doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Forcing cone alterations=out of proof?

From everything I've read here (and elsewhere) in numerous discussions of proof, I would have said no. However, someone recently brought the following quote to my attention, from McIntosh and Trevallion's "Shotgun Technicana", p. 78:

"English proof law holds that deepening a chamber renders a gun out of proof, but simply lengthening a forcing cone does not. (Or at least this is the London Proofmaster's position; we're told the Birmingham Proof House does not permit forcing cone alterations.)"

Based on the above, a friend who recently had forcing cones lengthened on a vintage British double emailed the Birmingham Proof House. He received the following as part of a brief reply from the assistant proof master:

"The gun would be out of proof by removing material from the forcing cone."

Mr. Trevallion, Dig, Hugh, others familiar with British proof practices . . . have we been wrong all along when stating that lengthening cones does not invalidate proof? And since there is no standard length for forcing cones (unlike chambers), and since there is no proofmark specifically relative to the forcing cone, how would the proofhouse know for certain that the cone had been modified?

That would be my same question, how would they ever know if its been modified?

Even if the gun were sent back for proof, is there a marking that indicates that cones of any specific length were proofed?

When forcing cone lengthening became fashionable here about 1988 or there abouts it was very common practice for gunshops and gunsmiths to advertise this work and to carry it out often on a while you wait process.
Indeed GMK the British Beretta importers used to have their resident gunsmith Gordon Swatton doing it at Game Fairs and competitions such as the British & English Opens.
It did not contravene Proof House regulations, maybe times have changed and Birmingham have bullied their way into the scenario.
It is typically a cock up, because Proof Law clearly says that if you alter a proved barrel in any way it renders it out of proof. That is why you have to have it reproved if you multichoke a fixed barrel.

It has always been my understanding that lengthening a chamber cone SHOULD render a gun out of proof as there is metal removed from the barrel . But came the argument ,providing the actual chamber has not been lenghtend byond the proof limits how dose any one know it has been done ?So it was not a consideration and as no directive was issued no one bothered .But Proof law is fickle, CIP has changed thinking in some areas. for instance I have threaded dozens of .22LR barrels to take moderators with out any problem ,Then a few years back it was decided that "as this could subsancialy weaken a barrel" these guns are now out of proof . This has been argued but to my knowlage no test case has been defined .
With proof we are at the discretion and mercy of the proof masters to some degree and often new proof masters make new policy . The answer to the original question must be yes it dose as a technical point but a point that has never been enforced .

Seems to me that certain alterations in a barrel do not impact proof. Honing/lapping, for example, as long as the bore diameter remains within the parameters for that particular bore (not more than .010 larger than standard). Likewise, I'd never heard that opening chokes impacted proof, although both Dig in his book "Vintage Guns" and Salopian above refer to a gun requiring reproof if it's altered to accept choke tubes. In the latter case, significantly more metal has to be removed to accept screw-in chokes than is the case if a gunsmith simply removes a few thousandths to open a fixed choke. Thus, requiring reproof in that case would seem more logical.

The key words are "substantially" reducing the strength of the barrel as fitting a multi choke boring out to cylinder dose not effect the overall integral strength of a barrel .British proof can be a mine field ,even those of us that are involved with regularly submitting guns get confused with 8[?] changes in proof law and stamps in the last 100 years .No wonder we can not keep up .Personally I think its time we started from scratch again ,but that's not going to happen so we just muddle along as best we can hopping we get it right most of the time.

L.Brown,
Lengthening the forcing cone results in a more gradual angle of transition between the chamber diameter and the bore.A more gradual transition between chamber and barrel diameters will result in a reduction in stress concentration in the barrel wall.In my opinion, lengthening the forcing cone will almost certainley increase the ultimate strength of a sound, in proof barrel.
One would think the Birmingham proof house would want to confirm the impact of lengthening forcing cones on barrel strength before making any ruling on the issue.

+1

Too many variables to say the effect is this or that. I have cut 3/4", 1 1/2", and 3" forcing cones in 12ga. What angle is on the outside and the forcing cone angle together with the chamber depth relative to the outside taper all play into the equation.

Roy, I don't disagree. However, it depends on how much steel there is at the end of the chamber, and how quickly the barrels begin to taper beyond the chamber. If there's a rapid taper (to reduce weight) combined with a long forcing cone, you could end up with a situation where barrel walls are of questionable thickness at a point where pressure is still fairly high.

I don't think it matters what the members of this Forum think.
The way I understand British Proof Law is that the Proof House will inspect a firearm and determine whether it can be Proofed or Re-Proofed and what they determine is Law.
If the Proof House is telling you that removing metal from theforcing cone takes the firearm out of proof, then I don't know what recourse you have under British Law.
Knowing or not knowing whether something was done is not the issue, it's whether firearm is considered in Proof(safe to my understanding).
Of course in the USA, British Proof Law does not have any Legal bearing.
From an Engineering perspective, at least my Engineering, anytime you alter the pressure retaining components of a firearm, you alter the design and call into question whether the firearm is safe.-Dick

"Personally I think its time we started from scratch again ,but that's not going to happen so we just muddle along as best we can hopping we get it right most of the time." Good idea Gunman, I feel like the British Proof Houses stance on removing anything "substantial" isnt a very substantial rule.

I guess I can see why the proof houses feel this way. They probably see hundreds of barrels a year fail that seem perfectly fine, obviously due to any number of conditions not related to removing "substantial" material, most are probably new barrels made of poor steel. In America its like talking to a cardiologist or oncologists, they will tell you everyone dies from cancer or heart attacks depending which one you are asking. If you ask the proof house modifying barrels makes them out of proof and therefore dangerous.

Has there ever been a major case where the proof house has provided evidence in a lawsuit that a barrel has been modified and out of proof? Living and growing up in America, shooting thousands of guns that have never been subject to a government proof, I guess I just dont get the point of the proof house. I suppose it gives me a warm and fuzzy feeling knowing that I am buying a gun that is in proof, but from a manufacturing standpoint it seems like a total waste of money and time, but thats jmho.

Channing

One other factor here is that the London and Birmingham Proofhouses apparently disagree as to the impact of forcing cone alteration on a gun's proof status.

A long time ago I knew a fellow that wanted long forceing cones on his Browning super 28 gauge. They hit air befor the reamer was all the way in. Out of proof?
bill

I'd suspect the forcing cone work was done well and safely, so it may come down to the perceived value of the proof stamp. If there's a question, I'd think send it to the proof house nearest the origin of the gun and let them use their expertise to either reproof it or provide documentation that the gun was still in proof.

Guys,
I really don't know why we are discussing this subject at all.
It as been proven beyond all reasonable doubt that lengthening forcing cones only lightens your wallet, that is not good and neither is lengthening forcing cones.

With all due respect, I'm going to disagree with you on this. My testing showed me that there was a small but measureable increase in the quality of the patterns in 12ga testing I did many years ago, comparing SAAMI spec cones to 1 1/2" and 3" long cones. My testing of .410's with SAAMI spec cones vs. 3" long cones showed a significant improvement in patterns with 3" shells.

It may be in the eyes of the buyer/seller as well. I bought a lovely Brit BLNE from a Seller of good reputation. The cones had been lengthened. I asked if it affected proof, and was told no. Couple years later when I was trying to trade the same gun with the same dealer, I was told value was diminished cuz the cones had been lengthened and it was out of proof...Heated discussion commenced..........

The chamber body itself tapers at a rate of about .005" per inch. Shotgun bbls having less taper than this over the chamber area are few & far between, thus lengthening a chamber is virtually guaranteed to reduce the wall thickness at end of chambder. The cone tapers by .070 �a thou or so over the length of the cone (to a std bore dia) There are not many bbls which will be thinned by cone lengthening unless it is a very long cone. Also the further down the bbl the lower the pressure. I cannot conceive of any ordinary cone lengthening creating a Danger Spot.
As to pressure, it has for years been standard practise to record max pressure 1" from the breech to ensure that max was always read. Pressure rises very rapidly, but it also falls rapidly. I nighly suspect that giveen proper equipment that would read the pressure curve from max to beyond the cone that change in max pressure from cone length would be essentially undectable. What would be more probable is that the presssure would simply be just a tad slower in falling with the short cone, vs the long one, but not likely anything to get up on your Tricycle about.
As to patterns the biggest aspect here would be of easing the shot from chamber to bore with the least deformation. Modern plastic wads have taken care of very much of this. With old style card & filler wads due to their lack of obturation a long cone often did more harm than good by allowing gas leakage, so its a good bit of a wash here.
The .410 in 3" is of course such an unbalanced & ridiculus load it does indeed "NEED" all the help it can get.

I'm not an authority on the subject like some. I just tested using the same barrel with the same loads cutting the cones progressively longer and shooting a gun. I didn't read about other peoples opinions on the subject. So other people likely know more about this than I.

Sherman Bell also did some fairly extensive testing in one of his "Finding Out for Myself" articles in Double Gun Journal. His conclusion was that lengthening the cone without lengthening the chamber did result in some reduction of pressure (usually a few hundred psi) when shooting 2 3/4" shells in 2 1/2" chambers.

LB
Bell's test, of lengthening the forcing cone, was to explore the specific pressure effect lengthening a forcing cone had when shooting an assortment of 2 3/4" shells in an English type 2 1/2" chamber originally with a very short forcing cone .

I'm interested the test parameters of Miller's work on forcing cone effect on patterns and how that compares to what I did.

I did stumble onto Jim Eyster's work from 2010 on forming a radius in the intersection "corner" of the cone and the bore on a 2� cone of a Blaser. My recollection is that a SAAMI cone is 5� (from the bore axis). So the Blaser has "long cones" from the factory. His rounding of the corner showed a significant benefit in pattern performance. What I didn't see in Jim's study was any dimensional description of how big the radius was, etc..
http://www.jimeyster.com/testing/documents/Forcing%20Cone%20Test.pdf

Actually Chuck, it had two purposes. One was to determine the pressure increase (if any, and there was always at least some increase) when firing various 2 3/4" shells in a 2 1/2" chamber, and then to see what would happen with the same loads when just the forcing cone (but not the chamber) was lengthened.

LB,
I was addressing only the lengthened cone tests, but you are obviously correct that he tested 2 3/4" shells in a 2 1/2" chamber first.

I had forgotten I had a somewhere acquired a pdf of ANSI/SAAMI Z299.2-1992, American National Standard Voluntary Industry Performance Standards for Pressure and Velocity of Shotshell Ammunition for the Use of Commercial Manufacturers. It shows a 5 degree (from bore axis) cone angle and a reference dimension that comes out to about a .417 cone length. The Blaser that Eyster tested and stated was a 2 degree cone angle would come out to about 1" long.

In my earlier statement about what I had tested, I mistated what I had tested. I tested standard SAAMI cone vs. approx 1 1/2" long cones and an approximately 4" (36 minutes of angle) long cone (not 3" as stated earlier),all in one 3" chambered 12 ga gun using 2 3/4" target shells. The 1 1/2" cone was cut with a reamer from Brownells, and the 4" cone was cut with a "taper-pin" reamer from a tool supply house and specially ground to fit inside the 12ga chamber by a professional cutter manufacturer who is a friend.

I have also tested an approximately 3" long cone in .410 3" chambers using 3" shells. I have tested at least 4 possibly as many as 6 separate .410 guns with before and after. I saw a benefit to the patterns.

Very interesting stuff, Chuck. Thanks.