doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Lengthening forcing cones....good or bad?

Hello Larry,

I did a comparison also and there were marked variations between different shells. I have a photo somewhere that I'll post if found.

JC

If one is planning on regularly shooting an American vintage SxS, and it has/had "short chambers," I see no harm in opening it to a standard 2-3/4" chamber AND adding longer forcing cones at the same time (assuming there is enough barrel thickness). I have had my gunsmith do so to several of my SxS shotguns.

Pressure is lowered at least marginally, and the useful life of the shotgun is probably extended. It does no harm...

gold40

I copied this from the Ceaser Guerini site someone linked to:

"The DuoCon forcing cones are a unique dual conical long forcing cone system designed by Caesar Guerini to produce optimum performance and maximum versatility. The first conical section maximizes chamber pressure and permits the use of fiber wad ammunition. The secondary 4.5" long forcing cone section reduces recoil and increases pattern uniformity."

It is interesting that we again get to GT Garwood's contention that gas bypasses the fiber wads in a long cone. I added the underline and italics

If anyone is planning to lengthen forcing cones, I propose we run a pattern test. Dr. Jones is willing to run analysis provided ten samples of each gun & load are taken. Less than ten doesn't meet the statisticalneeds of good research. Anyone want to do this?

Jerry, I think that advice is good maybe 95% of the time. Maybe even more. But I don't think it would be a good idea, for example, to take an Ithaca Flues 20ga, especially if it's one of the lighter ones, and punch the chambers to 2 3/4" and lengthen the cones. At the least, I'd certainly want to take some wall thickness measurements where I intended to have metal removed if I were to contemplate such a modification. You're certainly not making the gun any stronger by removing metal. And while MOST vintage American guns are pretty stout, some aren't. We've already heard of Flues frame failures, usually light 20's. The problem, at least in a couple cases I recall, appears to have been "light" promo ammo (which is not really light in terms of pressure, nor velocity), 2 3/4", in short chambers. If enough metal were removed to extend the chambers to 2 3/4" and lengthen the cones, I think one might have a legitimate concern whether even ammo at pressures appropriate to the pre-modified gun might not cause problems.

With regard to reducing the wall thickness, lengthening the chamber will push the corner of the chamber/forcing cone further forward. Since the outside of most sxs guns has a taper from the breach end, this definitely reduces wall thickness. However, I haven't run across a gun with a tapered outside that was steeper than the longest forcing cone reamer I have. Therefore, the minimum wall thickness will still be the corner of the chamber/forcing cone, regardless if the cones are lengthened with my reamers. I believe someone stated that modifying cones on a British proof gun doesn't take it out of proof? If so, this would be the reason, IMO.

It might be useful to remember that all "fiber" wads are not "created equal" when it comes to obturation. Cork wads, for example, do not obturate much, at all. Fiber wads can obturate a bit better, especially if they are properly lubed and/or "ventilated" (see Zutz). By and large the best obturators are felt wads, with obtuation levels varying according to the quality of the felt and the wad lubricant that is used (See Greener, Churchill, et.al.).

Chuck, it's correct that lengthening the cone doesn't take a British gun out of proof. But lengthening the chamber does. That's where the peak pressure always occurs.

Going back to my post about Flues frame failures, lengthening chambers would only increase the likelihood of those if, as a result of the longer chambers, someone were to use higher pressure shells. But most gunsmiths that specialize in doubleguns, I think, would like to see a minimum barrel wall thickness of .100 at the mouth of the chamber. I don't know whether another 1/4" added to the length of a 20ga Flues chamber would put you below that mark, but I think it would be something to consider.

Thanks Larry. I recall in some past posts in threads where there was concern of reducing minimum wallthickness with a long cone. I was pointing out that it just doesn't occur with a lengthening of a forcing cone on most doubles, since that outside taper is so much longer than even my reamer for a 4" cone in a 12g.

Who'd have thunk this little subject would get a bunch of old men seeing red enough to insult each other? I'm sorry if I did that, myself, to anyone on this subject.

This is great stuff to know! I was not aware of the proof distinctions between chamber lenngthening and forcing cone lenghtening.