[zippy13]

Originally Posted by darkgael

Zippy: "Double guns (SxS's) have a lateral eccentricity that's a component of the recoil dynamics."
Ok. You've piqued my interest but you are gonna have to say that in simpler terms. would you rephrase that for a simple soul?
Thanks.
Pete

Okay, Pete, I'll give it a go...

Recoil keeps reminding us, when a firearm discharges the laws of physics come into play. Action and reaction -- applied forces and resistive forces. The applied force is the easy part (it's covered in a topic we know as internal ballistics) since it acts in-line with the barrel. However, the resistive forces are not so simple. With a typical stance (no barrels between the toes) the hands, cheek and shoulder each provide components of the resistive force. Shooters of different sizes and weights will distribute the individual resistive components differently -- so will different stock dimensions. To keep the ghost of Sir Issac Newton happy, the combination of the resistive forces will equal the applied force. If the combination of all the restive force components is also in-line with the bore, then the motion under recoil will be straight back and forth (perfect reciprocation). Yet, we're all aware that this isn't the case, the motion is back and upward... why?

Look at a typical rifle, or shotgun, and its shape tells you that the center of the reactive forces is below the barrel's axis. This dimensional difference is known as vertical eccentricity. When opposing forces are eccentric, a rotational force, known as a couple is created. It's this couple that causes the barrel lift we're all familiar with.

The greater this vertical eccentricity, the more the muzzle will rise. This is why it's important when shooting an O/U that the lower barrel be shot first. It has lesser eccentricity than the upper barrel. So, it lifts less and you get on your second target quicker than if you shot the upper barrel first.

A benefit of SxS geometry is that both barrels have the same vertical eccentricity. So lift is the same for either barrel -- an advantage over the O/U configuration. However, the SxS brings in another factor: lateral, or horizontal eccentricity. For the sake of discussion, let's assume the center of the resistive forces is in line with the SxS's center rib. So, now we've added a lateral component to the couple. Since the lateral eccentricity is less than the vertical, you might not notice the left or right components of barrel lift. And, since you're usually swinging a shotgun, the lateral part becomes almost inconsequential.

In reality, SxS shooter's resistive forces may not be centered on the rib, and will change each time the gun is mounted. If the resistive forces are centered behind the right barrel, then only the left will have a lateral component to the lifting couple. In this condition, the right barrel will lift without a horizontal component, and the left's horizontal component will double. Don't worry, in the greater scheme of things, these SxS lateral forces are minor, won't change your shooting style, but should be understood.

Remember what vitesse9 said about his off-hand testing: "...the right bbl is dead on on elevation and about 2 inches right on windage. The left bbl is about 1 inch high in elevation and about two inches left in windage." How can we determine what might be due to an inconsistent mount? That's why I said: When evaluating your off-hand testing, don't forget: Double guns (SxS's) have a lateral eccentricity that's a component of the recoil dynamics. To test how the gun shoots, you really need a more uniform resistive force obtainable only with a rest.