doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com So, why wouldn't you?

I must admit that in the numerous times I've cleaned the barrels on my doubles, I've not paid a lot of attention to that dark ring about three inches in from the breech. Inspecting the new set of barrels from France, I used the summer sunlight to shed light on and in the tubes. That dark ring was so pronounced it looked like a shoulder instead of a slope.



A soft wooden dowel confirmed that it was indeed a slope, the half-inch or so of unpolished forcing cone.
So that got me thinking (usually bodes ill): If one uses a proper length shell that does not open into the forcing cone, and said shell's shot cup does not encase the full load, wouldn't there be the potential of some serious barrel scrub? So why wouldn't one polish the forcing cones? Just for grins, I looked at the second set of barrels for the Arrieta, made fifty years later than the Manufrance, and it had the same appearance. I cannot thus conclude that all doubles have unpolished forcing cones, but I can infer that it was/is a common practice.

Mike

Optical illusion.

Forcing cones have a major influence on patterns. We have improved the number of bb's in a 30 inch circle by 21 percent by polishing the forcing cones on Briley and Kolar skeet tubes.
Lightly drag a sharp point on a wire across the forcing cone. If you feel mild ridges it needs to be polished. It should be mirror if you want the best pattern.
bill

Does not appear to be so- I shined a very bright light in at the breech, and holding the barrels at the sharpest angle possible to still see the forcing cones, it is a cross hatch that is not polished to the degree of the chamber or bore.


Mike

Brownells sells a forcing cone polishing brush and it works well

Most likely what you are seeing is a shadow caused by light not being reflected at the same angle as the parallel angles of the bore and chamber.

Bill, before I did the copper wire test I put a magnifying glass on it in conjunction with a strong light source. The machining rings/lines are clearly visible, the chamber brought to a higher level of polish, but rings/lines still visible. The highest level of polish, of course, are the barrels- with no rings/lines visible to the unaided eye.

I cut the copper wire and put a 90 degree bend in the last half-inch or so. My lineman pliers leave a "chisel" cut on the wire end, so I made sure to orient the chisel to line up parallel with the grooves/rings left by the machining. The different level of machining/polishing was detectable.

A friend called today after seeing the topic, and said he had the forcing cones lengthened on one of his doubles, and they were polished to boot. He said no transition, dark donut, or anything is visible- from the chamber to the muzzle, it looks like one continuous shiny surface.

So now my question is, did your London gun come with polished forcing cones?

Mike

Polishing forcing cones in skeet tubes
We mount the tube in the head stock of the lathe and while it spins we use a small mandrel on a flex shaft Fordom grinder.
We polish 600, 1000, 2000 grit and then chrome rouge on a felt bob. Never dwell in one spot and make sure you are polishing the ramp. Keep before and after patterns so you can see what a major reduction in flyers does for patterns.
bill

I have polished my long forcing cones after reaming them, and still can spot it by the slope--it cannot completely disappear. Steve

Good reason to shoot 70mm hulls in short chambers!