Tell me about cylinder throats...

[fireman 9731]

First off, I'm fairly new to revolvers and know just enough to get me in trouble...

Over the past year I have been bitten by the Smith and Wesson bug. 38 Special has become one of my new favorite cartridges and I have been reloading it for a few months now. I have been reloading and casting other calibers for about 8 years now.

Up until this past week, I have been reloading the Hornady 158gr .358 swaged semi-wadcutters over light loads of Trail Boss. I have had minimal leading.

Well, being that making your own bullets is the only way to do it affordably anymore I bought a Lee 158gr .358 Tumble lube mold and cranked out a few hundred bullets over the weekend.

It was all going well until I thought I would try sizing a few just to see how much they needed it. I was using a Lee .357 sizing die and it was very difficult to push the bullets through. After some research I discovered that I should be using a .358 sizing die instead. I also began researching the relationship between chamber throats, bullet diameter, leading, and accuracy.

So naturally, out come the calipers and I start measuring. I know that all calipers are different, and my cheap digital one has its limitations, but it does show consistency. I know I should also be using pin gauges to measure cylinder throats but my calipers gave me, perhaps not accurate, but consistent results.

As best I can tell, my cast bullets measured .360.
The chamber throats on my Model 28-2 measured .357, My 10-5 measured .357, my model 36 measured .355ish, An old Rossi measured .360, and the wife's Taurus measured .360 (at least one chamber was out of round, and the barrel has severe leading problems, but thats another story)

I know I should slug the bores to see the whole picture but I really don't want to. I tried before with my Mosin and had bad luck.

I also know that I should load a few rounds up and actually try them out before jumping to conclusions.

So I guess my questions are:

What should my throat dimensions actually be?

Should I ream them to make them all consistent?

Or should I size my bullets differently to make up for it?

Is a .360 bullet too big to send through a .357 throat?

Bullets with be tumble lubed with Lee Alox. I also picked up some powder coat this weekend and plan on giving it a try too.

Also, mods if this should be in the reloading or gunsmithing section feel free to move.

 

[Jim K]

The easy answer is that the ID of the throat should be the same as the OD of the bullet, and that should be the same as the groove diameter of the barrel. But a fair amount of tolerance is acceptable where accuracy is not a major consideration (i.e., in most ordinary revolvers not made for match shooting). If the throat is too big, a lead bullet can expand under pressure in the throat, then be squeezed back down again in the barrel. If it is too small, the bullet is squeezed down and might or might not expand to fill the rifling grooves. Either way, the bullet is distorted and that is not good for accuracy.

Cast bullets out of the mold are generally oversize. Since you will have to lube them anyway, sizing to the right (throat) diameter should be no problem. It is where not all the throats are the same that you get a very frustrating situation.

As to reaming the throats, my advice is not to even think about it unless you are an experienced revolver smith and have the right tools.

Jim

 

[murf]

if it ain't broke, don't fix it. if the bullets go where you want them and the barrel doesn't lead, don't worry about it.

murf

 

[45_auto]

I know I should slug the bores to see the whole picture but I really don't want to. I tried before with my Mosin and had bad luck.

...

So I guess my questions are:

...

Should I ream them to make them all consistent?

You have trouble slugging a bore but you're considering reaming your revolver throats after measuring them with a cheap caliper?

An honest evaluation of your gun smithing equipment and skills may be in order.

Most revolvers are capable of MUCH better accuracy than the shooter. Have you ever fired a KNOWN accurate weapon (revolver or semi) for record and compared your results with what a good shooter can achieve? You may possibly be wasting your time and risking screwing up your guns for nothing if you don't have the necessary skills to take advantage of any theoretical accuracy improvements.

 

[edwardware]

Generally, lead bullets should be 0.001-0.002 over the groove diameter when they arrive at the forcing cone, so they, and the throat, must be sized to accomplish this.

Also, you'll get a much more accurate throat measurement by driving an oversized slug through the throat and measure that.

Ultimately, there's no need to change anything if your setup is working. . . unless this is your hobby and you're doing it for fun!

 

[TimSr]

if it ain't broke, don't fix it. if the bullets go where you want them and the barrel doesn't lead, don't worry about it.

Agreed. Size to the standard .358, and try them in all your guns. THEN decide if you actually have a problem to solve with custom bullets in particular guns, or gunsmithing. Standard .358" cast bullets will work just fine most of the time.

Often people overthink cast bullets way too much, before they ever run the first test.

 

[fireman 9731]

An honest evaluation of your gun smithing equipment and skills may be in order.

I am certainly no gunsmith. I have pretty good mechanical aptitude and enjoy tinkering with guns. I feel pretty confident that a I could ream a couple thousandths out of a chamber with some basic tools and not screw anything up doing it. But these answers are exactly what I was looking for. Like I said, I know just enough to get me into trouble. After reading Jerry Kuhnhausen's shop manual over the weekend, the notion of a "simple" revolver went out the window.

Seems that the general consensus is to size to .358, shoot them to see how they perform, and go from there. I'm not looking for precision accuracy, I just want an easy plinking load with minimal leading.

 

[reddog81]

I'd try sizing to .358. You should be able to push the bullets through the throat with a little force.

Next I'd drop the tumble lube bullets and move to powder coating. I tried tumble lube when starting out and had limited success in 38 special, but in 9mm & 45 had terrible leading. I tried powder coating and now wouldn't even think about tumble lube. Also the tumble lube molds aren't something I will ever buy again. I have achieved better accuracy with traditional molds even when using TL.

For the Taurus and Rossi I would use unsized .360 diameter bullets. Pushing a .360 bullet through a .357 chamber probably shouldn't cause problems with 38 special but I doubt it's going to be as accurate as a properly sized bullet.

 

[gwpercle]

Put a thin film of alox lube on them before running them through the sizer , and after sizing , give them another thin coat.
Thin the Lee liquid alox with mineral spirits, you want a thin even varnish coat and if too thick the stuff stays sticky.
I size mine .358 and have had success with it in S&W and Ruger revolvers . Never got around to slugging bores or throats.
The tumble lube design is not Known for accuracy , but can be made to work with a bit of trial and error experimentation.
Gary

 

[Vern Humphrey]

Originally posted by murf

if it ain't broke, don't fix it. if the bullets go where you want them and the barrel doesn't lead, don't worry about it.

Absolutely right. Work up some loads and see how well they shoot.

In general, a revolver should get smaller as it goes forward. According to this rule, a .3585 - .359 bullet, shot through a .358 throat, would be ideal for a .357 barrel. But not always.

So for most of your guns, your bullets should be fine as-cast. Use SOFT lead -- if you're casting from wheel weights, DO NOT drop the hot bullets into water -- that will make them too hard.

You may find that some of your guns (those with smaller throats) need a sized bullet, those with larger throats shoot better with unsized bullets.

 

[fireman 9731]

if you're casting from wheel weights, DO NOT drop the hot bullets into water

Well, I already did that... I had a mix of WW, range lead, some stick on wheel weights and a scoop of magnum shot. I really need to get a hardness tester. I have to say though, these were the prettiest bullets I have cast yet.

 

[Driftwood Johnson]

Howdy

As already stated, lead bullets should be .001 to .002 larger in diameter that the rifling grooves.

But the problem with slugging a S&W barrel is they usually have five grooves, and with an odd number of grooves it is difficult to get an exact measurement of the groove diameter. When you lay your caliper across the slug, you will be measuring from land to groove. Then to get the groove to groove diameter you have to add the depth of one groove. This is difficult to do accurately. There are ways to do it, but it involves a special V block and some calculations. Measuring a slug with an even number of grooves is easy, you just measure across the high points on the slug.

As stated, ideally, chamber throats should be the same diameter as the rifling grooves. Then, the bullet should be a nice slip fit, or maybe a slight interference fit in the chamber throats. The idea is if the chamber throats are too large in diameter, hot exhaust gasses can get around the bullet and soften its sides. Then you get leading. You do not want the exhaust gasses to leak around the bullet, they should stay behind the bullet as much as possible.

A caliper is the wrong tool to use for measuring the inside diameter of small holes, such as 38 caliber chamber throats. The inside measuring jaws usually have small flats on them. The flats will partially bridge the diameter, giving an erroneous measurement.

If you don't have access to a set of pin gauges, a set of small hole gauges is very useful for measuring revolver chamber throats. You insert the gauge, screw the handle in until the gauge contacts the diameter of the hole, then remove the gauge and measure it with your calipers. These can be very accurate and you can make multiple measurements to get a good average.

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Just for the fun of it, I grabbed three different S&W 38s out of my safe a few minutes ago, and measured the chamber throats.

One gun is an old 38 Military and Police made in 1908, another is a K-38 made in 1953, and the third is a Model 14-3 made in 1974. I have others, but these are what I grabbed.

The 1908 M&P and the 1953 K-38 both had chamber throats measuring .359. The Model 14 had throats measuring .358.

Very generally speaking, if you open the cylinder and place a bullet into a chamber, the chamber will tell you the correct diameter bullet it wants. Point the gun at the floor and drop a bullet in a chamber. If it falls right through, it is too small. If it hangs up but can be pushed through with a gentle push of a pencil, it is perfect. If it takes a lot of force to shove it through, it may be a bit too large.

I have been loading 38 Special for a long time, I always use hard cast bullets sized to .358. These bullet hung up perfectly in the two older guns (nothing is ever perfectly round). They were a little bit harder to push through the Model 14.

As I said, personally I always use .358 hard cast bullets for 38 Special in all my Smiths and Colts.

 

[fireman 9731]

Thanks again for all the information. I tried passing some .357 sized bullets through the chambers last night. On my Model 28-2 and 10-5 they dropped through. A .358 was a push fit. On the larger chambered Taurus and Rossi, they rattled through. I'm not concerned about them though, the Taurus is the wife's and will likely get traded in soon and the Rossi is a beater and will be my guinea pig for revolversmithing.

What does make me scratch my head some though is my Model 36. I couldn't push a .357 bullet through it, not even close. I have read of similar tight throats and bores on model 36s though.

And as far as bullet hardness goes... these seem pretty hard. Much harder than a swaged bullet. If they don't perform well they will just go back into the pot with some pure lead to soften them up.

 

[Driftwood Johnson]

Howdy Again

You are over thinking this.

Chamber throat diameters, just like anything else, can vary slightly as tool wear sets in over the course of a production run. This will depend on how often the tools are checked and replaced. Of course, the chambers of one gun should all be the same diameter.

I just measured some more chambers.

Old Model 36 Flat Latch - .358
Model 36 - .356
Model 60 (Stainless version of the Model 36) - .357
Model 19-3 - .358
Model 28-2 - .358
Another Model 28-2 - .358.

You want a hard bullet. Lead bullets are an interference fit in the rifling. So heat is generated by the friction created as the bullet rubs its way down the barrel. The softer the bullet (or more precisely the closer to pure lead) the lower the melting point, and the more likely the sides will soften and the lead may adhere to the barrel, causing leading. The tendency for a soft bullet to lead the barrel will increase as velocity increases because friction increases. Tin is added to lead to harden the bullet, antimony is added to make the alloy flow and fill up the mold better. I use pure lead in some of my Black Powder bullets, but that is a different story.

I don't bother casting my own Smokeless bullets, quality bullets are easily available and casting is a pain in the neck. Commercial hard cast bullets will come with a hard, crayon like bullet lube in the lube groove which lubricates the bullet as it slides down the bore, reducing the heat caused by friction. As I said before, I have been using commercial, hard cast bullets, sized to .358, in all the guns I just mentioned and quite a few more, for a long time now.

 

[Vern Humphrey]

I have shot a lot of swaged pure lead bullets, and a lot more wheel-weight, unhardened bullets. My experience is that hard bullets are more likely to lead, because they fail to upset and seal the bore. Gas-cutting occurs, which leaves lead in the bore.

 

[Walkalong]

Tin will slightly harden lead alloy, but its main function, and worth, is helping the alloy fill out the mold better. Antimony is used for hardening the alloy.

Too hard can be as bad as too soft.

But yes, many people over think it. Shoot some commercial .358 cast bullets and see if they lead. If they don't, your golden. If they do, a switch in hardness may cure it. If you try say 12BHN and 18BHN and they both lead, then you need to look at throat sizes, groove diameters etc.

If you are choosing a diameter, then match them to the cylinder throats. A tight slip fit is best. And of course the throats need to be bigger than the groove diameter by .001 or .002. Even .003 is OK.

The biggest problem you could run into is throats smaller than the groove diameter, and then you will have to have them reamed, or shoot plated or jacketed.