Centre of Gravity

[macguru]

Has anyone done any experimentation with regards to the ideal position of the front rest of an F class rifle with respect to the rifle's lengthwise Centre of Gravity ?? Particularly with target rifles that have some recoil.

I have 2 rifles, a savage with a kilo of lead in the hollow of the butt and a HV barrel
and a barnard with a heavier straight barrel and no lead in the butt

The latter is giving me some elevations, noticeably more than the former. Of course it could be lots of things but ..... I am thinking you would want the COG close to where the forend plate is resting in the front bag ??

[scott/r]

There is a write up about this on Accurate shooter which takes you through your questions. When I first started shooting full bore I shot in f standard with a light barrel omark, and this made a huge difference to vertical for me.

http://www.accurateshooter.com/shooting ... -on-rests/

[williada]

Andrew, (edit sorry mate I got my A's mixed) the most important rest is the butt rest not the front rest. Mann wrote about this early last century having done work on it in the late 1800’s and was mates with Harry Pope who followed his processes in the early 1900’s and to my knowledge Pope’s long 200 yard string has not been beaten.

It is essential the rear rest is very firm if you are going free recoil. The front rest may have just a little give so it is free to track unimpeded along its recoil line and not the line of sight because that is what happens even if it appears to track well. Unless you have a suitable muzzle weight, you may upset balance because the muzzle weight’s primary function is to control torque which it can do so most effectively at the muzzle due to leverage. I calculate the torque and use the appropriate muzzle weight. I don’t want to add to extra flog of the barrel if it is not stiff. The only other place to counter torque and the amount of lift is at the opposite end to gain maximum resistance to lift i.e. the butt and that is why it has to firm.

The lag time between barrel lift and rear movement varies with the stock centre of gravity from the rifle bore-line as well as a fulcrum point whether determined by the total mass of the rifle under free recoil or with addition a shoulder contact with the butt.

Because the rifle moves back, plenty of talcum powder needs to be applied to the rear bag to reduce friction so there is no impedance to stock movement which can snag a shot. Make sure you pound the butt into place on setup for a firm position. If it varies, elevation creeps in. That also happens when people use a firm grip with rifles of considerable recoil because mistakes in bag handling inadvertently take weight off the butt during a string. In that case the bag position forward and rear must take into account where you position your hand for reliable let off. If using free recoil and you shoot on a an uphill range like Moe where I shoot a better technique is to pin the rifle with your shoulder but keep your trigger hand well away from the stock.

We turn now to the harmonics, which travel as a shockwave up and down the barrel even before the bullet exits the barrel. We have three moments of inertia we want to consider and two are close together and the third is a long way apart from the other two. That means the balance point between them all has to be optimised. The first is where the powder ignites in the chamber, the second is the point where the bullet moves and the third is at the muzzle when the projectile emerges. The shockwave frequencies of the first two combine to form a donut which has concussive force which also changes bore dimensions. The frequencies of these forces speed up at the muzzle on bullet exit as gas escapes and in my opinion add to the chatter we see in group plots. The bullet exit timing to reduce vertical should occur when these frequencies are less excited in an OCW tune or in a nodal tune when they are in harmony.

What is critical to the rifle balance point as a primary factor, is to use the main inertia points and sources of vibration to advantage and two out of three are towards the butt. Then the next adjustment can be made for the third inertia point at the muzzle which ends up drawing an optimisation of the other two points forward a little as a compromise. The the resulting optimisation point is about a finger width or two in front of your action. That is the point you should balance the rifle weight fore and aft. Ever wondered why a barrel block performs best in front of the action by ½ an inch or more? It’s is not for balance but less disturbance of the harmonic formation.

Then fine tune this by moving the front rest position forward or back in small amounts by trial and error.

The general starting point for the trial and error testing should put about 60% maximum 70% of the rifle weight on the for-end not more, is sufficient to help it track well, as more is usually the case. So you want more weight on the butt than most setups have to counter a heavy for-end. This is where you need firm control so the bag does not compress with a little more resistance than you would otherwise have with weight. You do that by setting the front rest position near the for-end front. Otherwise if things are perfectly balanced they are easier to disturb on aiming and firing. It assists an unsteady position which would otherwise transmit into the stock and disturb things. That is why I say, “optimised” which is a trade off. (I edited that bit because it was not clear.)

You may find you need a greater percentage weight added to the butt. Too many people make a mistake overweighting the muzzle by weight or tuner. This leads to vertical assuming your rest is positioned to avoid bounce on their base which is problem with bipods. Bounce and un-weighting are different things. By positioning the front rest closer to the end of the forend you add to butt pressure.

Once set, it is a good idea to use a spring balance to measure the weight on the butt rest and the front rest once you have found a good tune area because mounds differ in height and alter the weight distribution. e.g 4.9k to: 2.7k or 4.5k to 3k. David.

[macguru]

David, you have put my ideas into an eloquent message, as usual. I just felt that my vertical was not a barrel /tuning issue but a weight distribution one. I moved the rifle back in the rest and my group closed up, but its the 6mm barrel at the moment. I think its much more important with the 7mm and the greater recoil.....

I am looking at ways of increasing the butt pressure, such as a lead spacer in the adjustable butt plate...

andrew

[williada]

Andrew, the lead weight is a good idea. Its still unfortunately a bit of suck and see depending on calibres as you say. Good luck.

[Nathan P]

On my new stock I made for my shehane I installed two 20mm brass rods into the butt both about 130mm long and all up weigh about 400-500 grams .. I would add more to balance it out more but the rifle already weighs 9.8kg

[williada]

Consider that butt weight placement may alter the distance between the stock centre of gravity and the bore-line in the vertical plane. If the weight is lower it will cause more barrel lift. If it is higher, more in line with the bore-line, you will get more rearward thrust and less barrel lift. Another trade-off. As Nathan is suggesting total rifle weight is an issue so stock design is very important. If there was room to move with the butt weight, it could slide up and down and be adjusted. This would give you another minor tuning avenue.

[johnk]

I've made butt weights making a mold by cutting the butt profile out of an appropriate thickness of scrap timber. fitting slab sides and casting a lead (actually Linotype, but you youngsters won't know what that was) spacer. Keeps the vertical balance relatively constant.

[macguru]

I will have a go along those lines and post some pictures in due course

[DenisA]

I've made butt weights making a mold by cutting the butt profile out of an appropriate thickness of scrap timber. fitting slab sides and casting a lead (actually Linotype, but you youngsters won't know what that was) spacer. Keeps the vertical balance relatively constant.

Sounds like a fetish you should seek some professional advise over.

[johnk]

Sounds like a fetish you should seek some professional advise over.

Says the guy who wears white velcro latched sneakers

[DenisA]

Sounds like a fetish you should seek some professional advise over.

Says the guy who wears white velcro latched sneakers

I'm just a grease monkey. I have trouble tying knots

[johnk]

Sounds like a fetish you should seek some professional advise over.

Says the guy who wears white velcro latched sneakers

I'm just a grease monkey. I have trouble tying knots

you win!

[RAVEN]

(actually Linotype, but you youngsters won't know what that was)

Well I do John
because I used to make my pocket money as a youngster melting it down for my farther into ingots to reuse through the hot metal linotype machine.
which my family donated to the Adelaide State Library and is still on display.
RB

[macguru]

OK I have made up a weight. I bought some air dry clay, 1 kg and put it in a shallow tin. The I removed the adjustable butt plate and pressed the butt, via a layer of glad wrap, into the clay a bit under an inch or so. Then I dried the clay a bit and poured a bit over a pound of molten lead into it, drilled some holes, and put it between the butt plate and the stock. looks ok with the lead taped over with velcro tape for a snug fit. , and to stop rubbing bare lead on my cheek when i fire the rifle.

The Centre of gravity is now about 2in in front of the action, and its a 9.5kg rifle instead of 9kg. We will see how it goes.....

If I had it to do over again i would try plaster of paris because the clay cracks when it dries which takes ages, and its essential the mold is dry or steam is released which is dangerous and messy when you pour.