Forcing cone measurements?

[user55645]

Hello. I'm going to be rebarreling a revolver in an odd caliber and there isn't a forcing cone plug gauge for it. Is there a standard list of dimensions for forcing cone sizes? I understand each varying angle (5,8,11,etc) would require a separate set of dimensions, but where are these? A plug gauge will have the "shelf" which shows you max and min. Where would I find a standard for what this should be, depending on caliber and angle?

[Outpost75]

Brownell's has Go/NoGo drop gages for forcing cone checks in most calibers.

Lacking one of those, I have used a new factory cartridge and inserted it, examining how far the case mouth enters. You want the bullet to enter, and the case mouth just barely. You don't want "slop" around the case neck to that it actually enters the forcing cone. See the examples. S&W 544 in .44-40 caliber a .45 ACP hardball round used as NoGo stands proud with the bullet sides showing. A .44-40 cartridge enters to the case mouth, but no farther.

Ruger Vaquero .44-40 swallows a loaded .44-40 cartridge neck and all and actually has the same forcing cone dimensions as my .45 Colts!

Ideally the major diameter of the forcing cone entrance should not exceed 1.05 times SAAMI max. bullet diameter for the caliber.

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[user55645]

Ideally the major diameter of the forcing cone entrance should not exceed 1.05 times SAAMI max. bullet diameter for the caliber.

If this is the standard, then it's exactly what I was looking for. Very much appreciated, thank you.

[Outpost75]

If this is the standard, then it's exactly what I was looking for. Very much appreciated, thank you.

I'm not sure it is an "official" standard, but back in the day before the Brownell gages were available to the gunsmith trade, that is how the late Bob Collins cut them and his PPC revolvers were among the very best.

As to the angle, an 11 degree will clean up tool marks in most factory forcing cones without enlarging the major diameter of the cone opening. If the ratio of bullet length to diameter is very short, as in 185-grain wadcutters in a .45 ACP revolver the length of forcing cone should not be so deep that the bullet base clears the cylinder face before the front driving band begins to engrave.

[user55645]

the length of forcing cone should not be so deep that the bullet base clears the cylinder face before the front driving band begins to engrave.

Again, very good info, thank you. As of now, the barrel will only see jacket projectiles with a rifle profile. I'll have to measure for base to ogive and work from there.

[Messy bear]

Outpost has it right! I would add that most times I find that they work better on the large side rather than small. Just my observation. Good luck!

[wonderwolf]

I'm not sure it is an "official" standard, but back in the day before the Brownell gages were available to the gunsmith trade, that is how the late Bob Collins cut them and his PPC revolvers were among the very best.

As to the angle, an 11 degree will clean up tool marks in most factory forcing cones without enlarging the major diameter of the cone opening. If the ratio of bullet length to diameter is very short, as in 185-grain wadcutters in a .45 ACP revolver the length of forcing cone should not be so deep that the bullet base clears the cylinder face before the front driving band begins to engrave.

Great info! I'm planning a few K frame rebarrels from a blank this summer (JG sales had frames less barrels cheap) and was wondering how deep I should go. I plan on making a 10-11° reamer to do the forcing cone but was wondering if I should worry about figuring out gauge depth and making a gauge as well. I may just make a few test barrels and come up with my own best practice depth. Going to be shooting 95% cast anyways

[Norske]

My gunsmith's 11 degree cone reamer has no stop on it. You need a depth gauge.

[DougGuy]

My gunsmith's 11 degree cone reamer has no stop on it. You need a depth gauge.

So you cut a little bit, stop and check, repeat until finished.

[wonderwolf]

So you cut a little bit, stop and check, repeat until finished.

I'm gonna look at my sammi spec books and see if by chance there is a max diameter in there for a 11° throat for 38....Can't seem to find any info in the matrix on what that number should be. I'm trying to build these guns on a shoestring budget and want to make all the tooling myself.