For example let’s say you have a short barreled AR-15 style rifle, 7 inch barrel.
You can have it in its default 5.56mm or change out the upper with a conversion kit to make it 9mm.
At close to mid range, which round would be more accurate? If the barrel was even shorter would that change things even more?
I’m asking because I see a lot of people with extremely short barrel rifles (under 7 inches) and am wondering if firing pistol rounds would be more effective in terms of accuracy (though not power)
Generally, the shorter the barrel, the less accuracy you have, just due to the reduced sight radius (the distance between the front and rear sights). With a shorter sight radius, if you have the sights slightly misaligned with your aiming, the shots will be further off-target. In other words, if you have an AR-15 with a 7 inch barrel and you move the end of the barrel up 1 mm, the deflection of the round will be much greater than if you have a 20 inch barrel and you move the end of the barrel up 1 mm.
A shorter barrel also results in lower velocities for the bullet since the powder doesn’t get a chance to burn as long and accelerate the bullet as much as a longer barrel will.
For close range, the shorter barrel and short sight radius probably aren’t going to mean that much. At medium range, the shorter barrel will mean less accuracy, and the slower round will be more easily deflected by wind or small branches, and the bullet drop due to the lower velocity will be more noticeable.
If the 5.56 and the 9mm both have equal length barrels (both short), the 9mm will be slower and will be deflected more by wind. Probably not much of a difference at close range, but at medium range you will notice a difference, especially with bullet drop.
If you compare it to a pistol, the short-barreled rifle will be more accurate at close and medium ranges just due to the better sight radius.
If you need to move quickly from target to target, the pistol will be faster since it is lighter and has less momentum.
The .300 Blackout is specifically designed to fulfill submachinegun-like performance (including good sound and flash suppression and limited penetration) in an AR-15-compatible package like the M4 carbine, just requiring a replacement barrel. It is not intended to be a ‘long range’ rifle cartridge, although with the lighter bullets you can get something approaching 5.56x45mm DM11 or GP90 speeds, albeit with less muzzle energy and more drag.
One of the keys to accurate shooting is when the gun cycles consistently every time. Bolt action rifles are known for their accuracy and the consistent lockup is one factor. A machine gun fires from an open bolt - the cartridge is not chambered until you pull the trigger and the bolt slams shut carrying a cartridge with it which immediately fires. The rifle barrel is not stable when the gun fires. For a machine gun that is fine. You don’t want all the bullets to hit in the same spot
A semi-auto rifle or pistol may look similar externally but it fires from a closed bolt so the gun is stable.Even if your two weapons are both based on the AR-15 platform they have that fundamental difference.
@mixdenny 's reply alluded to an issue with the title and question that has been bugging me. I am about to engage in what some who are not well-versed in firearms knowledge complain about: carefully clarifying our firearms terms so as to be exactly clear what we are talking about. My apologies to @Asuka if you already know all of this, but I feel the need to clarify.
A submachine gun (SMG) is not the same thing as a pistol caliber carbine.
The title of the question combined with the text of the question makes it unclear exactly what is being asked. An SMG is automatic fire or at least select fire. Shortening the barrel and/or chambering in pistol caliber an AR-15 pattern rifle/carbine does not make the firearm an SMG, if the firearm is not select fire.
Generally speaking, all other things being equal, firing bursts of automatic fire from a select fire firearm will be less accurate (because the firearm is more difficult to control) than firing the same number of rounds from a semiautomatic firearm of the same barrel length. There are muzzle breaks and compensators and the like to attempt to address this issue, but generally speaking I think that statement holds. I think to be clear we should clarify exactly what we are comparing here. Is it an AR-15 pattern semiautomatic carbine with a 16" barrel chambered in 5.56 \times 45mm NATO compared to an AR-15 pattern semiautomatic pistol caliber carbine with a 7" barrel chambered in 9 \times 19mm NATO? Because if not, the answer may change.
But I’m not sure that is what was meant in the question/title. I think the question was about barrel lengths and calibers only, not select fire versus semiautomatic fire. If I am mistaken, @Asuka , I hope you will correct me.
I think you kind of have to understand the history of the weapons to understand what they are.
SMGs are a WWI innovation for use in close-range combat in the trenches. In essence, they used the fully automatic fire of a machine gun, but in pistol calibers, because that’s what they had available.
Carbines were already around, but kind of by definition are/were in rifle calibers.
A pistol cartridge carbine is sort of a modern mash-up, both in terms and in the actual device itself.
What I understood the OP’s question to be is if we were to compare a CAR-15, which is a 5.56 select fire carbine developed from the M16 in the Vietnam era, and a say… Thompson SMG, both of which have 10.5" barrels, would the CAR-15 be more accurate?
I’m not sure… is the OP talking about CQB type shooting, or single shots at 200 meters fired from a rest? I suspect in the former situation, they’re probably similarly accurate, but the rifle caliber one will probably be more accurate in the second.
There are new rounds to shoot in both a carbine rifle and in a hand gun.The 5.7 x 28 round shoots very fast in a pistol and very flat and accurate. Low recoil, comparable to my .22 target pistol.. It was going to be the new NATO round but lost the competiotion.
I have a Ruger 57, 20 rounds, very light and accurate, best damn pistol I have ever owned. Light,fast, and you will hit what you shoot at unless you are blind or drunk, for 100 yards..