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Old 10-11-2018, 08:58 AM
Forrest r Forrest r is offline
Join Date: Feb 2007
Location: Ohio
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If there's a difference of 250fps (That's huge!!!!) between the 2 revolvers, it isn't the cylinder gap.

Cylinder gap ='s +/- 10fps per 1/1000th of gap.

Not only have I read the above statement, I have test it and found it to be true numerous time while working up loads for accuracy in my dw's.

It's common to move the cylinder gap in and out along with raising & lowering the tension on the bbl nut to tighten groups up with the dw's. Changing the cylinder gap has always changed the velocity over the decades and 3 different chronographs. It's always been +/- 10fps per 1/1000th's cylinder gap.

Myself I don't use/shoot anything over 7/1000th's cylinder gap. Start getting over that & things go south real fast. Never worried about squibs, was more concerned with things like flame cutting & forcing cone erosion.

Not a s&w or 586/686 expert by any means, but seeing how you've been so kind to me in the past I'll pass along what I've learned over the years and a little bit of ammo.
Bought my 1st 586 nib back in 1987, darn thing was like a laser.

Back in the 80's surplus powder was everywhere cheap, couple that with cheap primers and free lead. The end result is large amounts of range time. I kept tract of how many primers I bought to make reloads for that 586. At the 100,000 round mark I sent it back to s&w, the timing was wore out. S&W also set the bbl back and redid the forcing cone. At the 200,000 round mark that 586 needed the timing redone again. Got the work done and the forcing cone re-cut. At the 275,000 round mark I noticed the velocities were dropping when using the same loads (100fps drop) I've used/shot/tested in that 586 for decades. I used that 586 as a benchmark when testing firearms of the same calibers. Always take a known performer along with a unknown firearm to the range for the 1st couple range sessions. Anyway that 586's bbl was finely shot out, hence the drop in velocity. Keep in mind that firearm saw it's share of powder puff/target loads. But it also saw countless 1000's of fullhouse 357 loads. Here's what that 586's bbl looked liked when I pulled it.

As the cylinder gap gets larger the flame cutting & forcing cone erosion happens faster. This is why anything over 7/1000th's I don't use/shoot anymore.

It depends on what you want the 686 for. Right now I have 2 of the s&w l-frame series 357 revolvers. The 1st 1 is a 586 l-comp

It is designed for sd plain and simple. It's comped, has recessed cylinders that are cut generously (large at the back end/loads faster & cases fall out quicker) and a 6/1000th's cylinder gap with less than 1/1000th's endshake. Basically it's designed to load/unload fast, not bind & comped for hot loads. The bbl is listed @ 3" but is really 2 1/2" long with a 1/2" comp in the bbl shroud. Even with the 6/1000th cylinder gap & 2 1/2" bbl that 586 has no problem delivering 158gr hp's @ 1200fps on demand with loads that are under max with 2400 & home cast/coated bullets.
Now this 686 on the other hand has extremely tight cylinders. Spent cases from that 586 l-comp need a hammer to get then to seat all the way in the 686. It also has less than 4/1000th's cylinder gap and less than 1/1000th endshake.

Same bullet used in the l-comp 586. But now it's loaded for accuracy in the 686.

2 different l-frame 357's from S&W. 1 is designed/made for sd & the other for target work. One is loose & the other is tight. You need to decide what you want to do with that 686.

IMHO, 8/1000th's is just too much.
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