Originally Posted by TDC20
Shadow, I think that is absolutely a possibility. I would think that a locking block would break while the barrel was smacking it traveling back towards the shooter in recoil. If that is the case, then the broken locking block would allow the barrel+slide to remain locked longer than necessary. This would cause the normal locked dwell time to increase, not decrease, so that would actually make a KB less likely on that round. On the next round, if it chambered for firing, the barrel+slide might not be fully locked in the normal forward position, reducing the travel of the locked slide+barrel. For a typical handloaded hot 180 XTP at 1200fps, if the normal unlocking travel were reduced from .210" to .070", that would most definitely cause a KB! See explanation below....
Physics: It's not just a good idea, it's the law!
OK, I've been wanting to do this for some time now, but never sat down and took the time to do it. But what I've done is to create a spreadsheet to do some calculations on barrel-slide lock time, pressure, slide velocity, etc. Based on a realistic velocity for firing a 180XTP from a G29, and some assumptions that I've made below, I've checked my numbers a couple of different ways, and I'm 99% sure they're OK. It took a while to figure out how to work the math using the English (pounds, feet, etc.) system. The SI system (MKS) is so much simpler!
- Glock 29
- Barrel weight (weighed) 4.0 Oz. (0.25lb)
- Slide weight (weighed) 15.6 Oz. (0.975lb)
- Slide + barrel (locked) 19.6 oz. (1.225 lbs)
- 180gr. XTP fired at 1200fps plus 10gr. of powder(hot, don't try this without working up)
- Slide travel before barrel unlocks from slide (measured) 0.210"
- Spring force 17lbs
- Linear acceleration ***
- Distance bullet traverses barrel 3.145" (measured from where a 180 XTP would be positioned in the 3.77" barrel in chamber when shot is fired)
- Conservation of momentum, M1*V1 = M2*V2, where "1" is bullet and "2" is barrel + slide
- Neglecting slide friction and small amount of momentum transferred to the shooter in the first 500 us of shot initiation (time it takes to get the bullet out the bore).
- Neglecting force of "jet effect" of combustion gasses leaving the barrel after the bullet has cleared the muzzle.
*** The assumption of linear acceleration here is totally bogus, but it does fit within the use of the conservation of momentum equation. The problem with figuring actual time-based acceleration is having a precise pressure curve, then knowing what the force of barrel friction + bullet obturation is. This is still going to require some assumptions, because you can't precisely measure the time vs. pressure curve with load cells. A properly calibrated piezo could (could meaning "properly calibrated" and applied) get you very close. However, we can precisely determine what the velocity of the bullet is when it leaves the barrel, so that is what the spreadsheet is based on. The best method, IMO, would be to attach an accelerometer to the slide. Then you would know exactly how the slide velocity changes as the bullet traverses the barrel.
So what I did in my spreadsheet is to calculate, based on linear acceleration (assumption is a constant force is applied the bullet and powder mass to move them down the barrel). The counter-force to this acceleration must be an equal and opposite force applied by the system of the locked barrel-slide combo and the recoil spring. The resulting force in both directions turns out to be appx. 2320 lbs., and the bullet would leave the barrel appx. 437 microseconds after this force was applied.
In order for unlock to occur while gas pressure was still in the barrel (i.e., bullet has not left the barrel), the slide-barrel combo would have to travel the 0.210" backwards into the unlock point (locking block) faster than the bullet can traverse the barrel. Applying 2303 lbs of force to the 1.225 lb mass of the barrel + slide, the time it would take to reach the unlock point under 2303 lbs of force (subtracting the 17lb spring force) would be 761 microseconds. If I assume that the opposing forces are terminated as the bullet leaves the barrel at 437 us (neglecting gas jet effect), then my numbers for momentum are exactly balanced.
Here's some interesting observations from this exercise:
- Calculated slide velocity is 26.4 fps
- Changing the spring from 17lbs to 22lbs only changes slide velocity by 0.06 fps. (note that a 22lb spring will decelerate the slide motion much more quickly than a 17lb spring, that's really the only benefit to extra spring force)
- Reducing the slide mass by one ounce increases the slide velocity by 1.42 fps.
- Increasing spring force is not going to significantly delay unlock time. Completely removing the spring in this system makes almost zero difference in unlock time. No spring at all would not cause a KB.
- Mass is critical to control slide velocity, but it takes a huge reduction in slide mass for unlock to occur early. The normal barrel + mass of 19.6 Oz, all other factors held the same, would have to be reduced to appx. 6.5 Oz for KB to occur (assuming this "normal ammo" pressure)
- Slide travel to unlock is critical. In this case, all other things held equal, KB would occur if the distance was shortened from 0.210 to about 0.070" (could the KB round have fired somewhat out of battery/incomplete lock-up?)
I tried all kinds of practical changes to the system to induce an unlock condition, and I couldnt, even at 100,000 psi. Of course, I did make some assumptions on pressure-time curve, and neglected the "jet-effect" of gasses continuing to exert force on the barrel-slide combo.
So, I'm not claiming to be a gun expert, and I certainly could have made errors in my assumptions, especially with the linear acceleration, but it seems to me that the system is designed to never unlock under practical conditions, regardless of the chamber pressure.
OK, flame proof suit is on, fire away!