Re: Stacking tolerances and MOA - what happens?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ORD</div><div class="ubbcode-body">While I don't think you'll get any argument from anyone that every single aspect of a rifle from the receiver to the barrel to the trigger to the stock to...well...you get the idea...has an impact on the accuracy "potential" of the rifle. However, I am not aware of anyone with the time or inclination who has actually sat down and mathematically/scientifically attempted to categorize and quantify the precise figures you are looking for.</div></div>
RIFLE ACCURACY FACTS by Harold R. Vaughn
Publisher: Precision Shooting, Inc (October 1, 1998)
ISBN-10: 1931220085
ISBN-13: 978-1931220088
(1) Receiver Ring Movement(recoil lug) Recoil Effects page 46
"When the rifle is fired there is a net recoil force action on the rifle action that is equal to the force action on the base of the bullet, which is about 3,000 pounds at the peak chamber pressure of 53,000 psi. The force action on the bullet was shown in figure 2-24. This force is transmitted to the stock by the recoil lug on the bottom of the rifle action. Since there must be an equal and opposite reaction to any force, the stock exerts and equal force on the recoil lug in the opposite, or forward direction. This force results in a recoil moment being exerted on the forward receiver ring tending to drive the muzzle in an upward direction. "
(2) Scope Mount Motion page 93
The mounts on is particular rifle are Weaver Top Mounts with aluminum bases attached by two 6-48 screws. Now, there is just no way that two small screws can keep these bases rigidly fixed to the receiver under the loading conditions present on a rifle, no matter how tight you get them. This becomes very evident when one realizes that the axial load on the scope is roughly 500 to 700 pounds on a 270 sporter during firing. Well in fact the bases don't stay put, and you can prove this with a very simple test. All you have to do is tap on the front base with a small
hammer applied to a wood dowel so as to move it to the right, and repeat the operation on the rear base in a direction to move it to the left. Then you fire a shot, and repeat the operation three times for a three shot group. The whole operation is repeated again, only this time the bases are tapped in the opposite direction. If there is no effect, all six shots should be grouped together. When I ran this test, two distinct three shot groups resulted separated by .503 inches in the horizontal direction.
(3) Scope Mount Motion page 96
The action was held in a vice and a dial gage measured the deflection of the muzzle as the hydraulic fluid was heated causing a axial force between the front and rear scope mount. From the data in table 6 you can calculate the angle with respect to the vertical to be about 23deg, which is a little less than the angle seen on the targets. The reason for the Groups being canted at than angle is that the receiver is weaker on the right side than the left as a result of the loading port. Consequently, the receiver bends in a plane canted to the right with respect to vertical. One can also see from Table 6 that a differential force of roughly 75-100 pounds is all that is required to cause the amount of linear dispersion seen in Figure 5-2. I decided that the easiest way to solve this problem was to make a steel bridge mount and silver solder it at both ends (figure 5-4) The bridge mount worked and eliminated the problem. I repeated the test in Table 6 with the bridge mount and the muzzle deflections were reduced by roughly a factor of ten, which means that the vertical dispersion caused by the scope mount differential axial load should be less than .1 in at 100 yards. The bad part about this solution is that it interferes with the loading port and is a bit of a nuisance. It also adds about 1.5 ounces of weight, but I don't know of any other solution. However, the bridge mount does add considerable stiffness to the fairly flexible receiver, and should improve accuracy in other ways besides the scope problem.
(4) Chapter 6 Barrel-Receiver Threaded Joint Motion page 112
What all this comes down to is that the joint with an axial preload of 20,000 pounds has marginal stability under ordinary conditions, and is unstable when hot after firing two or three 5-shot groups without cooling. This roughly corresponds with our experience shown in Table 5, so I feel certain that this is a real effect. What is needed is an axial preload in excess of 24,000 pounds to assure that the joint cannot move under these extreme conditions of heating, shock, and vibration.
