocity.

And yes, i can shoot my 300 blk pistol as accurately as a rifle because a pistol brace is effectively a stock.

It sounds like you don't know what you want to me. So just cut it to 9" and be done with it because that is probably the best compromise between supers and subs.
 

MMH

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I really don't follow what you're trying to say...you said you were going to permanently affix a shroud to keep it legal. Then you said you were going to add a suppressor, but you don't want to do that to a 16" barrel even though that is effectively what you would have with a shroud. Also, there are no "heavy, high bc bullets" for 300 blackout unless you're shooting subsonics, which defeats the purpose of having a longer barrel for more velocity.

And yes, i can shoot my 300 blk pistol as accurately as a rifle because a pistol brace is effectively a stock.

It sounds like you don't know what you want to me. So just cut it to 9" and be done with it because that is probably the best compromise between supers and subs.
Yes, a permanent shroud that the suppressor would fit inside of, thus the suppressor would fit inside the shroud and thus not add to the length. With a 9" barrel and 7" shroud length the overall length would meet the 16" requirement w/ or without the suppressor. With a 9" suppressor, 2" would extend beyond the shroud and the overall barrel length would be 18".

I would shoot subs and supers, and would develop loads for both. Obviously, barrel length is not critical for subs. and virtually any reasonable length will do. For supers it is a different story. Shorter barrel length will result in less velocity. Do you have data to substantiate that "9" is the best? What would be the velocity difference for a 125 gr. supersonic in a 9" & 11" barrel? The velocity difference for a 155 & 178 super?

I know exactly what I want to achieve and am trying to get some input so I can make the best decision. The fact that you do not understand my question means just that - you do not understand.
 

C_R_Slacker

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Yes, a permanent shroud that the suppressor would fit inside of, thus the suppressor would fit inside the shroud and thus not add to the length. With a 9" barrel and 7" shroud length the overall length would meet the 16" requirement w/ or without the suppressor. With a 9" suppressor, 2" would extend beyond the shroud and the overall barrel length would be 18".

I would shoot subs and supers, and would develop loads for both. Obviously, barrel length is not critical for subs. and virtually any reasonable length will do. For supers it is a different story. Shorter barrel length will result in less velocity. Do you have data to substantiate that "9" is the best? What would be the velocity difference for a 125 gr. supersonic in a 9" & 11" barrel? The velocity difference for a 155 & 178 super?

I know exactly what I want to achieve and am trying to get some input so I can make the best decision. The fact that you do not understand my question means just that - you do not understand.
Haha, ok dude...I've never seen one of those shrouds before. Forgive me. Now that i have seen one, i wouldn't ever own one because I'm still just going to build a pistol with a real barrel at whatever length i want, but if that's what you really want then go for it.

And no, i don't have specific data for each inch of barrel length and it's affect on velocity with supers. In general, longer means faster. 300 blk was designed to be efficient out of short barrels, so it's a game of diminishing returns. I was told, in person, by the man who developed the round that 9" is the best compromise for use with supers and subs, so I'll just take his word for it. I believe he said 12" would be full or near full performance with supers.

Of course, if you're handloading, you could probably get creative with powder selection and get more performance with a longer barrel, but it sounds like you want to keep it light and handy. So, 12" for supersonic performance without being too long and unwieldy. 9" for a good compromise with supers and subs in a gun that mimics a standard 16" carbine in handling. 7" for a compact, handy rig that can still shoot supersonic ammo with some effectiveness. Final answer. Good luck with your build.
 
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MMH

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A pictorial for people.
Yes, some people need pictures...

Same concept as what I am talking about except I will attach the shroud to the barrel, not the upper. I don't want a dedicated upper as the host is a Sig MPX. I do not want to give up the interchangeable barrel capability. In addition, I have to believe that the hand guard and barrel need to be permanently attached to comply w/ the ATF. With one of these uppers if something happens to the barrel, or it just gets shot out, the entire dedicated upper will now have to be replaced.
 

MMH

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Haha, ok dude...I've never seen one of those shrouds before. Forgive me. Now that i have seen one, i wouldn't ever own one because I'm still just going to build a pistol with a real barrel at whatever length i want, but if that's what you really want then go for it.
And that is the difference - you want a pistol and I want a precision rifle.

[/QUOTE]And no, i don't have specific data for each inch of barrel length and it's affect on velocity with supers. In general, longer means faster. 300 blk was designed to be efficient out of short barrels, so it's a game of diminishing returns. I was told, in person, by the man who developed the round that 9" is the best compromise for use with supers and subs, so I'll just take his word for it. I believe he said 12" would be full or near full performance with supers.[/QUOTE] Still hoping to get some actual data so I can make a fact based decision. The 300BLK has limited powder capacity. With a .30 caliber bullet the barrel length at which you get diminishing returns is much shorter than most other rifle cartridges. What is that point? I would guess somewhere between 9" & 12" Even at 16", even with slow burning powders I would expect all the powder to be burned up so any additional barrel length causes additional drag resulting in less velocity.