Forcing cone angles

[Deleted]

I'm looking at recutting & lapping my own revolver forcing cones to suit my particular needs. I'll lead off by saying that Mom had me tested, and it is safe for me to own both guns and tools to work on them , and I do follow directions, except for a few embarrassing years in my early 20s, I'm much better now . Anyway, calibers would range from 38/357 up to probably 45 cal max. I shoot 90% cast, and am looking at Ron Powers (muh hero) recommendation of 11*. I could branch out to 18* sooner or later too, but it seems redundant at this point. Plug gauges would be used in this project as well. The disinformation super highway is full of contradicting information on forcing cone angles, but we have a number of very experienced shooters & gunsmiths here and I thought this would be a good conversation for us since many of us are stuck indoors. One point is that there seems to be some confusion as to which angles can be recut to both 11* & 18* without the risk of creating a compound angle. I am bypassing the 5* angle at this point because it appears to be limited to Rugers, but any input on 5* would also be very helpful. For that matter, if anyone wants to discuss Taylor throating as well, I'm game too.

[bradshaw]

Feb 2, 2020 10:25:53 GMT -5 @anachronism said:

I'm looking at recutting & lapping my own revolver forcing cones to suit my particular needs. I'll lead off by saying that Mom had me tested, and it is safe for me to own both guns and tools to work on them , and I do follow directions, except for a few embarrassing years in my early 20s, I'm much better now . Anyway, calibers would range from 38/357 up to probably 45 cal max. I shoot 90% cast, and am looking at Ron Powers (muh hero) recommendation of 11*. I could branch out to 18* sooner or later too, but it seems redundant at this point. Plug gauges would be used in this project as well. The disinformation super highway is full of contradicting information on forcing cone angles, but we have a number of very experienced shooters & gunsmiths here and I thought this would be a good conversation for us since many of us are stuck indoors. One point is that there seems to be some confusion as to which angles can be recut to both 11* & 18* without the risk of creating a compound angle. I am bypassing the 5* angle at this point because it appears to be limited to Rugers, but any input on 5* would also be very helpful. For that matter, if anyone wants to discuss Taylor throating as well, I'm game too.

*****

Any mention of forcing cones, especially ones given a name, must be defined as the angle or angles. To propose some works without defining terms admits ignorance. Loose talk may not in this instance sink ships, but it may doom ACCURACY. The job of the FORCING CONE in a REVOLVER BARREL is to admit passage of bullet from separate chamber----without altering BALANCE. The forcing cone must accommodate some CHAMBER-to-BORE MISALIGNMENT.

I know of no specific amount of chamber/bore OFFSET (misalignment) which dooms accuracy. It’s not that simple. Indeed, my old mantra revolver accuracy is an orchestra of dimensions holds true for chamber/bore alignment. When I give exact number, for instance, "runout greater than .006-inch negatively affects accuracy,” is based on experience with a number off revolvers with excellent bore & groove characteristics and a smooth, concentric forcing cone. The list of details is myriad and not to be ignored.

The so-called average shooter has no way to measure chamber/bore offset, nor have most gunsmiths, so the subject receives little attention.

Accuracy killers
* Lousy bore.
* Grossly oversize chamber exits.
* Lousy forcing cone.

Forcing cone
Mention of a forcing cone often includes a specific angle. The old 11-degree cone was mentioned for years as though it were some recent invention, or discovered by some living gunsmith. BULLROAR. The 11-degree cone has been around far longer than any of us have been alive. An industry standard, if not the industry standard. It is this writer who brought focus to the forcing cone as critical to revolver accuracy , and I went through battles with Ruger and Smith & Wesson over it. A steep learning curve forged on the firing line of IHMSA silhouette. Without which experience I couldn’t have raised an eyebrow at the largest revolver makers. It is precisely performance out to 200 meters which gave my argument leverage. There is a whole ugly backstory which I won’t go into here, but it only drove my argument home like a nail in a coffin.

Meanwhile, PPC shooting with refined .38 Special revolvers paid its own specific attention to the forcing cone. A world apart from handgun silhouette, PPC nevertheless demands extreme accuracy from the revolver to compliment precise marksmanship. The PPC boys & girls performed with a soft wadcutter, a bullet more sensitive to launch than the Pagan Pills hurled by silhouetters. The shooter of a properly made custom revolver may live in oblivion of such detail, you can get away with and what you can’t but the silhouetter tied to a factory stock revolver had better learn to sort what you can get away from dimensions which doom performance.

When Jim Stroh rebarreled my Freedom Arms .44 Mag to my specifications, Stroh cut a very shallow 11-degree forcing cone. The 10-inch Shilen barrel has a 1:16 RH twist----irrelevant to this discussion. Shilen had suggested Taylor throating. No jelly. Shooting the Sierra 240 JHC, this revolver has sandbagged 5x5 shots into 0.7-inch @ 100 yards before witnesses. The revolver has bagged sub-inch @ 100 yards with the following optics: Leupold 4x28mm LER, Nikon 2x, B&L 2-6x, and shot 1-inch with Weaver battery powered red dot. A 1-inch group with a revolver is never a given. The slightest wisp of breeze drifts a bullet traveling the length of a football field. To me there are no easy shots, no “on demand” shots. Each shot is a job. A shot is never celebrated before the results are in, a deer on the meat pole. There are no high 5’s, ever. It is just shooting.

Dimensional allowance shrinks as we demand tighter accuracy.

Unless you plan to use your barrel as a grain funnel, or to change the oil in your transmission, forget an 18-degree forcing cone. Don’t go a single degree wider than 11-degrees. A 5-degree forcing cone works well when chamber/bore alignment is close. Chamber offset allows a bullets to flounder in a deep forcing cone, obturating the projectile. Deformity is imbalance. Static imbalance becomes dynamic imbalance in motion----the flywheel effect. A bent bullet can’t fly straight.

A short 11-degree cone, cut on a lathe smooth & concentric & short, has record of introducing numerous bullet profiles into rifling with a minimum of distortion in the presence of .006” or less offset. Again, this is my idea of a general picture for straightening the trail to accuracy.
David Bradshaw.

[potatojudge]

I think Taylor throating, whatever that means dimensionally, as Jim Stroh espoused is a good option, but I don't think it's a better option than a revolver with good chamber to bore alignment and an 11 degree cone. I think Taylor throating helps some guns, but guns that don't NEED it suffer a little velocity loss with no gains.

I've attached pics as best I can get them. Apologies for not cleaning the barrels first, but I think what needs to be seen can be. The Colt has a typical 11 degree cone I believe, the Stroh 44 you can see an additional depth/angle that cuts into the rifling, and surprisingly the FA has about double the leade into the rifling as Stroh's 44.

Colt Bisley re-barreled by Stroh along with line-boring the original cylinder.



Jim Stroh's personal 44 Bisley that he re-barreled.


Freedom Arms 97 in 44 special.


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[bradshaw]

Feb 2, 2020 13:52:25 GMT -5 potatojudge said:

I think Taylor throating, whatever that means dimensionally, as Jim Stroh espoused is a good option, but I don't think it's a better option than a revolver with good chamber to bore alignment and an 11 degree cone. I think Taylor throating helps some guns, but guns that don't NEED it suffer a little velocity loss with no gains.

I've attached pics as best I can get them. Apologies for not cleaning the barrels first, but I think what needs to be seen can be. The Colt has a typical 11 degree cone I believe, the Stroh 44 you can see an additional depth/angle that cuts into the rifling, and surprisingly the FA has about double the leade into the rifling as Stroh's 44.

Colt Bisley re-barreled by Stroh along with line-boring the original cylinder.


Scoring rings common to hasty cutting of the forcing cone. A common outrage of ignorant manufacturing.
Jim Stroh's personal 44 Bisley that he re-barreled.
Time honored barrel face erosion from .44 Magnum. No affect on accuracy. Little or no affect on spitting, unless excessive chamber offset. Forcing cone appears a straight cut angle----not dual angle----although it could be hard to detect, even with gun in hand.


Freedom Arms 97 in 44 special.
Appears to be a very low angle forcing cone. Light gray ring at start of cone may be a combustion signature, have nothing to do with a convergent angle. Apparent radius at barrel face/forcing cone has no relation to accuracy, may hasten erosion.

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The LEADE----the ramp cut in the lands at transition of chamber throat to bore----is the closest thing a rifle has to a revolver’s forcing cone. The leade may have an angle low as 1-1/2 degrees, very gradual so as to not abrade the bullet. Absence of leade or an abrupt or rough leade instantly destroys accuracy of the finest barrel. Revolver makers should be aware of this, even as we throw slugs with the ballistic coefficient of a barn door. A low angle leade in a revolver should not be cut so deep as to accommodate excessive chamber-to-bore offset of .007 to .015-inch. The low angle must then be cut deep to provide forcing cone with enough root diameter to avoid spitting. Thus, a narrow-angle cone of, say, 5-degrees, works best with minimal chamber offset.

While a cylinder loose on its pin gives the bullet more chance to align passage from throat to bore. However, loose pin also allows cylinder more room to wobble under pressure.
David Bradshaw

[bradshaw]

LEADE, revolver barrel
As noted leade is an angle cut into the lands to permit rifling to smoothly engrave bullet. Long proven necessary in rifles, execution of the leade is a significant factor in accuracy. It should be considered likewise for revolvers. Any self-respecting gunsmith acts accordingly. The leade in a revolver barrel will be right when the forcing cone is done right.

LEADE, revolver chamber
A cylinder chamber which does not headspace the cartridge case has a leade angle to transition chamber wall to exit hole (throat). Leade must be smooth. A rough edge or ring amount to a knife which will tear off bullet material. Oddly enough, this may not adversely affect accuracy with jacketed bullets when chamber-to-bore alignment is extremely tight, and the bore extremely true. However, such a revolver will spit.

CHAMFER
To chamfer chamber exit holes on the cylinder face is a cosmetic affectation. Plays no part in accuracy, may contribute slightly to spitting. And to chamfer the root edge of the forcing cone is another affectation. To some pea-brain it may look attractive. Or, to a management type such a chamfer may be an expedient to remove a burr from a hastily cut forcing cone. In recent years, I have seen too many revolvers with lightly chamfered chamber exits, and barrel face chamfered, also. This includes some custom gun work, ill-informed.

EROSION
Mention of “forcing cone erosion” usually refers to barrel face erosion. The front of the cylinder is called the cylinder face. The rear end of a revolver barrel is called the barrel face. The two face each other and experience a blast of tremendous heat with each shot fired. Virtually all of the erosive force is directed at the barrel face. The barrel face receives six times the exposure of the chamber on a 6-shot revolver. Yet, numerical exposure doesn’t account for the far more erosive effect on the barrel face. I have long suspected that the tiny hard grains of ball powders exert a sandblasting affect on the barrel face of a magnum revolver, and have the same sandblasting effect on the bolt extension of the direct impingement AR-15/M-16, as well as flash hiders. Jim Stekl, Bench Rest Hall of Fame, Remington R&D and developer of 7mm BR, 6mm BR, .22 BR, described ruining a bench rest barrel with ball powder. Don’t remember which powder. During that conversation I maintained my belief in the all-round forgiveness of IMR 4895 and Hodgdon 4895 for the 7mm/308x1-3/4” (7mm Talbot), 7mm TCU, 7mm IHMSA, etc., but of course my silhouette loads were far below blue pill. Stekl said that while the particular ball powder which fuse-fouled his barrel was especially offensive, he would never go back to ball powder for a bench rifle.

Do not take this to mean I condemn ball powder. I shoot plenty, with Winchester 296/H110 and Win 231/HP-38 at the top of the list. Along with numerous other ball propellants for pistol, revolver and rifle. I pay attention to powder, and Jim Stekl’s cautionary tale from several decades ago lays etched in my approach to loading. And casts a shadow over Hodgdon Lil Gun, leastwise in a revolver.

Chamber pressure multiplied by times fired does not account for erosion. Factors include length & diameter of cartridge case, propellant burn characteristics & charge, bullet weight and seating depth, and cylinder length. Not a complete list but a start. heat goes up with pressure; even so, some fuels burn hotter than others. Nitroglycerine deflagrates HOT. Double-base powders contain nitroglycerine. I’ve watched what high explosives do to steel, and VOD (Velocity of Detonation) alone does not explain some of the action. I suspect extreme concentration of heat to be a factor in some high velocity explosives, as Powders of similar VOD exhibit different burn characteristics on steel.

Smokeless powder should not be confused with explosives. Deflagration characteristics differ entirely. Yo can’t make smokeless powder do the work of HE. By the same token, an explosive decomposes far to quickly to launch a bullet.

FORCING CONE---Savior or Epitaph
11-degrees is as steep as I want. Shallow may be better. Yet, to achieve root requires depth. That is the compromise. Quality is where it’s at. A finite measurement is meaningless without Concentricity & Smoothness.
David Bradshaw