PDW length barrels.....300BLK or 5.56?

[friendlyfireisnt]

Energy comparisons are useful, but aren't the full story.

Energy is only one component of what makes a bullet successful.

Let's give an example using .300blk. From a 10.x" barrel, with the 110gr Barnes you are getting about 2,200fps, and with a 147gr FMJ you will get around 1,900fps.

Plugging those velocities into JBM shows the 110gr Barnes as having 565ft/lbs of energy at 300 yards, and the 147gr having 700ft/lbs of energy at 300 yards. Yet we know that the 147gr FMJ is not nearly as effective of a bullet as the 110gr Barnes.

Best to look at energy as the means in which the bullet is able to perform it's work. That work can be categorized as penetration, expansion and/or fragmentation, as well as potential for tissue damage. But that ability to do work (energy) only matters in relationship to the materials available and the engineering (or design) of the bullet. The 147gr bullet has more material and more energy, but you can't say that it's going to do more damage to tissue then the 110gr Barnes, with less energy and less material, but a better design.

So comparing energy numbers isn't really ever an apples-to-apples comparison.

A better way of looking at it, in my mind at least, is to start the process with an identification of what you want the bullet to be able to accomplish at particular distances out of a particular barrel length, and then from there identify what caliber and bullet combination meets that requirement.

For example, to me a PDW weapon is one that is concealable, quick to deploy, should be accurate enough to 200+ yards, have solid terminal ballistics (ie. be able to penetrate at least 12" of 10% gel) and since a PDW is likely to be used in or around vehicles, perform even if the projectile hits an intermediate barrier (auto glass, sheet metal, etc).

Both 5.56 and .300blk have rounds that are capable of that.

So the question becomes which one does it better?

There are more than a few gel tests on both and from there we can get the following info:

The 5.56/.223 62gr Fusion (out of a 16" barrel) will have a max expansion of around 0.535", min expansion of .352". Average expansion of about .443". A shorter barrel will affect those numbers.

The .300blk 110gr Blacktip will have a max expansion of around 0.782" and a min expansion of .491". Depending on the test, the average seems to be in the .585" - .636" range. Those numbers come from 8"-10" barrels.

The .300blk has a larger temporary stretch cavity, which isn't an end all, be all, but it's something to notice. It also penetrates deeper.

The 5.56 will lose initial velocity faster as you go to shorter barrels than the .300blk, due to the power burn rate.

For those reasons, I keep a 10.3" .300blk (and am considering a 8.x" .300blk). At the same time, due to cost and availability of the 110gr blacktips, I also keep a pair of 11.3/11.5" 5.56 SBR's loaded with Federal Fusion.

[tom12.7]

There's a few other items to look at beyond terminal ballistics. Function is paramount for serious use, and some other considerations that we could look into. We want to fall into a place where we have the widest spans of requirements overlapping each other superimposed to better fit the role.
While a 5.56 carbine gas with the carbine action suit many, descending in barrel length to less than 11.5" can find many in a wider span using 300 Blackout pistol gas with a rifle like or A5 action.
The magazine issues involved favor the rifle or rifle like A5 to reduce MRBS issues for either chambering for function. The higher massed 300 Blackout projectiles benefit function with more time in bolt over travel to the magazine with a more rifle like action, than a shorter time duration carbine action for proper round presentation in the magazine to the chamber for function.
There's more than this that we can look at here.

[yoni]

I find this thread very timely. I am thinking about moving from 9mm mini uzi or MP5K both semi auto only to a PDW that is a rifle round.

I spent so much of my career with a 10.5 or 11.5 M16, and I know that we tried to get short 5.56 guns to work 100% and just stuck with rifles for overt work and mini Uzi for covert.

But I really would like to get a 100% reliable rifle with a 7.5 to 8.5" barrel that will not be the same as shooting a 22 magnum. I am lucky as I view this as a max range of 50 meter weapon.

[MegademiC]

What is the expansion threshold for 110gr vmax?

My current thinking is 556 down to 11.5, any shorter, get a 7-9” 300blk.
10.5-16” seems to be 6.8 territory if your into that (not for me).

I’ll be building a 300 upper soon, 8-9”, but I need to decide if I want a short gun with silencer capability, or build it as a subsonic, full time suprpressed gun with 13”handguard. From what I’ve read, supers with a setup like that are a no-go.

Anyway - full circle- it would come down to how far out your ammo will expand. Energy is potential, it doesn’t mean anything by itself. In other words, it’s not a wounding mechanism.

[Buckaroo]

Has anyone made up a Venn diagram that encompasses what we are discussing?
I'm convinced that the 300BLK outshines the 5.56 in short barrel applications but I don't have data to add to the discussion.

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[Todd.K]

What is the expansion threshold for 110gr vmax?

Doesn't really matter. Do not trust a 300 BLK load with an OAL much shorter than a normal 5.56, there is too much potential for them to move forward and jamb the stack.

[glocktogo]

What is the expansion threshold for 110gr vmax?

My current thinking is 556 down to 11.5, any shorter, get a 7-9” 300blk.
10.5-16” seems to be 6.8 territory if your into that (not for me).

I’ll be building a 300 upper soon, 8-9”, but I need to decide if I want a short gun with silencer capability, or build it as a subsonic, full time suprpressed gun with 13”handguard. From what I’ve read, supers with a setup like that are a no-go.

Anyway - full circle- it would come down to how far out your ammo will expand. Energy is potential, it doesn’t mean anything by itself. In other words, it’s not a wounding mechanism.

Depends on the can. A short can with a Ti tube and inconel or stellite blast baffle and inconel or 416SS baffles is going be fine for anything except multiple mag dumps. The issues you're talking about come from trying to run supers in a short barrel with a multi-cal can that's basically a 9mm can rated for .300 subs. Stick with a 7.62 based can and supers will be fine.

[ABNAK]

I always looked at anything under 10.5" in 5.56 as a range toy/noise maker. My personal preference is 11.5-14.5" in 5.56. I'd be willing to go shorter in .300blk, which puts it in PDW territory.

That was the small niche group of weapons I was suggesting that 300BLK could exceed 5.56 in.

[ABNAK]

What is the expansion threshold for 110gr vmax?

My current thinking is 556 down to 11.5, any shorter, get a 7-9” 300blk.
10.5-16” seems to be 6.8 territory if your into that (not for me).

I’ll be building a 300 upper soon, 8-9”, but I need to decide if I want a short gun with silencer capability, or build it as a subsonic, full time suprpressed gun with 13”handguard. From what I’ve read, supers with a setup like that are a no-go.

Anyway - full circle- it would come down to how far out your ammo will expand. Energy is potential, it doesn’t mean anything by itself. In other words, it’s not a wounding mechanism.

Pretty much what I was proposing, although I'd even use 5.56 with a 10.5" barrel. Shorter than that (and we're in PDW territory then) and I'll take 300BLK.

[tom12.7]

Has anyone made up a Venn diagram that encompasses what we are discussing?
I'm convinced that the 300BLK outshines the 5.56 in short barrel applications but I don't have data to add to the discussion.

Sent from my Pixel using Tapatalk

Variations of Venn's are used all the time. For something like this, you'd be looking at a comparison between some pretty complex diagrams to interpret for either. That is unless you go overly basic and overly basic is your best outcome for that.