9" PDW: 5.56, 300 Blkout or 7.62X39?

greentips

greentips

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The other day a friend of mine brought the question as to why 300 Blkout for 9" PDW, I came up with a few points. I think this is a great topic.

Blast:
223/5.56 is designed to burn in a 18 to 20" barrel. Lots of unburnt powder and therefore noise and flash out of a 9"
300 blk out is designed to burn most of the powder in 9", but we don't have suppressors so...

External Ballistic:
300 blk "point to shoot" range is somewhere around 165m - it does travel like a rainbow.
223/5.56 "point to shoot" range is just a tad over 200m, even out of a 9" bbl

Velocity
300 blk out is down to 1500fps somewhere at 175m'ish
223/5.56 comes down to 1500 fps at out to 375m'ish - so it will penetrate most flak vests down range

Terminal Ballistics
300 blk out - The most plain jane 125gr will trump a typical 5.56 FMJ at 9" velocity.
223/5.56 many bullets are velocity dependent, so ammo needs to be tested.

Barrier
300 blk out - Better for bricks and things like that
223/5.56 - high velocity usually better for steel, but 2450 to 2550 fps is not fast at all. It is a bit of a lame duck shooting 62gr or 55gr FMJ.

7.62X39 is basically a faster 300 blk out, but it will get more bang and flash. It also needs a "bigger receiver" and bolt to stand up for a longer time - magazine is longer and needs to be banana'ed, so it cannot be as compact as 300 blkout.

So:
223/5.56
The flat shooting characteristics of 223/5.56 allows the user to make way less error if it needs to go over 150m - and quite "shootable" to just under 300m. It will defeat soft vest at any range when the user can realistic hit anything.

300 Blk out
It is really a 150m max weapon, after that it drops like a rainbow and is not effective against soft vest. Within the shooter's ability, it does have better terminal ballistic. There is no option to go any further unless you are able to LR 25m increment and know your ballistic chart by heart. :p

7.62X39
The same as 300 blk. About 100 to 150 fps higher in MV, but the rainbow drops get delayed for another 25m or so. The long banana mag is not what I will call "efficient" storage.
 
7.62x39 advantage: lots of relatively cheap surplus ammo available
300blk: more choices for factory ammo and more reloading options (over 7.62x39) for those so inclined incl substantial super and sub-sonic data;
 
A bit of a moot point in Canada.

300blk was developed for use with suppressors in 230gr loading. It would be ideal in LEOs patrol carbine with suppressor.
 
greentips said:
7.62X39 is basically a faster 300 blk out, but it will get more bang and flash. It also needs a "bigger receiver" and bolt to stand up for a longer time - magazine is longer and needs to be banana'ed, so it cannot be as compact as 300 blkout.
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BillD said:
7.62x39 advantage: lots of relatively cheap surplus ammo available
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As a Kanuck suffering under our insane gun laws, limited to 10 round XCR mags in my range-only AR toys the taper of the x39 and the banana mags to accommodate it are moot points. What is not a moot point is the cost difference in ammo along with not having to scurry around picking up x39 steel cases because it isn't worth reloading.

I bought a 10.5" PSA x39 upper and have been completely satisfied with it so far. I get the superior AR ergos, controls and operation with the cost of shooting an SKS or CZ858/vZ58. I also have a Norc CQ-A (the 14.5" M4 clone) and a home assembled 16" heavy match barrel DMR style AR chambered in .223 Wylde so I do have a cross section of use and experience. I may add a 10.5" Norc AR in 5.56 just to complete the set but I still think I will shoot the x39 more than the others.

For a non-Kanadian hobbled 9" AR then I would say that 300BLK is probably the best choice overall.


Mark
 
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Canadian laws and realistic civilian practicality is being suspended for this post. Armchair tactician, ho! I like calibre design and discussion.

For the current thought exercise - a 9" PDW - there's two thought trains that I've observed. The P90/MP7 type filling the air with tiny, high-velocity rounds to turn someone into something resembling a screen door or the larger caliber, stopping power argument the 300 BLK can fit into. The 5.56 wasn't really designed for a PDW design (that was the point of a sub machine gun at the time) and performance drastically drops off under 14 inches. 7.62x39 was also designed for longer assault rifle barrels. So most of the potential of the cartridges is lost as blast and fireballs.

A personal defense weapon is supposed to be compact enough that it can be carried and used to counter a close in ambush. So it makes sense that the 300 blk's trajectory in a 9 inch barrel isn't much of an issue. Having less blast and fire is probably a plus at Franz Ferdinand assassination ranges. Cartridges that are efficient in short barrels lend themselves well to the PDW concept far better than intermediate rifle rounds used in that role. Out of the three in a 9" barrel, the 300 makes sense for that particular job.

For Canadian dudes at the range? Just a neat calibre we can't get the full potential out of due to suppressor laws. Still a great round to get near 7.62x39 performance without the reliability issues in an AR. And probably less obnoxious than a 7.5" 5.56 in the bay next to you. I don't actually know, I haven't seen one in the wild. Where are all these 300 blackout rifles?

I think I'm done thought vomiting on the internet now.
 
X-man said:
If suppressors were legal I'd be all over the .300 Blackout. Since they're not, I don’t see the point.
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This ^^^ as an "entry gun" the 8" is great suppressed, however after that I'm wondering why? low recoil and noise reduction are great at the CQB level but after that there are "big" trade offs IMO.
 
16" bbl 300 blk 110gr bullet gives me 2400fps muzzle velocity
that's 300+m range

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If you can't use a can there's no need to go so short. Giving up way too much performance for near zero gain. A 9" with a can is excellent. A 10"-12" without a can is excellent.
 
This caliber only really shines in a "suppressed" application IMO, maybe it was better than 6.8 who knows. Out to 300m it's not a "real" impressive option IMO. I'd take a 10" 556 myself.
 
mildot said:
This caliber only really shines in a "suppressed" application IMO, maybe it was better than 6.8 who knows. Out to 300m it's not a "real" impressive option IMO. I'd take a 10" 556 myself.
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well the 110 gr at 300m has more velocity and energy than a 4" 9mm 115gr at the muzzle
if that's good enough a stopper then the 300blk 110gr is as good at 300m
 
legi0n said:
well at 300m it has more velocity and energy than a 4" 9mm 115gr at the muzzle
if that's good enough a stopper then the 300blk 110gr is as good at 300m
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Point taken, but this caliber is some cases was to replace the MP5 in a CQB scenario and lessen the "collateral damage" of the 556rd in a CQB environment, that said it's not a "be all end all" IMO.I've shot it suppressed and it's quite, a great "entry" gun in an 8" barrel but that's about it IMO.
 
mildot said:
Point taken, but this caliber is some cases was to replace the MP5 in a CQB scenario and lessen the "collateral damage" of the 556rd in a CQB environment, that said it's not a "be all end all" IMO.I've shot it suppressed and it's quite, a great "entry" gun in an 8" barrel but that's about it IMO.
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agree, different applications = different requirements
 
I'd take a 10" 556 myself.
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I'll qualify that by saying that the 556 77gr rd offers more in the terns of "flexibility" at different ranges than the .300 can offer. While in specific situations the .300 Blk may be "optimal" IMO the 77gr has far more advantages. YYMV
 
mildot said:
I'll qualify that by saying that the 556 77gr rd offers more in the terns of "flexibility" at different ranges than the .300 can offer. While in specific situations the .300 Blk may be "optimal" IMO the 77gr has far more advantages. YYMV
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With less logistical drama and lower cost to operate..
 
Kidd X said:
With less logistical drama and lower cost to operate..
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We are on the same page, thinking "out loud" the 300 BLk seems to me to be a solution without a problem? I like the caliber and the CQB suppressor application but I'm thinking that if??I assaulting a compound and I have to make a 400m shot well I'm fu*ked. If I'm using a 556, I can just attache the suppressor when I get ready to do the entry? If I"m taking out "sentry's" then I'm going with a suppressed bolt gun, I don't see the advantages of a 300BLK? YYMV
 
mildot said:
We are on the same page, thinking "out loud" the 300 BLk seems to me to be a solution without a problem? I like the caliber and the CQB suppressor application but I'm thinking that if??I assaulting a compound and I have to make a 400m shot well I'm fu*ked. If I'm using a 556, I can just attache the suppressor when I get ready to do the entry? If I"m taking out "sentry's" then I'm going with a suppressed bolt gun, I don't see the advantages of a 300BLK? YYMV
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I feel the same way. the concept is great and the execution as well but there just isn't a role other than subsonic suppressed work. Which makes the 300Blk a one trick pony..
 
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