Coming back to Johnk’s comment, ”Dave Tooley claimed the deep butt "fin" on his stock design served to modify rotational moment, but without any particular mass to it, I wonder if it isn't accentuating muzzle flip.” Perhaps, is the answer depending on whether your bag moves? I was not game to answer it before. As Peter says, you need a well anchored bag. But our rules and on some range’s would limit spiking it down. If it was tightly packed in this area, so as to reduce the downward thrust it would reduce the muzzle lift. Using your trigger hand may be a better method of controlling the twitch.
As I said in my first post, what is old is new again. I would like to consider, Dr Franklin Mann and Harry Pope.
From Mann’s book, The Bullet’s flight from Powder to Target (circa 1907).
“As all riflemen know that the barrel recoils one-tenth of an inch before the bullet leaves the muzzle, they will readily recognise the desirability of a front brace which will properly care for this sliding motion, which occurs at its maximum just as the bullet is leaving the muzzle.
The late Horace Warner recommended that the arms of any front brace should invariable project from the body on a line that passes through the rifle bore. ......our “bob sled” device will give us will give us a balanced bob-sled front brace...
After all our devising and experimenting to present an accurate and convenient front brace to the rifleman, Dr. S. A. Skinner steps in with a simple block of wood, screwed to the rear end of the butt stock ... and eliminates all our front brace business.
In shooting with this butt brace, the barrel simply rests is a wooden V placed six inches from the muzzle.”
See Fig 1. Mann’s Bob-sled
Fig2. Butt Brace
Source: The Bullets Flight , Mann & Pope.
Fig. 3 Pope’s Bench Rest Source: R. M.Smith
Mann and the Pope later developed machine rests to test gear based on these principles of fore and aft control with the bore centre in the same place.
Fig 4. Mann’s Test Rig Souce
Source: R.M Smith
Fig. 5. Pope’s Test rig
Fig. 6. Mann’s Rest , The Bullets Flight
Also note, “...and allows the concentric rings on the rifle barrel to find an easy position, giving the barrel perfect freedom to recoil backward...or to rotate without changing its line of fire by a hair”, according to Mann.
If you look carefully at the heavy cast iron V rests, you will note concentric rings turned to the same diameter on the barrel and where they touch the V they are on the centreline of the bore.
Looking at another Dave’s picture of his rig and comments about Peter’s rifles, do I have to say more?
Take a look at Pope’s 200 yard group. How many could do that now with modern rifles and powder?
Fig.7 Harry Pope’s 200 yard group
Source RM Smith The story of Pope’s Barrels
Note: Mr Smith produced gain twist barrels in Canada.
I can’t help think that the Gene Begg’s (USA) gear confirms the old Mann principles of the “Bob-sled” together with the modern tuner concept. He is a gunsmith and legend in case design, uses his own tunnel to prove concept. Go figure.
Fig. 8 Gene Begg’s Gear
Mann and Pope concentrated on the short range, but it was the Fulton Family who made a name in long range a couple of decades later. I can’t find my picture of older Fulton using his rest, but it was essentially 2 V angles added to a beam. The rifle was pushed forward so the front of the trigger guard touched the rear V, but on the rear V a vertical stopper was offset to accommodate the trigger guard sideways to use the trigger guard as a reference point, but of note, the recoil from torque moved away from this point. Pretty close to the concept of free recoil? It was Robin Fulton’s work and documentation of compensation that took my development further.
As Robin Fulton said, “Compensation is the term used to describe the effect of barrel movement in the vertical plane. The vertical position of the vibrating muzzle when the bullet makes its exit depends on the bullets travel time in the barrel; and this depends on the bullets initial velocity and the amount of friction it has to overcome. Variations in these two factors will cause the bullets to emerge from the barrel at slightly different angles of elevation.”
Well there is a bit more to it in terms of accommodating muzzle jump as Border Barrels have examined with their devices and computer modelling of barrel contour and length. In Fulton’s day barrels were standardized.
For those that use barrel blocks, you can move the barrel fore and aft in these blocks to adjust the flip or you can add a fundamental weight to the muzzle. Mann indicated the force was greatest as the bullet exits the muzzle. So it’s not surprising a fundamental weight or an adjustable weight will work here to dampen recoil and torque. I think it works at the muzzle as a stabilizer because the initial torque is at the breach, the opposite end but if the tuner was to extend past the muzzle like an Obermyer tube or barrel extension it also has to counter the muzzle lift caused by escaping gas. As I said earlier there are 3 moments of inertia on firing. Also remember the bore diameter of the Obermyer tube has another function to condition the atmosphere behind the projectile. So there is another interrelationship in tuning. From memory, years ago I used 0.86" for a thirty calibre, then I trimmed the weight back on the extension to modify the lift. The Begg's tuner does control rotational forces to some degree and I use one. But the combination of a Begg's tuner on an Obermyer tube because the weight can be adjusted saves trimming the fundamental weight. The downside of the Obermyer tuner is cleaning and you have to be very careful you do not pinch the muzzle on fitting. In pinching the muzzle it not only increases fouling, an oval shape can be created at the crown and lead to gas yawing the projectile and the projectile itself can be misshaped so as to not fly to its potential. One final note and generalisation was, that the faster I fired from the machine rest the better the elevation. I fired at random times and set intervals and different ranges. It was a barrel heat thing about maintaining a range of temperature.
Just to confuse you guys, there is a school of thought that the pressure waves or vibrations due to twitch of the barrel can create an oval shape in the bore and a good nodal tune does not have this shape.
Obviously, the longer and thinner the barrel, there are more sine waves to tap into and conversely for short thick barrels. Perc Pavey told me how they had to cut down barrels in England once, to accommodate the ammunition that was somehow changed. Where did the knowledge go?
In further testing around 2002 -3, I used a Paramount with duel lugs on a machine rest. One of the first barrels tested was a new 34 inches 1-12 Kreiger. It had an extreme spread of over 65 fps, but at a hundred yards as Peter Pearce will testify it put everything into an enlarged hole. Not expected. So I took the rig apart and found only the rear recoil lug was contacting. Effectively I had added length to the barrel and although the front screw was set at 45 inch pounds, it was a floater in terms of flex. So changing the position of that front takedown screw without a front recoil plate would work or you could use a Barnard type recoil setup by moving it backwards and forwards. It’s just another way to skin a cat without changing an old stock too much.
I have to re-enforce, that with reloads and modern gear, there need only be a subtle change for the longs in terms of compensation, and more than subtle if you are a match rifle shooter at extreme distance for general setup. However, if you accept that heavy cold air near the coast or light air at altitude or hot moist air will require a barrel with a different twist rate for stability, then you might accept that air density together with mound angle will change your nodal tune or your compensation tune. I am reminded of radical air density changes in Malaysia for a Commonwealth event. Perhaps James C could comment.
Compensation
Fig. 9 R.G. Reynolds and R. Fulton
To determine a general starting point for compensation examination you need to know whether your rifle is negative, neutral or positive as I mentioned in a previous post (see details there) and determined by loading and shooting three different rounds separated by 1 gn. of powder at 25 yards. Remember you do not want a negatively compensating rifle as range increases. To fix this you can alter your fulcrum point with a butt plate or use another stock with the centre of the bore a bit higher or you can shorten the barrel, change powders or free flight of the projectile. Whatever your poison use it to increase or tap into a rising muzzle on bullet exit. If your rounds at 25 yards or 140 yards are leaving a black smudge on the paper target, may I suggest they are going too slow for accuracy purposes generally speaking irrespective of compensation?
You can conduct a different compensation test at 500 yards with your regular ammo or at consecutive ranges to confirm the direction of the spreads using a chronograph and the plot sheet I made below.
Instructions:
Set up a chronograph 15 ft from your muzzle. (n.b. your velocity at 15 ft is not actually muzzle velocity, it is less but won’t matter for the purpose of the exercise. Fire three sighters and adjust your sights for a waterline group. Shoot a string of 10 shots without adjusting your sights. Record each shot with a consecutive number in terms of elevation on the vertical scale and velocity on the horizontal scale. For a rifle that is compensating negatively slow shots will be low, fast ones high. For a positively compensating rifle, the slow shots are above waterline and the fast shots are below waterline. For nodal or neutral groups shots mostly hover around the middle within your rifle’s grouping capabilities. But note, separation of some clusters may have a bias higher or lower. i.e. not much difference between positive and neutral but a bigger difference between neutral and negative and sometimes vice versa. It takes practice to read them. Note these plots will vary due to air density changes. These records need to be kept. Mirage can alter elevation plots. So light angle, hence diffraction influences this. Do not test in boiling mirage conditions or fishtails. In this case go back to 25 yards. For rifles that place the first warm up shot within the rest of the group in cold conditions is a sure sign the rifle is compensating as one would expect it to go low. In my past tests I also measured barrel temperature. You generally need three sighters for the barrel to settle within a reasonable temperature range. So your first business, the 4th shot means something, then shoot another 9. That would be information overload now. But some of you could combine that test.
Fig 10. Williada
On consecutive ranges you can string a number of completed plot sheets side by side to see the overall trend.
To roughly simulate using a computer you need the angle your muzzle is pointing to the target for each shot, the muzzle velocity of the ammunition and the distance to 1000 yards plus. Plot the trajectories and you can come up with a compensation diagram like Fulton’s. If your program only plots one trajectory path at a time, print the result on acetate sheets and overlay them. Or you could use an angle gage on your action and point it at the target to get a base angle for mound changes and play with some fake angle changes to see what happens. Combine these with changes in velocity. You could add more to the fake trial by changing angles based on physical plots. Now, Ian’s use of a fast camera and a screen behind the rifle with graduations marked on it would be useful here. Or you could rig up a stop gauge suspended above the muzzle to measure its maximum height to work out angles. Those with mathematical skills can work back from the target plot using trigonometry if up close (less than 140 yards).
Then you can play groups around the nodes or do batch tests with 3 groups of rounds with an average 30 fps between them or set up a rest which mimics changes in mound angles.
Now I ask a final question, could a V rest on a round forend like Mann’s keep the rotational movement on variable mounds constant if you are worried about torque or use a Pavey grip?
I think I have said too much already.
