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Rifle Balance
- Thread starter penn63
- Start date
I don't have one piece rest experience, but I think rifle balance shouldn't be too different from two pieces rests...
Here's an answer I gave on another forum:
(Note: CG means centre of gravity)
"........
There is no correct CG positioning on a benchrest rifle. However, CG has a crucial part on the rifle balance.
To calculate your rifle CG location just used rule edges and, by trial and error located both CG points (X an Y). I always start with Y axle, the easiest, then the X axle.
Assume that the stock moving point is CG location. Well, in fact, it is a 3rd vector resulting from the CG vector and the applied force vector. But, for simplicity let's assume the stock rifle moves (turns) around the CG point. CG location is a combination of vertical and horizontal weight. Don't be fouled by static and dynamic CG. CG doesn't change it's location, but its vector force does.
If you take a car as an example, the more centred CG should give the best handling. Well, almost...
So, the rifle can turn in two panes (X and Y) being X from left to right or right to left (canting), and the Y going barrel up, butt plate down and vice-versa.
The least amount of stock movement for any applied force is desirable. Let's also assume as applied forces: firing and recoil; trigger pulling; induced torque.
Al of these forces can be minimised by correct technique, but recoil.
X axle: the lower the CG the better, because any canting force will have the minimum allowed movement by a lower CG.
Can be achieved by a low weight scope and rings, low profile stock, light trigger guard, weights below barrel, weight on the lowest rear stock portion. All the opposite will rise the CG, promoting an undesirable imbalance.
I tend to have it lining with bore.
Y plane: the more centred, between crown and but plate the better. Yes, for a static CG vector, but for a dynamic one no, because it allows easily a crown rise during recoil. This CG should be somewhere 10-20cm in front of centred distance. This is needed, to put more weight on the barrel portion to allow a more steady recoil. Too much in front, and the stock will rise on recoil, too rear and the crown will rise on shot. Both will lead to a vertical line on target. So, vertical lines, can, and often are, related to not a correct CG rather than a non tuned rifle. I know this is controversial, but competition results seldom are related to common sense...
Y axle CG can be modified by adding weight, either in front or on the rear. Please put it the lowest possible, to not imbalance X axle.
Than, other issues arrive...
Weight, sometimes it can't be added for a given discipline, or, it will imbalance in another way the rifle. Weight is a dead add-on, sure, but has big inertia. Hard to start moving and hard to stop. Think with me... you have added 400g of lead to rear of your stock, to bring the CG to the desirable position. Good! No so. That (too much) weight will not start moving on recoil, contrary to the stock, applying a static vector force, becoming a moving point. So, as the weight increase enormously the rear stock inertia, it will be like putting a stop on the butt plate, rising the crown on recoil. Vertical lines...
It's very important to choose correctly the rifle components, including shape and stock weight to build the most uniform rifle possible, to not need to add any weight.
I know, could be tricky...
......."
Here's an answer I gave on another forum:
(Note: CG means centre of gravity)
"........
There is no correct CG positioning on a benchrest rifle. However, CG has a crucial part on the rifle balance.
To calculate your rifle CG location just used rule edges and, by trial and error located both CG points (X an Y). I always start with Y axle, the easiest, then the X axle.
Assume that the stock moving point is CG location. Well, in fact, it is a 3rd vector resulting from the CG vector and the applied force vector. But, for simplicity let's assume the stock rifle moves (turns) around the CG point. CG location is a combination of vertical and horizontal weight. Don't be fouled by static and dynamic CG. CG doesn't change it's location, but its vector force does.
If you take a car as an example, the more centred CG should give the best handling. Well, almost...
So, the rifle can turn in two panes (X and Y) being X from left to right or right to left (canting), and the Y going barrel up, butt plate down and vice-versa.
The least amount of stock movement for any applied force is desirable. Let's also assume as applied forces: firing and recoil; trigger pulling; induced torque.
Al of these forces can be minimised by correct technique, but recoil.
X axle: the lower the CG the better, because any canting force will have the minimum allowed movement by a lower CG.
Can be achieved by a low weight scope and rings, low profile stock, light trigger guard, weights below barrel, weight on the lowest rear stock portion. All the opposite will rise the CG, promoting an undesirable imbalance.
I tend to have it lining with bore.
Y plane: the more centred, between crown and but plate the better. Yes, for a static CG vector, but for a dynamic one no, because it allows easily a crown rise during recoil. This CG should be somewhere 10-20cm in front of centred distance. This is needed, to put more weight on the barrel portion to allow a more steady recoil. Too much in front, and the stock will rise on recoil, too rear and the crown will rise on shot. Both will lead to a vertical line on target. So, vertical lines, can, and often are, related to not a correct CG rather than a non tuned rifle. I know this is controversial, but competition results seldom are related to common sense...
Y axle CG can be modified by adding weight, either in front or on the rear. Please put it the lowest possible, to not imbalance X axle.
Than, other issues arrive...
Weight, sometimes it can't be added for a given discipline, or, it will imbalance in another way the rifle. Weight is a dead add-on, sure, but has big inertia. Hard to start moving and hard to stop. Think with me... you have added 400g of lead to rear of your stock, to bring the CG to the desirable position. Good! No so. That (too much) weight will not start moving on recoil, contrary to the stock, applying a static vector force, becoming a moving point. So, as the weight increase enormously the rear stock inertia, it will be like putting a stop on the butt plate, rising the crown on recoil. Vertical lines...
It's very important to choose correctly the rifle components, including shape and stock weight to build the most uniform rifle possible, to not need to add any weight.
I know, could be tricky...
......."
Thanks Pedro for the detailed explanation. I have a light weight scope (March 48x52 HM) and a Rotex low profile stock. My balance point, butt plate to tuner, is well forward of the action face. I have been wondering if I should add some weight to the lower part of the stock to bring the balance point closer to the action face. It may lower the CG some also. I'm using a bungee cord to retard initial recoil which seems to help accuracy somewhat. Thanks again. Les
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A bungee cord puts another force vector on the rifle, mimicking a lower CG. I assume the bungee is from the trigger guard, pointing down, because if not pointing well down, it will worsen things.
If your rifle balancing point is anywhere around 10~20cm in front of middle full length distance, is ok. If it's too far font bring it aft by adding weight to the lowest stock part possible. I tried several weights until 250g and, for an heavy rifle, not disturbing much the recoil vector direction. But if, after adding weight, a vertical line appears on target, you have too much weight, and inertia is playing an important role.
If your rifle balancing point is anywhere around 10~20cm in front of middle full length distance, is ok. If it's too far font bring it aft by adding weight to the lowest stock part possible. I tried several weights until 250g and, for an heavy rifle, not disturbing much the recoil vector direction. But if, after adding weight, a vertical line appears on target, you have too much weight, and inertia is playing an important role.
Thanks again Pedro. I have measured my balance point to be in front of the action face by 3 inches. I will measure it as you have indicated and if I need to add weight I have a tungsten rod that measures .379 x 2 inches. That may be enough weight to get me in the ball park. Thanks so much for your time. Les P. S. Yes the bungee cord is from the trigger guard to the front post on the rest angled down.
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