Slide Stop Pin Fit Causing Vertical Stringing?

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Slide Stop Pin Fit Causing Vertical Stringing?

Post by Bigtrout on Mon Aug 24, 2015 8:22 am

My 9mm RO has occasional vertical stringing (thanks Rob) and I've ruled out lug bind and barrel spring (thanks Doc).   The thread regarding S/S pin fit leads me to ask the question; has anyone has experienced this phenomenon?   My pin seems to be fairly sloppy in the link bore, not so much in the frame.  I haven't measured yet nor tried moving the barrel when in battery without the recoil spring but want to throw out the question in the event it may generate some discussion regarding a solution.  Is there an aftermarket link I should try? The pin shows very little wear.   Thanks.
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Re: Slide Stop Pin Fit Causing Vertical Stringing?

Post by Dr.Don on Mon Aug 24, 2015 9:36 am

A question I should have asked earlier:  Does your RO have the factory barrel, or have you had a true fitted barrel installed?  If you have had an oversize barrel fitted by a gunsmith, the link is not involved in the actual lockup.  It is essentially free-floating when the barrel is locked up; some gunsmiths even elongate the link hole to accomplish this.  The link's function is to unlock the barrel.  If you still have the factory barrel, which is tighter fitting than most factory jobs but not a true fitted barrel, then anything is possible with regard to the link.  Also note that the 9mm with a standard twist of 1:16 is notoriously difficult to achieve high 50yd accuracy, much more so than the 45.

Again, we need the true gunsmiths to chime in here.......

Don
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Re: Slide Stop Pin Fit Causing Vertical Stringing?

Post by Jon Eulette on Mon Aug 24, 2015 9:39 am

In my experience vertical stringing can be caused by the link being too short and allows the slide stop pin to hit radius on the bottom barrel lugs which caused a speed bump so to speak that causes erratic cycling. Also if disconnector hits disconnector rail on slide too hard it can vertical string. Also too light of a recoil spring. If your slidestop pin isn't bearing against the lugs firmly in battery and doesn't have an engagement surface of .050-.080" thsn it can string the shots. Did you use a ransom rest?
Jon
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Re: Slide Stop Pin Fit Causing Vertical Stringing?

Post by Bigtrout on Tue Aug 25, 2015 8:49 am

Don, I do have a factory barrel.  I sent the RO back to SA in June for grouping issues at 50 ft.   They refit the slide to the frame and now it is snug but easily rackable and doesn't have any play when in battery minus the recoil spring.  They also installed a new barrel.  I never did get an explanation as to what they found wrong with the old barrel (600 rounds).   Regarding the 50 yard accuracy; my limited 50 yd. experience seems capable of 1 1/2".     My last shoot at that distance grouped 7 out of 10 shots in 1 3/8" with 3 flyers.  Pardon my ignorance, but what holds the lug lockup in fitted barrels if the link "floats"?  I can't seem to understand what keeps the barrel in the uppermost position.

Jon, I do not have access to a Ransom rest.  I built my own rest for the frame underbelly and firm cushions for my forearms but the RO pistol is held and trigger released manually.  Not the best of conditions but the Ransom is way too expensive for my limited use.   Is there a way to evaluate the recoil spring and disconnector impact on the lockup?  Also, pardon my ignorance again, where does one measure the .050-.080 surface contact?

I'm just a 1000-round greenhorn and my self-taught knowledge of accurizing a 1911 is quite limited as you can tell.  I do have Jerry Kuhnhausen's 1911 A-1 manual but the knowledge you folks have is not in there.   Thank you for your patience with this rookie.  Your comments and suggestions are invaluable to me.
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Re: Slide Stop Pin Fit Causing Vertical Stringing?

Post by james r chapman on Tue Aug 25, 2015 9:45 am

The link is designed to do nothing more than pull the barrel out of vertical lockup. The lower lugs on the barrel slide over and up when the recoil spring forces the barrel forward onto the slide stop pin which in perfect harmony forces the barrel up into the slide recesses. JB was such a genius,,
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Re: Slide Stop Pin Fit Causing Vertical Stringing?

Post by Dr.Don on Tue Aug 25, 2015 9:47 am

A fitted barrel is held in lockup by the slide stop pin pushing upward on the lower lugs.  That is what much of the care in "fitting" addresses.  Material is gradually and carefully removed from the lower lugs until the slide stop pin locks the barrel into place firmly, but not so tightly that the gun does not cycle smoothly.  The link functions to pull the barrel down out of lockup as the slide recoils, and it also lifts the barrel upward over the "corner" of the lower lugs as the slide moves into battery so they don't "bump" as Jon indicated.  So the fitting of the link requires care too.  It must be long enough to prevent the lower lugs from "bumping" the slide stop pin, but not so long that it prevents the lower lugs from resting firmly on the slide stop pin.  When the gun is in lockup you want all of the pressure on the barrel to be coming from the slide stop pin, and none from the link, hence my use of the term "floating".

Kuhnhausen's book is excellent in my opinion.  Individual smiths will differ from him on minor details, but overall it is an excellent reference.

It's just my opinion, but if you are getting less than 1.5" at 50 yds for 7 of 10 shots, that is probably about what you could expect from a barrel fitted by a production shop.  They cannot afford the time to hit all of the minor areas and some of those can cause flyers.  That is why a true bullseye smith costs what he does.  It sounds like this gun could take you pretty far as is.
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