Locked breech vs blowback / HK94 vs Uzi Carbine

[firearms_instructor]

I have wondered for some time how much muzzle velocity is lost in a blowback-operated pistol caliber carbine, such as the Uzi carbine, versus a locked-breech design such as the HK94? Is there anyone here who has (or would) chronographed two such firearms using the the same ammo? Inquiring minds want to know...

Also, is there someone here who could tell me the idea barrel length for a 9mm carbine? .45? .223? .308? Ideal length being the length at which all the burning powder has been used to accelerate the bullet to maximum velocity with no velocity loss from friction due to excessive barrel length.

Call me picky, I know, I hate the idea of wasted energy... (he says, knowing full well that this post itself is probably wasted energy)

And for extra credit for those of you who have owned or fired an HK94 and an Uzi carbine, any additional thoughts on the pros & cons of one vs the other?

Thanks,

an infinitely curious FI

 

[natedog]

The handgun calibers (9mm and .45ACP) were designed around shorter pistol barrels, about 5". They don't really gain anything out of a longer barrel, and I've heard that a really long barrel, like a 16", may actually slow it down. Handloads are exceptions, if you load up, say, a .357 Magnum with slow burning powder, you can experience something like a 113% increase in energy (http://www.thehighroad.org/showthread.php?s=&threadid=45283&highlight=chrono). I don't think all of the powder in a .308 will burn in a barrel under 24 or 26 inches, but most consider 20-22" to be ideal. For .223, velocity is important, and 20-24" is considered ideal. Generally, you loose about 100 FPS per inch of barrel (with rifle calibers).

 

[mete]

There isn't a significant difference in loss between blowback vs delayed blowback because in each case some energy of the burning powder is directed backwards which operates the action. That doesn't 'subtract' from the velocity. ...Ideal barrel length for the handgun rounds is probably 8-10" since the powders are fast burning. For the 223 and 308 it's probably 18-20"......HK is considered the best smg on the market and comes in many variations. It fires from a closed bolt which means more accuracy. The UZI smg fires from an open bolt, "slam fire".

 

[firearms_instructor]

Thanks for the replies. Mete, for clarification I was referring to the semiauto Uzi carbine, which has a 16" barrel, and a semiauto HK94, which also has a 16" barrel, IIRC. The semi Uzi carbine that I'm referring to fires from a closed bolt. The Uzi subgun has a 10" barrel, which is probably the ideal length for most NATO standard 9mm ammo. I'm not interested in doing or having anything which requires permission or a tax stamp from the Feds.

I imagine that pushing a standard 9mm load through a 16" barrel probably loses some velocity. If I were a reloader I might try to squeeze as much performance out of that long barrel as possible, or maybe +P+ ammo would do this. The only advantage of a 16" barrel on a 9mm carbine (besides legality!) is no muzzle flash.

 

[shoobe01]

One thing is that HKs are also not strictly locked. The bolt starts moving right away, and is simply delayed thru mechanical disadvantage just barely enough. The flutes and such promote gas leakage around the case (to assist with primary extraction). Most open-bolt guns use "advance primer ignition" where the firing pin is arranged to strike just before it runs to battery, so its pretty well sealed at peak pressure, though it then opens right up.

In reality, hardly any of the gas (as a total proportion) is wasted on action. Chrono speeds for gas ops with the port open vs. closed are generally statistically insignificantly different. I should think this is the same sort of thing, and nothing important is wasted.

I hear tell that about 8" is ideal for .45. Around 12" is too long (starts slowing down), but I don't know where for sure. At 16" a .45 is slower than at 8". 9mm runs great out of 16" barrels. Faster than 8", so it must be good. I have heard several writers expound on the value of 9mm as a general carbine round, especially for accuracy, so long barrels seem to serve it well. I think max length for rifles starts to depend too much on powder, and be way on out there. Remember, everything we carry is a tiny carbine to a 17th c. hunter. They had 30-36" tubes and /longer/. This with lead balls (paper or linen patched, but still). I'll bet you would see no ill effects from a 40" .308. Vibration would kill your accuracy though, if you could carry it.


I love the HK94. Fires wonderfully, and not too heavy, which some blowbacks can be. I would rather have a quality gun than a quality design though, so vote for getting a good Uzi over a crappy MP-5 clone, any day (I say as one who had a troublesome SW-5).

 

[Jim K]

The UZI is locked, it is just not mechanically locked. It is what is called a blowback action and is locked by the mass of the breechblock which is held closed by its own inertia until the bullet is out of the barrel, so there is no energy lost. If the breech opened before the bullet left the barrel, the full force of the pressure in the barrel would be released into the action with interesting results.

On an open bolt gun, like the UZI SMG, the bolt can be lighter since advanced primer ignition fires the round while the bolt is still moving forward and before it loses its inertia, so it acts as if it were heavier than it is. In this case, also, the bolt remains closed until the bullet leaves the barrel.

More powerful cartridges cannot be used in blowback actions, because the breechblock mass would have to be too great. Someone calculated that for a .30-'06 blowback, the breechblock would have to weigh 28 pounds, obviously impossible in a rifle or even machinegun if it is to be of some reasonable weight.

Jim