Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

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Ghost12three
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by Ghost12three »

dellet wrote:You will need to move the gas port back at least 1” from pistol length. A 4 1/2” barrel with a normal pistol length port location will only have about 1/8” of barrel in front of the port.

Image

Keep in mind barrel length is measured from the bolt face.

Muzzle pressure with supers will increase somewhere between 3-5000 psi. to 20,000+ when going from a 5.5” barrel to a 4.5”. Basically about like a 300 Wmag with a 14-16” barrel.
I appreciate this information. I'm looking forward to move from a medium to a heavy contour (same length), having such high pressures in mind now. Do you think it is a good safety measure or it would just add unnecessary weight?
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Flatliner
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by Flatliner »

With no intent to offend at all, I am going to give some advice that is related to what I think may be your underlying thought process. I say this because my son in law was quite hung up on the idea of a super short AR platform PDW.

First, I don't have the expertise to say it CAN'T be done but as Dellet pointed out, there are more considerations, not the least of which is the need for a heavy duty suppressor because of the radically increased muzzle pressure at that length.

Second, I am guessing that concealability and maneuverability are major considerations.

So here is my advice:

1.) Take a dynamic carbine or cqb class in which there is a lot of movement, barricades, etc. Beg, Borrow, or steal several different length ARs to see what works for you and what doesn't. My educated guess is that you will find that a somewhat longer barrel is really usable and actually more comfortable for the application.

2.) Consider some alternative ways to get shorter, for example, I have a LAW adapter on one of my ARs but there are other ways to skin that cat as well.

If you proceed with this project, you will find the bunch around here to be EXTREMELY helpful and you really won't get 'flamed' so long as your are respectful and willing to listen. There have been some crazy projects that the team here has developed, things I would personally never consider but it makes for a fun read. The reason for my post and advice is that I tend to gravitate to what I KNOW will work for my goals when I am spending money on a project.

Welcome to the forum.

(Edit: Regarding your original question, I am not the best to answer but I am guessing, in addition to shortening the gas port location by the 1" suggested, you may also want to consider a larger port and a set it and forget it AGB.)
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Flatliner
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by Flatliner »

One other consideration to think about. While IIIA is readily available, Plan on encountering a much higher grade armor in real life. My suggestion would be that if you are serious about injecting yourself into an active shooter situation you would be well served to also consider these two things in training and preparations:

1.) How not to get shot by the responding police. I am not being flippant, this is a HUGE consideration. How are you going to identify yourself as one of the good guys quick enough to NOT get killed by another good guy.

2.) Train with the mindset that your body shots put the shooter off balance but you need to be good enough, consistently enough, to deliver an effective head shot in follow up.
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Omega
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by Omega »

Might like to take a look at this thread, gds has a real short barreled AR in 300Blk, not sure the specs as I can't get to the videos here at work, but if I recall it's pretty damn short.
viewtopic.php?f=128&t=102049

He may have insight in what a short barrel will/will not do.
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dellet
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by dellet »

Ghost12three wrote:
dellet wrote:You will need to move the gas port back at least 1” from pistol length. A 4 1/2” barrel with a normal pistol length port location will only have about 1/8” of barrel in front of the port.

Image

Keep in mind barrel length is measured from the bolt face.

Muzzle pressure with supers will increase somewhere between 3-5000 psi. to 20,000+ when going from a 5.5” barrel to a 4.5”. Basically about like a 300 Wmag with a 14-16” barrel.
I appreciate this information. I'm looking forward to move from a medium to a heavy contour (same length), having such high pressures in mind now. Do you think it is a good safety measure or it would just add unnecessary weight?
A thicker barrel will not have any plus or minus on muzzle pressure.

Pressure inside the barrel will peak around 55,000 psi and drop with barrel length. This and heat is what will put stress on the suppressor. Heat is often more of a problem than pressure, but in this application they will come as a package. Muzzle pressure difference between 8" and 16" is probably about the same difference a 4.5" and 5.5". And the difference between 8" and 4.5" will be in the 15,000 plus range.
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cdl
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by cdl »

1 in 12 seems pretty slow for a twist. Even the Rattler is 1 in 5. Be careful or you might nickname your build the Tumbler...

MILLER TWIST RULE (My translation...)
s=(30*m)/((T/d)^2*d^3*(L/d)*(1+(L/d)^2))*((V/2800)^(1/3))*EXP(0.00003158*a)
m bullet mass (gr)
d bullet diameter (in)
L bullet length (in)
V velocity (fps)
a altitude (ft)
T twist (in per turn)

s gyroscopic stability factor

1.3 to 2.0 is marginal.

Thanks for the Military Arms Channel Rattler link.

4.5" is pretty short.

I've put about 2K rounds through my Rattler now. It's been FLAWLESS. I'll run my Omega with subs, but not suppers and I haven't had any issues with gas or watery eyes. Didn't even know I was suppose too. Works great, awesome even. There is noticeably less crude when cleaning the receiver. I use an ASR blast can on the flash hider with supers. It makes a big difference. I handload so I actually get 1000fps with a sub if I want and 1920fps with my favorite 110gr VMax load. I would think that running factory ammo designed for a 16" barrel in a 5.5" or 4.5" barrel would be a waste of time. My setup weighs in 6oz more than my 5.5" AR9 before ammo, so it's not light. Neither is an AR9 for that matter. The adjustable gas block is awesome. Easy to operate and easy to clean. I figured muzzle pressure for a sub in a 5.5" was 2/3 a 308 in a 16" barrel and a super was 50% more than a 308 in a 16". Your on your own about that though. I was perfectly convinced I didn't need a Rattler and could build something similar till I picked up a side folder at a dealer. It's short, real short. I'm a sucker for those impulse items. This is one I'd do again though.

I think a lot of people are rooting for you to make a 4.4" work. Keep us informed.
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plant.one
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by plant.one »

bad guys dont wear body armor on their heads. #justsayin
Reloading info shared is based on experiences w/ my guns. Be safe and work up your loads from published data. Web data may not be accurate/safe.
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withoutremorse42
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by withoutremorse42 »

KAK has a 4.75" barrel that I've tested for accuracy and velocity, you can glean some of their data such as gas port location (or just buy one) for similar effect
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Ghost12three
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by Ghost12three »

withoutremorse42 wrote:KAK has a 4.75" barrel that I've tested for accuracy and velocity, you can glean some of their data such as gas port location (or just buy one) for similar effect
I checked out their gas port location. Thanks, man! It was very helpful.
withoutremorse42
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Re: Supersonic load on 4.5" barrel able to defeat IIIA body armor?

Post by withoutremorse42 »

Ghost12three wrote:
withoutremorse42 wrote:KAK has a 4.75" barrel that I've tested for accuracy and velocity, you can glean some of their data such as gas port location (or just buy one) for similar effect
I checked out their gas port location. Thanks, man! It was very helpful.
No problem. It's a good little barrel, favors the lighter rounds. I'm working on getting some 97gr or 78gr rounds to test through it
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