jacob,

  The idea is to place the "sled" at the harmonic nodes, or "dead spots". There are a coupla ways to determine this. The one you read a lot about, and possibly you can find the threads on here discussing it, is to coat your barrel with chalk dust and tap it with a hammer. Where the barrel vibrates the chalk will "walk" around and the dead spots won't move. Some say it works, others say it doesn't, I've never tried it. The way I do this is by sliding the "sled" up and down the barrel in small increments until best accuracy is found. 

   This can take a long time so here's what I've basically found out. Put the back edge (edge nearest the chamber) 6" from the muzzle. You'll have to adjust this a little but 1/2" either way has worked for me on three rifles that are capable of sub MOA. One will even do sub 1/2 MOA. Then I wrap a piece of electricians tape around the barrel so I can find that spot the next time i put the "sled" on that gun.

  Actually I'm not sure if this is all that critical as accuracy really depends on quite a few things other than barrel vibration, so I doubt that if you don't hit a node dead center you won't be able to tell the difference in accuracy.

  The above will apply more to short (28") 3 wgt. barrels, and heavier, than those "buggy whip" barrels. 

PETE

Brent,

  After serious consideration and much experimentation I settled on the 6" + or - 1/2" cuz it allows the muzzle to stick over the end of the bench. This allows the dirty wiping patches to drop into my "catcher" instead of all over the bench top, creating a mess. 

  Then there was an experiment that Mann did that seemed to show that if your muzzle is within 6" of the bench top, and behind the front edge, that there are shock waves(?) that will disturb the bullets flight at it's most critical point. 

  So. There is method in my madness!

PETE

I always heard it was more like 1/4", but........

There is a well-known aeronautical phenomenon known as 'ground effect' that comes into play here but, like Brent, I believe it's largely mitigated if not eliminated by supersonic speeds.

Facts would be good here.
Regards, Joe

Brent,

  Maybe shock wave isn't the correct word. You're right in that any kind of disturbance wave behind the bullet can't do much in the way I'm talking about. But...... What about that air column in front of the bullet?

  But.....  It is well known that the gas pressure coming out behind the bullet is coming out at a pressure/speed faster than the bullet. As witness high speed photo's showing the smoke surrounding the bullet at the muzzle. This dissipates rather quickly so might not affect the bullet much, if any.

  What Mann did was to place and fire a barrel just off a plank to some distance above. Apparently this affected accuracy. So what was the cause? The air column moving in front of the bullet is traveling as fast as the bullet, and the gas behind is traveling faster once it exits the muzzle. So it could be that both have some effect on it.

  It's probably a moot point these days as a normal front rest is well off the bench top and most shoot with the muzzle at the front or hanging over.

  We also know that the first 20-30 yds. of a bullets flight, before it goes to sleep, are the most critical so it probably doesn't take much influence to alter it's flight. It wouldn't take many thousandths of an inch to disperse a group noticeably.

  JB,

  You might be right. 4" is stuck in my head but that doesn't make it the gospel. Possibly that was as high as it needed to be before no effect was noticed. I just recall the experiment more for being one of those "Gee! I didn't know that!" kind of thing rather than the exact particulars.
 
PETE

first,i want to sincerely thank you for the most knowledgable answers i ever heard... i'm rally leagly blind,and am having to start shooting with my left side now,,my right eye left me,,,,, and well,,i am kinda starting all over....and my rifles are all mostly without the forarms,and the barrels are just exposed,and i have tried to put the rest,,i don't have a sled,,andi put the rest at 6 inches from the mozzel,and i have tried the rest all over the barrel,and i have found the best place to get groups,,is right in front of the reciever,,under the front of the scope,,on my 223,22-250,and my 270.... on the barrel there... i sure am sorry for the trouble,,and i know all of you have many ,many years,experience,or know the right people to help you,and know the right books,,,but is it  alright to start shooting from this place in front of he reciever,and not on the end of the barrel,,?... thanks for putting up with me,,men,,,

Jacob, let me add a bit of perhaps helpful info.  Steve Earle produces a special set of offset scope mounts for the Lyman/Unertl style bases to enable the right handed shooter to mount the scope so as to use his left eye.  They were designed for folks in situations similar to yours.  I reviewed them for the SSR Journal about a year or so ago and then passed them on to a fellow shooter who had gone through exactly what you describe... loss of vision in his right eye.  While I didn't shoot them due to time constraints and the fact that my right eye remains dominant, I was able to get a good sight picture with my left eye when I installed them, even when I held them up to shoot offhand style.  I don't have his address in front of me, but check any issue of the SSR Journal for telephone and e-mail contact info.

Regards and best of luck,
Charlie the Frog