"Forced barrel deformation" -- Varmint Al?

[Charles E]

From another thread

The clip shows how the barrel and action are forced to rotate around the rifle's CG. There is no natural frequency with exactly this mode shape.

OK, let's suppose that the rifles GC was at the center of the bore -- well, or less. Would we now have a barrel that exhibited far less motion?

I imagine that for a perfect model, the CG would have to be evenly distributed both radially and longitudinally, in order not to set up what I think of as a "rocking couple" (probably the wrong term).

I have noticed that as benchrest trends went from 1 pound scopes and 2.5 pound stocks to 1.5 pound scopes and less than 2-pound stocks, the aggs seemed to get smaller. Of course, there could be many reasons for that.

In 1,000 yard shooting, where there are no stock-taper rules, there have been a number of very successful rifles built around heavy tubes, some with tensioned barrels, some not. A common feature of these rifles is that the rifles CG is much closer to the center of the bore than with other designs. It isn't usually a perfect distribution, but far closer than with a conventional stock.

Oh, and assume the rifle is fired "free recoil," meaning essentially that there is no shoulder against the butt for a distance much longer than the time it takes the bullet to clear the bore.

It seems clear that we want some muzzle movement. If we couldn't use that to compensate for velocity variations, we might be worse off. But I wonder if what we really want is to have the amplitude fairly small. Could one calculate the amount that would give the widest latitude in compensating for velocity variations?

 

[Charles E]

Even with the CG on the axis, part of the mass that puts the CG there is much more flexible than the barrel steel. For short time high-level loads the CG of the flexible system would not be located in the same location for static loading like suspending from a string.

Well, and I suppose only FYI, the 1,000 yard heavy guns I'm talking about use something like a 3.5- to 5-inch o.d. steel or aluminum pipe. If steel, the wall thickness is usually around .25 inches, if aluminum, anywhere from .5 to .75 inch wall. Reasonably stiff stuff.

The barrel is centered inside the tube, and guide rails for the bags are mounted on the tube. Weight of this sort of rig is anywhere from 70 to 170 pounds.

The light guns are much more conventional in appearance, and can weight only 17 pounds. The tube is usually aluminum, about 1.375 to 1.5-inch o.d., with a maximum wall of about .125.

The interesting thing is these rifles, when the barrel is unde a bit of tension, shoot round groups with varying loads. On my 1K heavy gun, I can vary the powder charge 3 grains or so (4+ percent) and still get nice round groups. I should stress that I don't mean one group of differing loads (haven't tried it), but that "tuning" seems to just be for the overall size of the group, it stays round through that range of charge increase. Maybe more, again, I haven't tested greater variations.

The Australians Jeff Rogers and Tonz Z have reported similar results with their HGs.

The speculation on my part is this: we need some muzzle movement to compensate for the velocity variations in our loads. If we didn't get that, we'd have vertical stringing from velocity variations alone. But we don't. I can't think of anything that would explain this except that we have in some sense "broadened" the ??? (forced barrel deformation?) area and are working in a fairly linear area. Otherwise, how to explain two groups of equal roundness in shape, but coming from a 4% variation in charge.

 

[Yukon Kid]

Amplitude in ???

We see sine curves on the scope as two demension and sound graph the same, why must the barrel be only looked at in only this same manner ???? Can not the vibration be say left to right or better yet in a oval configuration ???

 

[Vibe]

We see sine curves on the scope as two demension and sound graph the same, why must the barrel be only looked at in only this same manner ???? Can not the vibration be say left to right or better yet in a oval configuration ???

Absolutely. But the main discussion has been centered around tuners and the function of reducing vertical distribution in POI. In that discussion the topic is simplified to "just" 2 dimensions (well 3, if you include the time variable).

 

[Yukon Kid]

Torque ???

How about the torque of the bullet down the barrel ??? I can think of others but how small do you pick, just like "string theory" some things are just too small to measure, let alone S@@ ??????

 

[Jerry H]

? for varmit al

In a BR rifle, a .1 grain change in the average charge weight can take a .250+ group and cut it in half. Since this is only about a 400 PSI change or about 12 FPS MV difference, what frequency or barrel vibration would this be relative to? Any idea if you have one would be appreciated.

 

[Jerry H]

I have used an Oehler 43 in years past and recall those figures from memory. I don't use one anymore and have long lost the data, but I felt that the consistancy in rise time as indicated by that device was the singularly most important factor regarding group size. This is obviously an internal ballistics issue as this area is half of the in barrel time but occurs in the first inch or so of bullet movement. I think the total in barrel time was .00125 (approximate) @ 65K peak pressure. The 43 defines rise time as the time it takes to go from 20% to 80% of peak pressure.

 

[Dusty Stevens]

sometime could you model what a v-block on a railgun does to the barrel. what I mean is how far does a 1.450" barrel expand and what does those 3 contact points do to the barrel as the bullet is traveling. I need some strain gauges again I guess. I'd like to see how much difference a v-block and a round delrin sleeve in the block has if any at all.