doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Laser Cartridges

This stems from the mention of these devices in a previous post regarding the potential purchase of a Darne. The potential buyer mentions checking to see if the barrels are regulated through the use of the laser pointing cartridges. Somehow I always imagined a regulation problem would stem from something wrong at the muzzle end like chokes, etc. Does a regulation problem in a shotgun originate at the breach / chamber area? I have never used these laser pointer cartridges nor have I ever owned a shotgun that did not shoot to the same point of aim on a patterning board, so my question is an honest one. All answers appreciated.

Convergence starts at the rear.

If you could get some sort of concentricity between the chamber and the center of the muzzle, you could definitely jig up the shotgun, and see where the barrels converged in a static condition.

I have drilled a LOT of small, deep holes and re-barrels my share of rifles over the last 40 years. Bores are rarely straight. On rifle blanks, you have to indicate the bore at both end and skim the OD in order to get the bore close to center. Even after, the bore will have a banana. Some worse that others for a variety of reasons. With that in mind, I think the last few inches of the barrel would be where the issues would come about.

I once re-barreled a rifle with a bore that I knew had a banana. I indicated everything within tenths, so I knew the machine work was good. Sighted it in. I then removed the barrel, machined it to index 180 degrees and reshot it. It did exactly what I anticipated.

No direct experience with shotgun barrels but I do know Briley makes eccentric chokes to change POI and they work well. That tells me the last 3" are critical.

I could give other examples with trepanning drill collars over 30 feet, pull bore vs push bore blah blah blah, but I think ya'll get the idea.
YMMV!

Can you explain how chamber convergency matters? Those laser cartridges will tell you where your chambers are pointing, not the barrels or patterns. If you think that does not matter why do we use eccentric chokes to correct a point of impact problem. They do not change chamber convergence. The only way to check for barrel convergence is to use a pattern plate, shooting the gun repeatedly to get as complete a picture as possible.

the rifle laser cartridges that I used pointed to different points depending on the orientation when inserted into the chamber. In other words the laser wasn't leaving the cartridge from the exact center or was not parallel to the bore.

If you are looking for barrel convergence, then the longer the distance between sight and aperture, the smalller the measuring error.

Drill a hole through the center of a cork and stick it in the muzzle as close to center as possible.
Insert the laser in the breech.
turn the laser emitter in the chamber until the light comes out the hole.

Hold the barrels, level and steady in a jig, and at a distance of 16 yards mark the point of impact on a sheet of cardboard

Do the same thing for the other barrel and you will identify whether or not the barrels converge.

You will have to measure the muzzle centers and compare that to the distance between your laser impacts.

A little math and you have point of convergence.

Definitely close enough for shotgunning.

I think I have about three dollars into the two corks, and laser pointer that I use for training.
It’s for dry firing exercises, and Mount practice.

I have used a laser "cartridge" to pre-zero a gun. What I've found is that the chamber fit has some looseness and the red dot will move from insertion to insertion unless one indexes it in the chamber the same way each time--and this was at about 10 yards. In other words, It's no substitute for lead downrange. YMMV and OMMV Gil

None of the above will predict where patterns will land when the gun is actually shot. The reason is that recoil is not factored into the deal. Recoil is what causes side by side guns to need barrel convergence. And, different loads will cause the patterns to "over-converge" or "under converge". When the right load is found many guns will converge properly, printing right and left patterns dead on top of each other. Then, the gun is regulated, with that load. Regulation of side by side barrels is a complicated affair and subtle changes in loads can affect that regulation boldly.

Photographing lots of shotguns, lot as in thousands, my observation is that most double shotguns have muzzles that touch and the spacing ot the breech ends depends mostly on the type of construction. Monobloc barrels impose wider spacing at the breech than chopper lumps. With chopper lumps there can be filing in the inner surfaces before brazing. This brings the central axes of the barrels closer than in monobloc construction. Despite these differences in convergence both types seem to hit OK on the pattern board.

A person could very easily mark the edges of the laser cartridge, and then rotate it in the breech while recording the eight places it hit the cardboard.
Draw lines between each of the opposing eight spots and where the lines cross is the centerline of the bore.
Do both barrels.


Once you mark your spots, you can figure out how old the two barrels shoot relative to each other.

Lateral dispersion or convergence is one thing, but if there is a substantive difference in elevation, and the breach was level, you have a real regulation problem.