Locked vs. Blow-Back Design Velocity

[Robert101]

I am curious to know if the action of a firearm design affects the velocity of the round. For instance, some 45 Automatic handguns are locked (like a 1911 lug type) and some are blow-back breech like the Hi Point design. Does the different design affect the final velocity of the bullet exiting the barrel if all other variables are similar like barrel length, and bore diameter. And, what might that difference be?

I assume in rifles between a bolt action and a semi-auto gas system, that the gas system similar to an AR-15 does negatively affect bullet speed as the gas required to operate the action has to cause some loss relating to bullet velocity. Any knowledge of how much?

This is just a matter of curiosity and does not affect my choice for various weapons.

 

[Drail]

No. The problem with blowback pistols is you must have a lot of mass in the slide or the gun will beat itself to death in short order. The other problem is if the action starts to open before pressures have dropped it creates a huge pressure blast to the shooter. Hiram Maxim actually built a large caliber blowback handgun back in the late 1800s but no one wanted to shoot it for more than 2 rounds. The blast was supposedly horrendous. You need to hold the action closed until pressures have dropped significantly like the Browning locked recoil system. In a gas operated rifle like the AR the gas is not vented back to the action until the bullet is almost out the muzzle - the effect on the bullet is insignificant.

 

[reddog81]

I'm interested to see what others have to say, but I'd guess it would be hard to quantify. Your going to have variations from Hi Points and 1911's and others due to items like you mentioned - bore diameter, groove diameter, smoothness of the barrel, dirtiness of the barrel, variations from gun to gun regarding lock-up, variations from gun to gun regarding chamber size and how good of a seal you get from the brass and the chamber wall.

With out extensive controlled testing any answer only a guess.

 

[Robert101]

And, if the answer is very little let's say 5% as an arbitrary number, then it really doesn't matter. That 5% would be: if the velocity is 1,200 fps X .05 = 60 fps which is really minimal in the grand scheme of things. If the round is traveling 900 fps, then the same 5% loss would be even less at only 45 fps.

 

[Carl N. Brown]

Blowback versus locked breech. There is the Savage Model 6 .22 rifle "Triple Action" -- the bolt handle can be pushed in to lock the breech or left pulled out to function blowback semi-auto.
Gas operation versus locked breech. There are straight pull bolt action ARs made for jurisdictions that ban semi-auto ARs. The Yugo SKS and M70AB rifles have a gas valve cut-off.
Someone with a chronograph could test the difference in velocity, locked breech versus blowback or gas operated.

 

[Jim Watson]

I had a gunzine article in which the writer blocked the bolt shut on a Ruger Mk I. Velocity was HIGHER with the gun shot in regular semi-auto than with the action locked. Maybe a centerfire with a big powder charge would be different, but... Somebody recently calculated that it took 5 to 7 foot pounds to cycle the action of a typical autoloader. With muzzle energy in the hundreds to thousands of foot pounds, that little bit is lost in the noise of ammo and gun variability.

 

[Sunray]

Blowback versus locked breech is about the pressures generated by the cartridge. The greater the pressure, the greater the strength required of the return spring and weight of the slide. Reaches a point of diminishing returns in blow back. The firearm gets too heavy/difficult to use.
Hasn't got anything to do with velocity though.

 

[Rock185]

I haven't addressed this particular question. Never gave it much thought, but have done a lot of chronographing over a good many years. I agree that any difference would be insignificant. I have learned that I cannot reliably predict results. Sometimes a shorter, sometimes much shorter, barrel will give higher velocities, two guns of the same model giving significantly different velocities,etc. There are so many variables that it would likely take a well funded government study to determine the answer to your question. Then of course, they'd get it wrong and fund another years long study

 

[Jim Watson]

Hey, maybe we could get a grant for the THR Institute to study the matter. It could fund gas operated guns with cutoffs, recoil and blowback guns with bolt and slide locks, multiple makes and models, multiple examples of typical models of each type, an Oehler Ballistic Lab. Travel and per diem compensation.

 

[DownInTheDark]

I read an article looking at 7.62x39 in a semi auto with the action cycling and also fixed closed. The cycling function was 1/200th slower.

 

[Deus Machina]

While it may make a difference with a long enough barrel and fast enough action, the difference in anything portable on your hip (or probably any vehicle, for that matter) is utterly insignificant.
The bullet is out of the barrel before the pressure overcomes the inertia. Otherwise you'd be losing pressure out the back. Which you can imagine isn't a good thing on the shooter's end.

 

[JohnKSa]

Does the different design affect the final velocity of the bullet exiting the barrel if all other variables are similar like barrel length, and bore diameter?

Yes.

And, what might that difference be?

If you made a list of all the things that affect the velocity of a bullet, and put them in order, the difference between blowback and locked breech wouldn't make the top ten.