doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com M 21 question re bbl alignment

At what range do the 12ga bbls converge?
Are CSMC 21s the same?
danc

perhaps no one knows??.after 150 looks,& no answers.... why is is important?, just curious,danc, no harm meant mate
cheers
franc

40 yards is where Winchester patterned all their guns, so I think it likely that is where they set up the regulation of the barrels. As far as CSMC goes, Lord only knows. Do they even pattern the guns before putting them out there?

I called CSMC. The answer I received was that he thought that it was 28 or 30 yards.

An online search found statements that Benelli was 25 meters and Beretta was 30 meters.

Krieghoff uses a #4 hanger for trap guns and a #5 for sporting.
They must have a reason for the difference.

I have used my CSMC 21 on tower pheasants at an estimated 30 to 45 yards with awesome results. [#5- 1 oz at 1150 RST-LM&M]

I will be using my CSMC 21 for sporting with the belief that a sxs is the better choice for sporting and that is why I wanna know.

Dan Crowley Florida AA SSV 7 punches

I have a question of opinion on barrel convergence. Consider that the barrels are about one inch apart on center at the muzzle. Consider that the only curve in pattern flight path is ballistic drop. IMO, barrels should shoot to the "same" (within one inch) point at all distances. If not, there must be a built in divergence or cross-fire. That is to say, you cvan't have a multiple inch divergence at 20 yards followed by a convergence at 40 yards.

What think ye?

DDA

There was a long discussion on this bbl convergence a while back which ran into many pages. I believe that it was finally & generally conceded by all that the bores on a SxS double do converge from breech to muzzle in as straight a line as the makers could accomplish. This convergence was given in the Plans & Specs for the L C Smith FW as about .011" per inch of barrel length. An extension of the b static bore axis of the two barrels would thus converge about 6' from the muzzle & at 40 yds would print about 15" apart, the axis of the right bbl being to the left & that of the left bbl to the right.
That one would not expect the two bbls of a well made SxS to print its patterns 15" apart at 40 yards is I believe conclusive evidence the barrels do not remain static during the shot's travel down the bore. The spacing of the two bores of a 12ga would be expected to be around .800" depending upon actual bore size & actual wall thickness. Bores of .730" with wall thicknesses of .035" assuming they touch at the muzzle would result in this .800" spacing. I can't recall that I have ever seen a 12ga ouble with the bbl centers being anywhere close to 1.00" at the muzzle.

Agree with your 0.800", 2-p. I used one inch only as a scale place holder. Certainly the barrels are not static during discharge and shot passage. They will, in fact, have reached maximum movement velocity at shot exit but not maximum movement.

The "fly in the ointment" of barrel convergence regulation and barrel flip regulation is the individual shooter. Factory made regulation is approximate unless fired by the shooter.

Using 2-p's numbers above, we see that barrel movement is expected to be about 5/16" for 30" barrels: 15"/ (40 yd X 3 ft/yd X 12 In/ft) 1440" = bbl move/30"; bbl move = 15" X 30" / 1440" = 0.3125" + 5/16". I suspect "average" is generally close enough.

So, unless you have shot patterns for the guy regulating the barrels, you are sorta getting "luck of the draw."

DDA

If we're going this far to account for factors affecting lateral barrel movement, matching the weight/velocity of the load used for regulating is another.

Glad I bring enough random factors to my shooting to confuse all this beyond investigation.

Jay

Strange how the vertical alignment which is potentially more troublesome is hardly ever mentioned. The one and only time I had alignment problems was with vertical alignment, ie the left barrel shooting noticeably low and left when testing for point of impact from a steady rest. There is no measurable indication of any construction misalignment on this gun.

The other oddity is testing with rifled slugs. Guns that partially cross printed their shot patterns put rifled slugs from both barrels into a 10cm circle at 40 meters.

The above and many other tests indicate that the only way to know is to test.

Thnat is for certain an absolute truth.
Compared to normal shot loads slugs most often have a higher velocity (Shorter bbl time) plus are lighter than the standard shot load. It is not at all uncommon for them to cross fire when used in a SxS.