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

I do not contend that longer forcing cones do not improve barrel performance. What I said was is that there is no empirical evidence to substantiate that claim. If someone is happy with the reamer results - cool. Forcing cones are difficult to eyeball and a crappy one could be difficult to detect but one improved by recutting could be a boon. To my eye, the Perazzi cones look very rough but the results on the board and targets completely negates any desire on my part to change that.
So I currently have nothing that feels the need of the time and expense involved. Maybe if I could qualify for it I might be tempted hahahaha or not

Dr.WtS

I would tend to agree . . . except for the fact that I don't believe he ever recorded an INCREASE in pressure as a result of a lengthened cone. There was always some decrease. I do agree that in order to determine how much of a decrease one is likely to get, you'd need to fire several shells (all the same loads) through the unaltered cone, then do the same through the altered cone and take an average.

Since SAAMI is only a industry org, it's obvious they have no legal authority. On the cone length, the only information I have is a hard dimension, in this case an angle with a reference length for the finished cone.

Hello 8B,

Here are some quotes from the article related to different tests involving a
comparison of same shells before and after the lengthening:

"Modifying the 2,5 inch chamber with a long forcing cone dropped the breech pressure slightly to 11,045 psi (from 11.125 psi), The pressure at the cone dropped more significantly to 7500 psi (from 8,100 psi), or almost down to 2,75 inch pressure value."

"When the long-coned figures are in, we see a drop of only 220 psi, to 8545 psi at one inch but again, a nice fall-off of 800 psi to 5500 psi at the forcing cone."

"The much-anticipated data with the long forcing cone was a pleasant surprise. The breech pressure returned to 6385 psi, virtually identical to the longer chamber figure. At the front of the chamber the number is 4700 psi, which is even lower than in the long chamber".

But, having re-read the article I must now keep in my memory bank (don't know for how long it will stay) Mr. Bell's last remark:

"But I might have the forcing cones lengthened, assuming the barrel wall thickness will allow it. I wouldn't bother for the slight pressure benefit but as a fussy handloader, I hate it when my hulls have that nose-cone shape. I have no proof, but I think patterns will be better when the shot charge has a smother, unrestricted ride into the bores".

I other words I am leaving my Manufrance Id�al as is and shall continue to shoot 67 mm CIP shells from it galore.

And thank you for making me re-read the article and mend my ways.

JC(Alway Learning)

For those who may find the partial Bell quote confusing, let me say that the "nose cone shape" of the lip of a shell is not from a short forcing cone, but from firing the shell in a chamber actually shorter than the case. Short forcing cones don't do anything to a shell.

8B, you are spot on on that (as usual). Thanks for clarifying.

JC

P.S. For those interested the complete article is in "The Double Gun Journal",
Volume Twelve, Issue 4, Winter 2001. jc

At least with plastic shells, you seem to get some "nose-coning" after you've used the same hull several times. I can notice a definite difference between once or twice fired hulls and 8 or 10 times fired hulls. And that's in 2 3/4" chambers.

I doubt I'd have cones done for the relatively little pressure reduction either, but it was an interesting result of Bell's tests.

I am firing a shell in a chamber actually shorter than the case and I get no "nose-coning". Clearly 67mm shells in 65mm chambers are nothing to worry about.
And no SAAMI shells for me thank you!

JC

I'm using a full 2 3/4" paper in a 2 9/16" chamber and I get a little shredding of the hull in the I.D. of the crimp. I load them twice and toss them. There's a definite indication of the chamber being short on the hull outside as well. I'm running a measured 6500psi load.

JC, at one time I assembled quite a collection of fired hulls. I discovered that some 67MM hulls were as long as some 2 3/4" hulls (none of which actually measured as long as 2 3/4"). If you measure your chamber and measure your hull, you may also find the hull isn't actually any longer than the chamber. I was surprised at the amount of variation.