Chuck -- Mike --
The barrel convergence angle is difficult to accurately determine. As Mike suggests in his appended website thread from 16ga.com, the needed measurements could depend on the design/manufacturing methods used to achieve the needed convergence of the barrels at the muzzle. The barrels could be angled to achieve the convergence with the chokes mounted in line with the barrrel bores. Alternatively, the chokes,alone could be angled to provide for the convergence, while the barrel bores are essentially parallel. Of course some combination of both procedures / designs could be used to achieve the desired final result.
But for the purpose of the measurements -- no matter which procedure is used to achieve the desired convergence for the angle at which the shot charges exit the muzzle -- the angle between the choke bores at the muzzle is the necessary parameter to measure. But, without some special measurement equipment / fixturing / tooling -- this is a difficult measurement to make. I know that I cannot do it well with what I have at home, here. It is essentially impossible to do this in a store, when considering the purchase of a new gun.
The suggestion that the results of careful patterning of the gun for the points of impact (POI) for the respective barrels is best --- is probably the most pragmatic suggestion / approach. But, the intrinsic difficulty here is that one probably owns the gun by the time the patterning is undertaken to assess the guns point of impact (POI) performance. Without a specification for minimum patterning performance, the manufacturers can and do say that the "gun is within spec." -- as can clearly be gleaned from the above postings in this thread. Catch-22, all over again.
If the necessity for barrel convergence does, indeed, derive from the recoil motion of the gun before the shot charge leaves the muzzle -- then there are several parameters that - in principle - affect the necessary convergence to supperimpose the patterns from both barrels. These parameters include:
1. gun mass / moment of inertia for rotation about the stock heel.
2. shot charge mass / weight
3. shot charge velocity at the muzzle.
4. barrel to barrel spacing
5. stock dimensions.
In principle even the physical attributes of the shooter could affect the patterning outcome. This suggests that the patterning should be undertaken off-hand and with the gun mounted in the "normal" way.
The B. Rizzini POI patterening problem I encountered may have been the result of a poor B. Rizzini design, re: convergence ( most important); 1 oz. field loads used in the patterning; and the gun is light (part of the design considerations).
To prove the point, concerning the importance of / necessity for barrel convergence, I may try patterning the B. Rizzini gun with 3 inch - 1-1/4 oz loads -- when I have time (after the wedding). These loads should significantly amplify the problem, if this barrel convergence, design, deficiency is the origin of -- or substantially contributing to -- the problem.
Regards,
Don
