when the VIS and frame bed get added to the mix, I do get more confused than usual.
It's not complicated, Jerry. The hard part is trying to describe in detail and simple function via the written word. Sometimes that gets tough, especially when ya try to juggle 5 different things while ya bounce ack and forth between the daily scoot and the keyboard.
I'll try to do it in as few words as possible...
When the slide draws the barrel backward, the link swings through a short arc. That arc is necessary to give the bullet time to exit so the breech can open safely.
When the link reaches the limit of its arc, the barrel...still moving rearward...starts to pull on the link, and begins to swing downward on an even smaller arc.
When the upper lugs have vertically cleared the slide's lugs, the rear face of the lower barrel lug strikes the vertical impact surface...halting all rearward barrel movement...and the barrel drops the tiny amount remaining to the frame bed. This small amount clearance between barrel and frame bed is necessary to prevent stressing the link, slidestop pin, and lower barrel lug. It's not necessary to have a lot of clearance, as long as the barrel doesn't hit the bed before it hits the vertical impact surface. Even a thousandth of an inch is good, but I prefer to see a little more than that because as the bed and the barrel become carbon fouled...the clearance goes away, and the barrel then hits the bed at the same time as the VIS. While that isn't as destructive as striking the bed first...over time, it does overstress the areas mentioned...and can cause a premature failure in any of the included parts.
How soon depends greatly on how well the gun is maintained and how often it's cleaned. Clean it every 50 rounds, and you'll never have a problem. Be like me and clean it every 2,000 rounds whether it needs it or not...and you will sooner or later.
This is another reason that I tend to set my guns up a little on the loose side. Laziness. I don't like having to stop 2-3 times during a 500+ round range session to clean my gun.
Now then...You understand that when the barrel comes out of the slide, before it stops on the VIS...there must be a little clearance between the top of the barrel lug and the bottom of the slide lug. How much clearance that's necessary depends on how well the gun is fitted. A precisely built gun like Jerry Keefer builds can get by with less clearance than one that I build.
This clearance must be present even if the barrel is forced upward when it's fully linked down and on the bed.
Gravity will let the barrel drop enough to fool you into thinking that you've got enough clearance when you actually don't. A quick'n'dirty way to check for it is to whack the muzzle sharply with the heel of your hand and hold it back hard when it stops. At that point, the barrel is against the VIS and has linked down as far as it can go without gravity helping it. The slide is at the timing point for full barrel linkdown.
Now, check for the clearance. If it's there, you're probably golden.
If it's not...the barrel linkdown timing needs attention.
I know you're wondering how to know about bed impact before VIS impact...
Coat the bed or the bottom of the barrel with layout fluid. A sharpie marker will work. Assemble the gun. Whack the muzzle with your hand sharply about a half-dozen times and look at the coated area. If ink has been removed...even in spots...you need a little clearance there. If not...you're good to go.
