Forcing cone angle question

I am hoping the experts here can clarify something for me. I have been reading around about forcing cone improvements because I have a Smith and Wesson Model 15 that has a very rough forcing cone.

What I am having trouble understanding, and there seems to be a lot of contradictory info on, is the forcing cone angle. I have gathered that Rugers have 5 degree angles, and that the popular angle for shooting lead is 11 degrees. The Smith angles seem less clear - I have seen 8, 9 and 18 degrees.

The first question is whether these angles are per side (i.e the angle between the bore centerline and the cone surface) or are the angle between the two cone surfaces, which would be twice the angle between the centerline and the cone face.

Put another way, is the ruger cone 2.5 degrees between the centerline and cone face, for a total of 5 degrees. Or, is it 5 degrees each, for a total of 10? Likewise, is the 11 degree cut that is common 5.5 degrees per side for a total of 11 degrees, or is it a total of 22 degrees? Finally, does anyone know whether the Smith cone is 8-9 degrees per side, for a total around 18 degrees?

The reason that that matters is that I don't want to enlarge the wide part of the cone at the cylinder gap. It is already ~0.020" larger than the throats, which I understand is about what it should be. If the existing angle is steeper/greater than the cutter angle, then the cut can be made extending the cone further into the barrel and leaving the diameter the same at the cylinder end. If the existing angle is shallower/less than the cutter, using that cutter would either leave a compound angle or enlarge the opening at the cylinder end, neither of which I am OK with. In the later case, the barrel would have to be set back and the barrel face re-cut to set the cylinder gap. That isn't likely worth it.

Thanks in advance for any education on that. I am just trying to make sure I understand the mechanics there before I decide whether I am going to mess with it. If my description doesn't make sense, I can draw it out and add that. Also, if this belongs in gunsmithing, feel free to move it.

beag_nut said:

Is there a practical reason for you wanting to ream the forcing cone? Are you imagining using only lead bullets? For more info I would recommend you visit the website a long-time user and retired gunsmith seems to be happiest with, by the name of Iowegan. He has certainly helped me in the past. I don't know if actually quoting a competing forum is allowed here. One could simply google that name. More info: send me an email.

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A couple of things are driving my interest in doing that. I picked up a Model 15 pretty cheap in pretty rough shape aesthetically, but seemingly fired very little. I picked it up as a bit of a project to tinker on, and would like to optimize it for shooting wadcutter loads. The throats were quite tight (0.3565 - 0.357) and it leaded pretty badly. I opened that up such that a slug pushed through the bore is an easy push through the 0.358" throats. I am still getting a little lead, and the forcing cone is visibly rough, with concentric ridges left by the tooling the entire depth. My hope is that cleaning that up will get rid of that last bit of leading. I am going to shoot it with a few different loads before I do anything, I am just making sure I am properly educated on the subject before I do anything with it. The loads I was shooting the other day when it leaded up were MBC DEWC over 4.2gr. Unique. I have some HBWC over 2.7gr Bullseye and some of those DEWC over various Bullseye charges in the 2.8 to 3.5gr range I am going to run through it to see what happens.

Thanks for the reference, Iowegan's discussions on this and so many other subjects are incredibly helpful and I will keep digging through his comments on it. Thanks also for the invite re: the PM, I will try not to bug you but will if I am still lost after digesting all of the readily available info.

243winxb said:

https://www.thehighroad.org/index.php?threads/forcing-cone-results.882502/#post-11806592 Good Question.

http://smith-wessonforum.com/s-w-smithing/641406-m29-2-44-mag-forcing-cone-specifications.html


View attachment 977479 I am guessing, this is 11 degrees??

44 mag Forcing Cone.
A .430" flat base bullet goes in about .258" deep with a factory forcing cone. A .451" flat base bullet goes in about .100" A 11 degree may go .265" from barrel face to the .430" barrel diameter? Would seem my forcing cone is ok. Using very crude measurements & eyeballing it.

How i see it when trying to figure out mine. My barrel was set back by the factory 2 years ago.

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This is an interesting way of looking at it. By my calculations, based on the different penetration of those two diameter bullets, that is right around an 8 degree angle total (4 degrees per side), which makes sense since it was cut by Smith at the factory.

The forcing cone on my Ruger SP101 definitely looks like a shallower angle/longer cone region than my Model 15. So this seems to support that the 5 degrees for Ruger, and 8 degrees for S&W is the total angle (2.5 degree and 4 degrees per side, respectively).

Based on that, it seems that there is no way to cut a 5 or 8 degree cone to 11 degrees without enlarging the entry at the gap. I want to look more closely at how to manage that, since it seems lots of people have made an 11 degree cut on S&W revolvers.

For those of you that have done this, I assume you cut until the fresh cone was all the way up the lands? Did you measure the change in diameter of the opening at the entry?

I know there is a plug gauge to check the depth of the cone, but I am not sure what the dimensions of those are.

I spent a healthy amount of time reading what I could find on this, taking measurements, and giving it some further thought. Since I was confused by it, I am going to summarize it in hopes that it saves someone else the headache... Sorry if this post gets long.

I have confirmed that my concern is correct: if you cut a forcing cone to a steeper angle, you will by definition enlarge the opening at the breach face in order to cut the cone to at least the depth of the existing cone, which is required to avoid a compound angle. If the original cone is undersized, you may be able to stay under the recommended 0.020" larger than the bullet diameter even with the steeper angle. If it is at or over that 0.020", you will be cutting the cone oversize.

There are two problems with that: (1) you will loose more pressure at the gap than intended, and, perhaps more importantly (2) you will make the barrel at the breach face thinner and therefore more susceptible to damage. Depending on the gun and the dimensions of the cut, these could be dangerous conditions, and you must absolutely 100% ensure that you are not creating those problems BEFORE cutting. If you do cause those issues, you may have to have the barrel set back, or end up with a cracked forcing cone.

In the case of my Model 15, the opening to the forcing cone is right at that 0.020", or just over, from the factory (my measurements seem to put it at 0.379" +/- 0.001"). I can also confirm that it is an 8 degree cone, 4 degrees on each side. Unfortunately, in addition to cutting it to the maximum depth, the factory also left a lot of tooling marks. Visible below.



Based on my rough measurements, if I cut that cone with an 11 degree cutter deep enough to just remove all of the factory cut, I will open up the the entry of the cone from approximately 0.379" to approximately 0.387". The breech face is 0.065" thick, with a flat at the bottom that is 0.045" thick. This cut would reduce the breech face thickness at the thinnest point to about 0.041". I have no idea if that is a concern or not. I know that the K-frame magnums had problems with cracking forcing cones shooting hot 125grain loads. Bottom line, that will make the barrel thinner at that point than it was originally. Incidentally, my SP101 has a cone opening that is ~0.383", but it doesn't have that flat at the bottom of the barrel like a S&W K-frame where the cone it thinner, so that isn't that helpful.

Another option would be to cut it with a 5 degree reamer, which would allow me to keep the same opening diameter at the breech face, but would make for a longer/shallower cone. My understanding is that a steeper cone is better for cast bullets. But I am not sure I have a firm grasp on why that is, and am still trying to get an understanding of that. If an 8 degree reamer were available, I would just use it to clean up the cone without really changing the geometry any significant amount, but that reamer does not appear to be available.

Thanks for the photos 35 Whelen, those are great. That lower one is purdy!

I am confident that you are correct, I am way overthinking this. I'm an engineer, that sort of goes with the territory.

Since I started down that path, I wanted to share what I have gathered from reading as much wisdom as I could find in various corners of the internet from the likes of Ed Harris and other quite qualified contributors.

It seems that in a perfect world, the cylinder aligns perfectly with the barrel, and the throats are exactly the size of the bore, and the bullet could pass seamlessly from one to the other. It is not a perfect world, and there is always going to be some misalignment of the bore and throat and some change in size of the bullet as it passes into the bore. So, there has to be a forcing cone to correct any (hopefully small) misalignment of the throat and the bore, and to allow the bullet to be sized down to the bore without shaving material off of it.

The goal of the cone is to achieve that goal with as little unsupported distance traveled as possible, and with the resizing happening gently enough to allow the bullet to maintain a concentric shape. Lead is relatively soft, so it can be reformed more easily than a jacketed bullet. Therefore, a steeper angle cone allows for a shorter distance between the support of the throat and the support of the bore. 11 degrees seems to be the angle most feel achieves this goal. For jacketed bullets, a shallower angle allows a more gentle transition into the bore, but the unsupported travel is further. I have not done testing or otherwise verified those comments, they are a summary of my understanding from readings and pass the "sniff test" (at least to me) from a mechanical sense.

Bottom line, the general consensus seems to be that a cone that is concentric to the bore, and smooth, is more critical than the angle (within reason, which anything in the 5-11 degrees is). The cone should be as small as possible at the opening, while being large enough to correct for any misalignment and bullet size change. On a custom, hand built, revolver, the potential misalignment or slack in the lockup could be as little as 0.001", so not much cone entry diameter would be needed to make sure the bullet doesn't strike the breech face, so even at 5 degrees, the cone could be quite short. On a production gun, there is a higher chance for misalignment from the mass production process, so they cut a larger forcing cone. Often that is done a bit haphazardly, leaving a rough oversize cone. To really correct that on a competition gun for example, the barrel has to be set back and a proper cone cut at the desired angle.

Barring that, on a S&W with an 8 degree cut, an 11 degree cone could be cut, resulting in an even more oversize cone, or a 5 degree cone could be cut resulting in an excessively deep cone. The resulting unsupported length is less with the 11 degree cone (slightly), but the breech face is thinner and there is some loss in velocity due to the volume. For cast bullets where velocity is not critical and hot magnum loads behind shorter bullets won't be used, the 11 degree option seems to be preferred. The most important thing is that the resulting cone needs to be concentric to the bore and smooth. Given the quality of the machining (or lack thereof) on my S&W cone, I would not be at all surprised to find that it is not concentric to the bore.

Anyway, sorry for all the pontification on the subject. Hopefully it helps someone understand the geometry and mechanics of the forcing cone. Again, I am not an expert, so all of that is information I have gathered, together with measurements and calculations I have performed using modest measurement tools and the two revolvers I have mentioned. Since I want an 11 degree cut ultimately, I am going to cut the forcing cone on this barrel using an 11 degree cutter. If I have to go so large to clean up the factory cone that I am not comfortable with the result, then I will have to set the barrel back and recut the breech face. I want this gun to shoot full power DEWCs accurately and without leading. If I have to do some work to get it there, all the better, the learning to be done on that journey is half the point anyway.

Thanks for the great additional info. 35 Whelen, nice Uberti, I love those SAA clones.

I am somewhat concerned about the support of the bullet, more in terms of wanting to maximize it, rather than concerned about not having enough. Along those lines, here is a picture of how deeply the bullet sits in the existing cone of my Model 15 and SP101 (sorry for the crappy cell phone picture). It is ~0.1" further into the cone (or further out of the throat), which is more or less consistent with what you would expect from the 8 degree cone cut to an opening of 0.379" on the S&W and the 5 degree cone cut to 0.383" on the Ruger. As between cutting the existing cone to 11 degrees, with an oversize entrance, or cutting it to 5 degrees without enlarging the opening, the bullet will make it to the rifling earlier (and moving slightly slower) with the 11 degree cut.



I am more concerned about making the bottom of the breech face, where there is the machined flat at the bottom of the barrel, too thin. I measured my S&W 442 (which is +p rated) forcing cone, and it is 0.043" thick. My estimate is that the Model 15 will be around 0.041" thick, but I won't really know that until I do it. Bottom line, I think that it will be OK. So, I have an 11 degree reamer on the way, and will be giving it a try soon. I will post results when I get that done.

GooseGestapo said:

S&W used an 18deg angle back in 90’s and earlier.

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I have read that, but this revolver was made it 1977, and really looks to have an 8 degree cone by my measurements. If I'm wrong and it does have an 18 degree cone, that will clean up nicely and all my consternation is for nothing. I will know depending on whether the reamer starts cutting at the mouth of the forcing cone or the rifling first. Any chance you know of a reference that would show what the cone should have been with the various models over the years? Not sure if that level of machine detail info is readily available.

I have checked this revolver for end shake and cylinder gap, all well within tolerance. The lockup seems centered in the bore visually, but I have not confirmed that with a range rod. I am considering ordering one to confirm the lockup.

Thanks all again for the excellent information, I will update as I get this done and get out to the range.

Sorry about the delayed follow up on this, it took a looong time for the chamfering tool to get to me. Spent some time this evening getting the forcing cone cleaned up.

I used the 11 degree cutter, and it started cutting at the entrance edge of the cone, so the cone was definitely 8 degrees as anticipated. It took a lot more cutting than I expected to get it cleaned up because the mice that chewed the original cone into the barrel did so off center by a good bit. The side that got the worst of the leading was the side that it was off center to, so the bullet would make contact on the other side first presumably wrecking all kinds of havoc on the bullet.

I got it cleaned up nicely though I think. I put some before and after shots below. I would be happier if I could lap it, but it is night and day better than it was.



(ignore the lint from the patch I ran through before snapping the picture)

The opening seems to have been expanded to about 0.384" which is a little better than I expected. It is very hard to accurately measure that, so that is approximate, but I am at least confident it isn't way worse than expected. A bullet goes in a little further, but still less than my SP101 pictured above.



In doing this work, I did notice that there is a definite tight spot in the barrel where it threads into the frame. It is minimal, I would guess less than 0.001", so if it still leads that is likely the culprit. My hope is that the forcing cone clean up will get it. I can clean a little lead if I have to, as long as it is light enough that the barrel isn't leading to the point where accuracy falls on its face after 30 or so rounds like it was before. I have to travel some for work over the next week or two, so I am not going to get it to the range for a bit. Looking forward to seeing how it performs.