Cylinder Throat Size vs Bore Size

[Werewolf]

I've got a question regarding revolver cylinder throat size vs bore size.

To make a long story short most of what I've gleaned from various sources (mostly from reading newsgroups) says that the cylinder throat (exit side) diameter should be at least 5 tenths (.0005") bigger than the bore size.

1) Is the above reasonable and if so are there any sources (i.e. books, magazine articles etc) to back that spec up and what are they?

2) Assuming the spec is correct why then would Ruger consistently make the cylinder throats .003 less than bore diameter (again - this is what I've picked up from various forums and newsgroups and based upon using an inside mic to measure cylinder throats on my own Rugers).

3) I've got two Ruger Vaqueros. Both have cylider throat diameters that run .449 to .450 and the barrel bore on both is .452. Should I bother getting the cylinders reamed to .4525?

 

[BigG]

It stands to reason the throats have to be larger than bore diameter so the cartridges will drop in the cylinder. The cartridges should slide back and forth freely so there is no chance of cylinder binding up. Now, the exact specification for the size of the throats - ya got me.

 

[HSMITH]

If you shoot lead bullets then YES get it reamed!!!!! Leading will drop dramatically and accuracy will improve as well. Throats should be at LEAST 5 tenths over bore diameter, and 2 or 3 thou over is not going to hurt a thing. I would have them reamed to .454" myself making typical 45LC bullets the same diameter as the throats, it would also make bullet sizing less critical for top accuracy. With jacketed bullets this is all a lot less critical.

I would call Ruger, tell them about what you found and tell them to send a call tag and a box for both guns. Let Ruger fix what they did not do right in the first place.

 

[C.R.Sam]

Tight throat can put a fin of extruded metal on the base of the bullet.
Can allow gasses to go past the bullet in the bore.
Raise pressure a tad.
Cause leading.
Cause copper fouling.
Destroy accuracy for all the above reasons.

Sam

 

[Jim K]

Maybe I don't understand the terms as used here, but both the bullet and the cylinder throat should be GROOVE diameter, not bore diameter.

Jim

 

[Werewolf]

Jim Keenan said:

Maybe I don't understand the terms as used here, but both the bullet and the cylinder throat should be GROOVE diameter, not bore diameter.

Jim

If by groove diameter you mean the widest diameter of the bore (i.e. not the rifling diameter - example 45 Colt bore is .452 - the diameter if measured across the rifling is considerably less) then you are correct in your use of the *term* groove diameter. FWIW in thirty years of being around weapons bore diameter has always been the term I've heard used. To be honest I've not heard groove diameter used very often though I have heard it on occasion.

I asked the question because in 30 years of shooting rifles (a 105mm one when I was a tanker in the Army) and pistols I never once owned a revolver so was unfamiliar with cylinder specs vs bore specs.

 

[BigG]

Whatever the semantics, the cylinder throat has to be larger than the bullet diameter which is usually groove diameter or a tad larger. The catridges should slide back and forth with in their headspace limits so the cylinder does not bind.

 

[Jim K]

If the cylinder throat is larger than the bullet/groove diameter, the bullet will upset in the throat and have to be squeezed back down to groove diameter, which causes some distortion.

Ideally, for a groove diameter of, say, .452, both bullet OD and cylinder throat ID should be .452. In the real world, such precision isn't possible in mass production of either guns or ammo.

Hi, Werewolf,

Yes, the bore diameter is the diameter of the hole bored and reamed in the barrel before the rifling grooves are cut. Bore diameter and land diameter are the same. For example, in the U.S. .30 and 7.62mm rifles, the bore diameter is .300, while the bullet diameter and groove diameter is .308. This means that the groove depth is .004.

In a breechloading rifle, the bullet is always groove diameter and is forced into the barrel so that the lands cut into the bullet and cause it to spin. In a muzzle loader, the bullet must be smaller than the bore (land) diameter so it can be pushed down to the powder charge; the pressure from the powder gas expands it into the grooves.

Jim

 

[Old Pete]

Most of the recent discussion on this topic concerns the undersized throats in .45 Colt revolvers. There is a much more common problem with undersized cylinder throats in S&W J-frame .38 Special revolvers. Apparently, there are hundreds of thousands of J-frames out there with nominal 0.3555" throats, about 0.001" smaller than the barrel groove diameter.

I started a thread on this topic about a month ago on the S&W Forum (in Revolvers 1945-Present), but didn't get much response. The search engine doesn't work for that section of the S&W Forum, but those interested can go directly to the thread at the following URL:

http://www.smith-wessonforum.com/ubb/Forum13/HTML/007541.html

Pete

 

[Jim K]

I know what I said about the ideal, but I would consider a throat diameter .001 smaller than the groove diameter to be nothing to worry about, especially in a J frame. I have seen throats on S&W and other makes of revolvers that were too small/too large by a lot more than that.

Jim

 

[Old Pete]

Jim Keenan - Jacketed bullets for the .38 Special are usually 0.357", and hard cast bullets run 0.357" or 0.358". Would you be concerned about the increase in chamber pressure involved in "dynamically resizing" these hard bullets to 0.355" in the chamber throats? In the case of hard cast bullets, will gas cutting in the barrel (around the now undersized bullets) lead to barrel leading? Would sizing the bullets for my J-frame .38 Specials to 0.356" be advisable? Thanks.

Pete

 

[Jim K]

I don't think firing bullets .001 or so oversize for the chamber throat will cause any real problem, though it might not help accuracy since the barrel is presumably .357 groove diameter. It would not be like, say, a throat running .008-010 undersize as was being discussed in reference to .45 revolvers (a .449 throat and a .452-.454 bore diameter, which would mean a groove diameter of around .460).

Gas cutting of the barrel cone and the topstrap will occur regardless of the throat diameter. You could, in theory, get gas cutting in the barrel if the bullets did not fill the grooves, but that only happens with thick jackets (like the .45 GI ball). Most pistol bullets have thin jackets and are extremely elastic under even moderate pressures.

My statement about the J frame was simply because very few people shoot a J frame enough to worry about fine accuracy or even a little erosion. Target revolvers used to have problems over many years and thousands of rounds, even with lead wadcutters, but I doubt many J frames get that kind of treatment. Guns I experimented with in .357 and .44 got gas cutting on the bottom of the top strap, but S&W assured me it went only so far and stopped, and I found that to be true, or at least none ever came apart. (S&W used to put a dish-shaped cut in the topstrap at that point, but dropped it as one of their early cost-cutting measures.)

HTH

Jim