Dampening harmonics

C

cdoc42

New member
A little history: I bought a Rem M700 in .270 in 1968, could never get groups under 1.5" but I never failed to score on whatever big game I hunted. Eventually I started to get groups that climbed vertically and burn marks were evident at the end of the stock. I fiberglas bedded and free-floated the stock. Changed the heavy trigger to a Timney at set it at 3 lbs. Still nothing impressive, so I figured after 20 years I burned the barrel out; returned it to Remington and they sold me a new barrel. Impressive. Now getting small groups, including a 3-shot , 1-inch group at 200 yards.

Recently the bolt stop spring failed and I replaced it tonight, reset the trigger for 2.5 lb and I'll see what my groups will do tomorrow now that I took it all apart.

I then got lost in thought. I have Browning BOSS rifles and I think I understand their concept of dampening vibrations so that the bullet exits the barrel at approximately -or hopefully-the same spot in the rotary motion of the barrel. Free-floating eliminate contact with the stock but some rifles still shoot better if there is contact with a spot in the first few inches of stock.

That led to the thought: if we want to dampen the vibrations, has anyone ever played with the idea of bedding the barrel entirely with a rubber material or some other shock absorbing material?

That leads me to this post, looking for opinions from those far more expert than I.
 
Anything that transfers external pressure through the stock fore end to the barrel will not be the same from shot to shot. Totally free float the barrel because stock fore ends are flimsier than rifle barrels; they bend more with a given force against them.

A totally free floating barrel vibrates exactly the same for each shot. Barrels touching something besides the receivers don't. Repeatability is the fundamental element of accuracy.

A bedding pad under the barrel's chamber area causes vertical shot stringing as the barrel heats up. Pressure points at the fore end tip partially correct big problems caused by poorly fit actions to the stock. They transfer uneven external forces to the barrel.

BOSS equipped barrels don't make bullets leave at the same place in the barrels whip, wiggle and rotate cycle. All bullets don't leave at the same speed nor take the same amount of time to go from case mouth to out the muzzle. Browning designed it so people shooting factory ammo could tune the barrel whip frequency so slower bullets left at a higher muzzle angle than faster ones to compensate for their greater drop down range.

Browning beds their BOSS barreled receivers in a rubbery stuff. I don't think it'll keep the same rubbery properties in a wide range of temperatures. Someone needs to test it in a 100 degree F range starting at 10 then going up in 10 degree steps.
 
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Full length bedding of rifles is one way to go about doing it, and some gunsmiths prefer that route. Most of the time free floated is better I think, but with some very thin hunting barrels such as those on a Remington MT Rifle you may see better accuracy with full length bedding.

The concept is not new. If done right it can be accurate. At least as long as nothing is putting external pressure on the stock, which will then transfer that pressure to the barrel. Floating allows a little stock flex without it contacting the barrel.
 
If the barrel is full contact bedded in the fore end, be sure the fore end never touches anything when holding the rifle to shoot it; that includes a sling or a hand to support it. And verify the fore end doesn't bend as it changes temperature.

How much the barrel bounces off from it will be hard to measure, but I guarantee it'll be a few times before the bullet leaves the barrel. It's the same thing as resting the barrel against a hard object to steady one's aim.
 
I found that a rubber barrel damper helped the accuracy of my Vanguard 257 Wby. Those rifles have a fairly whippy barrel burning a bunch of powder-recipe for significant barrel vibration.
 
I don't think it matters how much or how fast a barrel whips or vibrates. When totally free floated, they all whip, wiggle or vibrate exactly the same for each shot. How much they vibrate is load dependent, but for a given load, they're all very repeatable for every shot.

One can add a weight to the front part of the barrel to change the frequency it vibrates at so bullets leave at a more desireable point in their cycle that compensates for muzzle velocity spread. It's just as easy to change the charge weight; and a lot cheaper, too.
 
Bart,
Thanks for the clear and understandable explanation. Dr. Gerald Bull wrote a paper on a similar subject but was no where as readable as this.
Thanks!
 
TXAZ, you're welcome.

There are 30 caliber 22" skinny featherweight barrels that are just as stiff and rigid as 26" heavy sporter or medium target barrels. Both have the same resonant frequencies and harmonics thereof.
 
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Years ago I bought a device called a "Supertune" by Time Precision. I put it on a '96 mauser in 6.5 swede and I can testify that it worked remarkably. I used a load that I had worked up to near max (for a '96). I shot five shot groups and turned the device in small increments after each group. Groups shrank from just over moa to under 1/2 moa and then began to grow again. I used it for some time and still have it. The only way I could fault it is for offending my sense of aesthetics... it was as ugly as Fido's ass sitting out there on the end of my barrel looking like some kind of goofy thing.
 
We, who tune our handloads to accomplish tight groups by varying seating depth, bullet selection, and powders/charge weights, etc., sometimes forget that it's often more difficult for factory ammo users to achieve better groups with their rifles.

Pressure points in the forend can be installed, providing 4 lbs. or more, of uplift, by means of bedding compound, moulded about an inch long near the forend tip. After setup, grind or file the pad into into two 3/16" wide sections (4 and 8 on the clockface). The trick is to provide uniform and sufficient upward pressure as to NOT allow the barrel to bounce off the pads.

A shooter once told me that the older Ruger 77s respond well to pressure-pad bedding, due to the action's flexibility. Some folks try folded paper, thin cardboard and other materials to make a pressure pad, but I've never had good results because it's impossible to achieve consistency.

As others have mentioned, group size can improved by this method, but point of impact of the group may vary, depending on sling pressure, position of the forend on a rest, or different rest densities/angles. If that's the only way to achieve improved accuracy, the stock's forend must be relatively stiff. Injection molded or thin wood forends are not usually good candidates for pressure pads.

Regardless of forend stiffness, consistent forend holds/rests are important. One means to improve more consistent POI is to sight in a rifle to be used in the field with the forward hand grasping the forend in the normal field hold, but resting on a relatively soft surface for stability. Always check the sighting-in by shooting it using typical field rest positions. (Using a sling for tension when a rifle has a pressure pad or non-free floated factory bedding is not recommended.)

(As always...your results may vary with pressure, materials, or location of bedding pads. NOTE: The only rifle I own with a pressure pad is a Ruger 10-22, which screams for one, since there's only one bedding screw, so free-floating is not an option, IMHO.)
 
The most consistant zeros are had when the barrel's totally free floating. That way, nothing will interfere with its normal wiggling, whipping and bending it goes through when the round's fired and before the bullet exits.

The most accurate fore end tip pressure bedded rifles I know of are the M1 and M14 semiauto service rifles; their lower band has downward pressure from the stock ferrule that changes with how much their fore ends bend. But they are notorious to having zeros change when the fore end's hold is changed and supported. Which is why many people will have a different zero at 200 yards for two different shooting postions; standing without a sling and sitting with one. All in spite of their receivers being very solidly epoxy bedded in the stock.
 
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