Throating Cylinders and Using Jacketed Bullets

[RG1911]

I've been thinking about having the cylinders of a couple revolvers checked and, if necessary, throated. (If that's the correct term.) I am *assuming* (there's that dangerous word) that the cylinder mouths would be cut to the diameter of the widest mouth.

My understanding is that uniforming the mouths of the cylinder benefits cast boolits (which I use) *if* you cast and size boolits to the new diameter. (I'm not sure what happens if the new diameter is more than maybe .001 or .002 over groove diameter.)

One question then is, if jacketed bullets are .357 in diameter (for example) and the cylinder mouths have been opened to .358 or .359 (for example), is accuracy downgraded?

Thank you,
Richard

[tazman]

For cast boolits, if the boolit fits the throats and is more than .002 larger than the groove diameter, there is no significant problem until you get much larger than .005 bigger than groove. You can sometimes lose a bit of accuracy due to boolit distortion when the boolit gets swaged down by the barrel. Not always.
Colt revolvers used to be set up that way and they shot just fine.

As far as jacketed, I suspect it won't bother them much. I really don't have any experience to go by with jacketed since I shoot very little of it.

[Outpost75]

For best accuracy with jacketed bullets the ideal situation is that the ball end or cylinder throat would be either the exact size or very slightly, about 0.0005" smaller than bullet diameter. When shooting jacketed, it is OK if the cylinder throats are slightly, about 0.0005" smaller than barrel groove diameter. Ruger .45 Blackhawks come this way from the factory and shoot jacketed ammunition extremely well.

That condition is not ideal for cast lead bullets, which is why I had DougGuy ream my Ruger .45 cylinders to .452"...

For target use with lead hollow-based wadcutters in .38 Special the fellows building accurate PPC revolvers found that cylinder throats which were the same size or up to 0.0005" larger than the typical .358-.359 diameter of factory Winchester or Remington factory lead bullets were ideal. It was perfectly OK for the bullet diameter to exceed barrel groove diameter up to 0.003" as long as cylinder alignment was correct, the forcing cone concentric, and the forcing cone angle and depth such that the wadcutter bullet was guided straight into the bore and began to engage the rifling before the bullet base cleared the front of the cylinder.

The major diameter of the forcing cone entrance must not exceed 1.05 times bullet diameter. THAT is important! If you recut forcing cones, buy the Go/No-Go gages. If a factory forcing cone has circumferential tool marks, recommended practice is to use a forcing cone reamer of more gradual angle to clean up the radial tool marks, without enlarging the base diameter of the forcing cone. You use an 11 degree reamer to clean up a factory 18 degree or 16 degree forcing cone, and a 6 degree reamer to clean up an 11 degree cone. This saves the time and expensive having to pull the barrel, set it back a thread and refitting it, and cutting a new cone. However, if your factory barrel was collapsed at the root of the thread by over-torquing, and shows a constriction or "thread choke", pulling the barrel, setting back and refitting is the best fix.

The most accurate PPC revolver I ever had was an S&W Model 14 Target Masterpiece in .38 Special, which had .3585" cylinder throats, which had been fitted by Bill Davis with a 6" Colt Python barrel with 14" twist and a barrel groove diameter of .355". That gun would shoot 12-shot machine rest groups under 2" at 50 yards with any US make 148-grain HBWC wadcutter ammunition. The best lots would shoot under 1-1/2" for 12 shots at 50 yards from a Broadway machine rest.

I would not be at all concerned shooting cast lubricated lead .358" bullets in .3575" throats into a .355" barrel if everything was lined up correctly.

Nor would I be overly concerned shooting similar cast lead lubricated .360" bullets into .359" throats with the same .355" barrel, as long as the barrel forcing cone was 11 degrees or less. Jim Clark cut many PPC guns with 6 degree forcing cones and they were tack drivers. You will see some Ruger GP100s turned out that way also.

[DougGuy]

The problem with a .38 or .357 caliber revolver isn't the throats, it's the chamber right behind the chamfer in the end of the chamber. Ruger had a nagging habit of using a reamer until it just wouldn't cut any more, which by the end of it's cycle was pretty worn down in size and it cuts the chambers themselves smaller. I had to buy a .357 finishing reamer to "fix" one customer's cylinder so all 6 holes would chamber a .3585" boolit seated and crimped normally.

If you have any doubt about what your cylinder will chamber, say if you are curious as to will it chamber a .359 boolit seated in the case, you can seat the boolit backwards and push it down flush with the case mouth and then crimp it. This will tell you, BEFORE you ream the cylinder throats, if the chambers themselves will accept a round loaded with that boolit. You can't check for this, with a boolit seated normally, IF the boolit will not seat in the cylinder throat. You seat it backwards in a dummy round to make a go/no-go gauge out of it so that the throat diameter does not come into play while checking the chamber.

I have good success with .3585" cylinder throats, .358" boolits, even with a 5° forcing cone this works if the forcing cone is concentric and done right and is smooth of tool marks. I also have had good luck with .360" boolits in guns that would chamber them, as a companion to a Marlin rifle in the same caliber that won't hit the side of a barn with anything smaller than .360" so it just goes to show that none of this is really cast in stone, and with cast boolits it's a combination of things that make it work and shoot accurately, the throats being one of the most important components of that combo. It is more important that the throats be -even- in size than what size they actually are, because you can size a boolit to fit the throats OR with enough pressure like a magnum load and a soft enough alloy, you can let the pressure slug the boolit up to fit the throats which they will gladly do without issue.

J-word bullets will obturate too. Generally the core in a j-word is dead soft swaged lead and there is less of a pressure spike to fire them through tight throats or a thread constriction. This is why there is no degradation of accuracy to fire a .357" j-word through .3585" throats because by the time the boolit gets into the throats, it's already bumped up as far as the throat will let it.

The above may not be true with a .38 Special, a .44 Special, or .45 Colt using standard pressures because there isn't enough pressure to cause obturation in the throats. Also, if the throats are excessively large, as in .005" or .006" greater than boolit or groove diameter, the gun may never shoot well until a new cylinder is fitted with smaller throats. Ask me how I know this one...

[bcp477]

Hmmm. All of this is interesting. I am trying to make sense of the "rules" regarding revolver accuracy, but I admit to being a bit confused. Somebody correct me if I am wrong, but it seems that......

A) Best accuracy is obtained when the cylinder throats are sized so that the chosen boolit dia. just barely slides through, with absolutely no wiggle room. Very slightly oversized boolits (0.005" or something like that) are acceptable...but not much larger.

B) The groove dia. of the barrel must always be smaller than the dia. of the cylinder throats. If possible, not severely smaller....but smaller.

C) The forcing cone needs to be smooth, concentric and cleanly cut, with no damage to the beginning of the rifling lands just beyond.

D) The forcing cone angle is tricky - a lesser angle, such as 5 degrees, is ultimately better for accuracy - BUT, can be difficult sometimes, unless only jacketed or plated bullets are used. The narrower the FC angle, the more perfect the timing of the revolver must be. With cast, a wider angle may be better, such as 10-11 degrees.

As I said, I am still trying to get all this straight in my mind. I gave up on the Ruger SP101's I had recently, because they seemed to be too touchy as regards getting cast to shoot well, without FC leading. Even after getting the chamber throats right and polishing the FC's....the problem still wouldn't go away. Especially with the 2" 357 mag one I had. I had a 3" 38 spl. one, which had a very different FC profile (much shorter)...and it was better, but still not to my liking. The 2" 357 FC was a mile long. Both were 5 degrees, or thereabouts. Probably, I would have been better off if I'd had the FC's opened to 10-11 degrees....but I didn't.

I ended up getting another K-frame Smith, to complement the 10-8 I already had. They both have 10 degree FC's....and they shoot ANYTHING well. The chamber throat/ barrel dia. sizes are nice as is, very consistent for all 6 chambers, too. The FC's are not smooth - nevertheless, they both shoot like wildfire with no leading issues.

The "outlier" for me (and a source of confusion) is a slightly used Smith 442 I picked up as a sometimes carry gun. That little thing has a horrible FC - HUGE angle, probably 18 degrees or more - and ROUGH as heck. The lead in to the rifling is even damaged (a couple of the lands have machine marks across them). Overall, it looks as if the FC was cut with a ROCK. The chamber throats on the 442, however, are nearly perfect - all 5 are very consistent and the dia. is a perfect fit for 0.357" dia. boolits. The groove dia. of the barrel is a bit more than 0.355" Regardless, the 442 shoots very well - not a trace of leading, quite accurate and it gobbles anything I feed it very happily. Even 0.356" dia. boolits do just fine in it.

Go figure.

[tazman]

Your A,B,C,and D are essentially correct. Each gun makes it's own rules, so there will be exceptions to this, but in general those statements are correct.
As you have found, some guns just shoot well.

[Char-Gar]

I am not an engineering type and all of this stuff that involves numbers seems to go in one ear and out the other. I have shot cast bullets in revolvers a right smart amount though. I would just uniform my cylinder as you suggested and shoot the darn thing. I doubt you would be disappointed with either cast or jacketed.

Do anybody around here still shoot those dreadful little yellow thingies anyway? If they do, they should consider stopping that silliness.

[tazman]

I think the yellow color is from the powder coat people wanting zombie killers.