Bedding revisited

chino69 said:
... Make a removable shim under the chamber area, in front of the recoil lug, to provide a degree of support. ...
Click to expand...

Interesting idea. For a stock with good strength under the action, the extra support isn't needed. But anything that changes the connection between the barrel and forend will affect dynamic response and tuning. Who's to say your shim wouldn't improve accuracy for a particular rifle, although it might be preferable to use an elastomeric, rather than hard, material so that it stays in place and doesn't rattle around and cause inconsistent force on the barrel. A damping material like Sorbothane might help, too. You can buy sheets of this and cut to fit. Let us know how it works.

Cheers,
Keith
 
Damon,
Good point, although I would modify a bit to say that since the stock material is considerably less stiff than the action, what we want is for the action to move consistently relative to the stock each time. Vaughn tested a stack of flexible shims to allow the action to recoil rearward a lot relative to the stock, while still constraining lateral and rotational motion. I wonder if anyone has tried this with a BR rifle.

Stress-free is still a goal so that things like bolt fit and lug contact are not affected.

Nice article. I was initially concerned that thread pitch is not in your equation for bolt force, but I see that it simplifies to your equation for small pitch angle. For those who want to split hairs, an equation that includes thread pitch is at http://www.engineersedge.com/gears/screw-axial-thrust-load-calculation.htm

Cheers,
Keith
 
Keith,

Indeed, the pitch doesn't really matter. That was surprising to me as I learned all about bolts back in engineering school. There are more involved methods of modeling a bolt, but they get can pretty esoteric. The lowly bolt is a complex beast.

Also, good point re: Vaughn's experiment. Maybe I should expand:

What we want is for the barrel to vibrate consistently from shot to shot. One way to achieve that is to make sure the stock is "welded" to the action. Another might be to make sure the barrel recoils with total freedom relative to the stock (Vaughn's approach). I would love to see someone expand on his idea - I bet it can be done a lot better.
 
mks said:
Interesting idea. For a stock with good strength under the action, the extra support isn't needed. But anything that changes the connection between the barrel and forend will affect dynamic response and tuning. Who's to say your shim wouldn't improve accuracy for a particular rifle, although it might be preferable to use an elastomeric, rather than hard, material so that it stays in place and doesn't rattle around and cause inconsistent force on the barrel. A damping material like Sorbothane might help, too. You can buy sheets of this and cut to fit. Let us know how it works.

Cheers,
Keith
Click to expand...

Keith,
I've already done this with another stock and barrel combo. The shim was made out of Acra Glass with powdered stainless which is what I do for all my bedding. When I use the term shim that is a bit of a misnomer as I actually bedded this portion with release agent in the barrel channel of the stock as well as the barrel, making it removeable. The barrel is a light varmint. The shim mimics a permanent bed approx. two inches in front of the recoil lug. I originally went this way as a means of comparing with and without. The problem is the rifle, a trued 40X in 6mmBR, shoots so damned well I'm reluctant to change anything. I realize removing the shim will affect dynamic response and tuning so I don't want to screw with success.
 
Speedy and McMillian had this discussion many years ago. Speedy sent out copies of the letter from Pat M. but I have lost it. Bascially, a few differnet rifles were bedding with a portion of the barrel bedded in front of the reciever/lug. They were tuned as best as they could be and then the the bedding removed. Every single time, the gun shot better without.

Hovis
 
Barrels swell a little under pressure, and the peak pressure occurs when the bullet is only a few inches down the bore. It seems to me that giving it something to "push off of? may be counterproductive when trying for the highest level of accuracy.
 
HovisKM said:
Speedy and McMillian had this discussion many years ago. Speedy sent out copies of the letter from Pat M. but I have lost it. Bascially, a few differnet rifles were bedding with a portion of the barrel bedded in front of the reciever/lug. They were tuned as best as they could be and then the the bedding removed. Every single time, the gun shot better without.

Hovis
Click to expand...

Hovis,
Thank you as I can now save myself much time and trouble; appreciate that piece of info.
 
I think a lot of people complicate this simple process. If your using clamps or screws, then just snug them and check often by re-snugging, the bedding will be fine. Using old inner tube works but it s alittle thick, I like using the rubber bands that are at rehabiliation places, wrap it around the action and it will apply a nice light and even pressure down to settle the action. If I have the barrel handy, I like wrapping electrical tape around the barrel in two places to set the height and center the action, but you first must make sure the channel is centered.

On a side note, I have bedded dozens of rifles, both pillared and glue-ins. At one time I wondered what the best way was so I started a test project, same stock/barrel/action, etc but just different bedding methods. I started out with glue-in and tried, within reason, to stress it and then rebed without stress, I also did this with three different ways of pillar bedding. What I found out was as long as you didn't stress the action, they all shoot the same, however, anything over .001 in stress will effect the rifle. My wife still shoots this test rifle and if you glance at the pillar bedding, if might frighten a normal person but this is the best shooting rifle I have ever had. One other note, I even applied cleaning fluids (wipe out, butches, shooters choice and my own mix) to the bedding and it has been moist for over three years between the bedding (jb weld) and the action. Nothing has softened at all. If you don't believe me, see me at a shoot and I'll take it apart and you can see for yourself, it'll be wet and solid. That's one reason I like final bedding with JB. Not all expoxies acted that way.

Another note, it's not hard most of the time to find bad bedding or a bad stock, but it is usually the last place someone looks. If the bedding is stressed to the point of effecting accuracy, normally it will show up in bolt opening/closing quickly. Cracked stocks, which there are more out there than someone would think, are hard to find and diagnosis.


Hovis
 
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First, bed a dovetail rail to the action. The action is out of the stock for this. Take the base off, then do the bedding. After that, put the base back on but only snug the front screws, check for gaps with shims (very thin .001 to .0005). Repeat with rear.

Hovis
 
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