Importance of "dwell time" before unlocking?

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Importance of "dwell time" before unlocking?

Post by GrumpyOldMan on Sun Sep 28, 2014 10:06 pm

Fast enough high speed video turned out to be a bit harder to find than I expected.  The shot of a Beretta type here about 1:21 in: https://www.youtube.com/watch?v=vozrLZclXfg shows the guide rod becoming visible as the gases ahead of the bullet are exiting.

So the barrel and slide are moving backwards maybe 1-2mm (sorry, I'm not visually calibrated for less than 1/8-inch) before the bullet leaves.

I'm familiar with how the 1911 type rests the barrel legs on the slide stop pin for about that distance rearward when properly match tuned. Poked around enough with a P38 to understand how something similar can also be done with the Beretta--tight lockup but everything gets generous clearances for reliability after that initial unlocking.

But what about the SIG? There is no similar forward-only "slide" motion into final lockup. The front camming surface on the barrel just hits the takedown lever pin. I don't have access to a SIG X-5 or X-6 to see whether the reported 2-inches or better groups they are capable of are done with that simple angled plane of the lockup surface, or they rounded it out for fitting (or even put a small flat on top of the takedown lever pin).

So at the risk of getting all mental and affected my next test results (barrel rotation in the slide/barrel lockup is now gone), who important to accuracy is that little bit of slide/barrel unity in the first small rearward movement before the bullet exits? The SIG releases the barrel from that sort of coarse wedge fit immediately on the first tiny bit of rearward slide travel.

GrumpyOldMan

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Re: Importance of "dwell time" before unlocking?

Post by Jon Eulette on Sun Sep 28, 2014 10:21 pm

I've done trigger work on Kalifornia legal X-5's.  The barrels are fit extremely well. Excellent hood fit and upper lug and lower surface lockup/fit. They stay in battery very well until the barrel drops down.  Don't remember the distance traveled before unlocking but it was excellent.
Jon

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Re: Importance of "dwell time" before unlocking?

Post by Guest on Mon Sep 29, 2014 7:21 pm

Thanks for the video https://www.youtube.com/watch?v=vozrLZclXfg
Neat to see how the gas exits first.  I thought the bullet would seal the gasses behind the bullet.

Chip

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Re: Importance of "dwell time" before unlocking?

Post by davekp on Tue Sep 30, 2014 8:25 am

ChipEck wrote:Thanks for the video https://www.youtube.com/watch?v=vozrLZclXfg
Neat to see how the gas exits first.  I thought the bullet would seal the gasses behind the bullet.

Chip

The gas you see exiting ahead of the bullet is the column of air in the barrel ahead of the bullet. The bullet has to push it out.

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Re: Importance of "dwell time" before unlocking?

Post by GrumpyOldMan on Tue Sep 30, 2014 7:14 pm

Continuing the thread drift, there is visible there some of the powder gas which gets past the bullet as the case mouth releases it and as the bullet is moving into the throat and later (not very much later!) sealing the leade.

Same stuff that sometimes leaves visible soot on the fired cases.

Same stuff that some high-speed video shows escaping the rear of the chamber.

And no, it's NOT awful to be old enough to have seen this much stuff. Still ain't seen it all, but I've seen a lot...

Got me an idea to put some straight-back but still locked up tight travel into the SIG lockup, but it's gonna take a little weld-up and then a little ream-out. I'm hoping that circles and tangents are our friends.

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Re: Importance of "dwell time" before unlocking?

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