Why are Locked Breach 'Softer Shooting' than a Blow Back Action?

[Slamfire]

The pressure curves for gas operated guns are kind of a red herring. The gas is tapped off while the bullet is still in the bore and the pressure is still significant--it has to be since the gas pressure is what works the action. In Browning's pistol design (and variants of it) the action is operated by momentum/recoil, not gas pressure and the design is specifically set up to insure that pressure is not a factor by the time the action unlocks and extraction begins.

There are plenty of blowback, retarded blow backs, where that pressure curve drop is critical to the extraction timing. It is not just a gas gun thing.

Let me add this, my data:

Kimber Custom Classic 

230 gr LRN Valiant 7.7 grs Blue Dot  Mixed Brass WLP 
21-Jun-06 T = 97  °F  OAL 1.250" taper crimp .469"

 Ave Vel = 797.1   
Std Dev = 29.68   
ES = 121.1  
High =  855.8  
Low = 734.6  
N = 32  

accurate, not as accurate as Bullseye Load
long ejection distance stouter recoil

230 LRN Valiant 5.5 grs Unique lot UN331 1989 Mixed brass WLP (brass)
16-May-09  high 83  °F  OAL 1.250" taper crimped 0.469"

Ave Vel = 827.4   
Std Dev = 17.63      
ES = 85.68   
High =  871.6   
Low = 785.9   
N = 31   
    
230 LRN Valiant 6.0 grs Unique lot UN387 6/21/93 Mixed brass WLP 
18-Mar-07 T = 62 °F  OAL 1.250" taper crimped 0.469"

Ave Vel = 898   
Std Dev = 21.44    
ES = 75.58   
High =  944.3   
Low = 868.7   
N = 25   
   
 about 4" high accurate lots recoil 15' foot ejection


230 LRN Valiant 6.0 grs Unique lot 6/21/1998 Mixed brass WLP (nickle)
16-May-09  high 83  °F  OAL 1.250" taper crimped 0.469"

Ave Vel = 885   
Std Dev = 16.79      
ES = 67.26   
High =  917.8   
Low = 850.5   
N = 30   

  strong recoil   15' foot ejection

230 LRN Valiant 6.5 grs Unique lot UN387 6/21/93 Mixed brass WLP 
18-Mar-07 T = 62 °F  OAL 1.250" taper crimped 0.469"

Ave Vel = 926.4   
Std Dev = 16.64   
ES = 71.16   
High =  963.4   
Low = 892.3   
N = 32   

 light leading accurate centered hard recoil  20 foot ejection

The Bullseye load I was comparing against was a 230 LRN with 4.5 grains Bullseye. Just at 800 fps. Now, the weight difference between 4.5 grs Bullseye and 5.5 grs Unique is one grain. Then it goes to 1.5 grs for 6.0, and 2.0 grains for 6.5 grs. I don't know what the sensitivity of humans is, but could I feel the difference between 0 and 1 grain, or 2.0 grains?

(Is this not at the level of force of a mouse fart? And if all the mice in the world farted counterclockwise at the same time, would the world stop spinning? Should we be worried? LOL )

I am skeptical that the additional amount of powder mass is why the slide is recoiling more with the heavier charges. I think it is due to pressure curve peak, dwell, and residual pressure at unlock.

Chin Vol IV has a bunch of representational pressure curves, which I bet are based on real data. These are from the blow back section. The projectile is out of the barrel long before unlock occurs.





But, without data and a timed pressure on a 1911, I can't prove harder recoil is due to the burn rate of the powder, assuming the peak pressure is the same. But also, based on my experience, I believe this.

 

[JohnKSa]

There are plenty of blowback, retarded blow backs, where that pressure curve drop is critical to the extraction timing.

1. The pressure drop is ALWAYS critical to extraction timing. If the pressure isn't low enough by the time extraction starts, bad things happen.

2. The guns we're talking about are recoil-operated, locked breech guns. In that design, unlocking is performed by slide movement which is a function of recoil--muzzle momentum. By the time unlocking starts, pressure has dropped to nearly zero because (by design) the bullet is out of the bore and there's nothing to hold the pressure in.

Then it goes to 1.5 grs for 6.0, and 2.0 grains for 6.5 grs. I don't know what the sensitivity of humans is, but could I feel the difference between 0 and 1 grain, or 2.0 grains?

It's not the weight of the powder, it's the momentum. It's very light, yes, but the powder gases move very fast so the overall momentum (mass times velocity) can be significant.

https://www.shootingtimes.com/editorial/gunpowder-contribution-to-recoil/328788

Note that the chart indicates that for the .45ACP, the powder charge weight can contribute around 8% of the overall recoil.

Ran the numbers for your various loadings through this recoil calculator.

http://www.shooterscalculator.com/recoil-calculator.php

When the powder charge is taken into account, the first loading is not the one with the lowest recoil. The second one is.

 

[toivo]

That's why I'm a fan of the few .32 ACP locked breech guns that are out there. Most folks know about the KelTec P32, and some know about the Taurus TCP732.

And the Colt 1903 Pocket Hammerless.

 

[Mosin77]

And the Colt 1903 Pocket Hammerless.

Colt is a blowback. Still a pleasure to shoot however.

 

[toivo]

Colt is a blowback. Still a pleasure to shoot however.

My mistake. I was thinking of the .380 Pocket Hammer.

 

[Johnm1]

This has been a very informative thread. I have learned a lot. Thanks to all that contributed.

I'd still be willing to help in a totally subjective test if someone has one of the blowback Llamas in the Phoenix area to compare against my locked breach model. All we would need is a scale to measure the weight of the reciprocating parts and some way to measure the spring weight. At least comparatively. I'd be willing to travel a bit.