Chamber throat diameter vs boolit size

[AggiePharmD]

Okay so I understand that you want the throats for a 41 caliber cylinder to normally be .4125 but does this preclude the use of .4115 boolit diameters? What happens if the boolit isn't sized to the .412? Asking for a friend.😁

[Tatume]

Okay so I understand that you want the throats for a 41 caliber cylinder to normally be .4125 but does this preclude the use of .4115 boolit diameters? What happens if the boolit isn't sized to the .412? Asking for a friend.��

Not necessarily. My 41 Mag Blackhawk has chamber throat diameters of 0.410+", and my S&W Model 57 has 0.411" throats. Both are greater than or equal to the 0.410" barrel groove diameter (the Blackhawk by only a couple of tenthousanths). Both shoot very well with 0.410" cast bullets. The Blackhawk is the most accurate single action revolver I own.

If your cast bullets are 0.001" smaller than throat diameter they should shoot well, provided the throat diameter is greater than or equal to groove diameter.

[243winxb]

I size to the barrels groove diameter. Good advice above.

[salvadore]

Really, I always size to the throat. Any expert opinions?

[Tatume]

Yes, several in this thread.

[DougGuy]

In a perfect world, a revolver should have the boolit sized .001" to .002" greater than groove diameter, throat size should be .0005" to .001" greater than boolit diameter.

This works for all centerfire calibers regardless of maker.

Having the boolit at least .001" greater than groove diameter ensures there is enough boolit material to swage into the rifling and make a good seal, prevent gas cutting and leading, etc..

Throat diameters that let the boolit pass through with a light drag fit ensure the boolit is not sized down as it is fired, so you deliver the boolit to the bore as intended.

Cylinders with throats that are smaller than boolit diameter are merely a multi port sizing die.

There are no mathematical exceptions to this logic, but there are workarounds. One workaround is using an alloy soft enough that it obturates or bumps up to fill the bore after leaving the cylinder, jacketed factory ammo works very well for this because for a pistol caliber those are generally dead soft lead cores swaged into a gilding metal jacket. These will gladly squeeze down to go through an undersized throat, and then IF there is sufficient pressure, they will bump back up once leaving the throats and make a decent seal in the bore. Of course, within limits, i.e. couldn't expect a difference of several thousandths to seal with standard pressure loads, where magnum loads and Ruger Only loads will definitely bump up even a ridiculous amount of difference.

Cast loads, if they are soft enough, will also bump up to fill the throats, example a 44-40 cylinder with .445" necks will only accept a boolit of .431" at the largest or it won't chamber. You can load a .430" boolit so it fits in the chambers, and hone throats to .431" or .4315" and the boolit will bump to throat diameter, and you will then present a boolit large enough to seal in the .429" bore, be accurate and not lead the bore, yet it started out sized to .430" which will gladly chamber in a cylinder with a .445" neck. Alloy has to be fairly soft for this to work but it works great.


I size to the barrels groove diameter. Good advice above.

^^ This is an often overlooked part of handloading. You can size to groove diameter, but if cylinder throats are smaller, you are sizing the boolit down to throat diameter each time a round is fired. You cannot fire a .430" boolit through a .429" throat and expect it to remain .430" once it enters the throat.


Okay so I understand that you want the throats for a 41 caliber cylinder to normally be .4125 but does this preclude the use of .4115 boolit diameters? What happens if the boolit isn't sized to the .412? Asking for a friend.��

I often size 41 cylinder throats to .4115" *IF* none of the throats are already larger than this diameter. I see no reason to arbitrarily choose .412" unless the shooter prefers that diameter boolit, maybe they only have a .412" sizing die, or they have several thousand boolits already cast and sized to .412" or maybe they have a companion lever gun that likes .412" and they want to use the same boolit for a revolver as well as the rifle.

Afaik, there is not any real advantage to going .002" over groove diameter as opposed to .001" although some guns will shoot a .412" boolit through a .4115" throat more accurately than they will shoot a .411" through a .4115" throat, it is often said to let the gun tell you what it wants and I subscribe to this theory as well as I subscribe to the mathematical relationship between groove, boolit, and throat diameters. Some things just don't conform to logic. You have to shoot the gun, try different things, and go with what works best.

[str8wal]

What is more important is throat diameter relative to barrel groove diameter.

[Larry Gibson]

Have offer an alternate to the perfect world sizing. The three most accurate revolvers I have are a Colt Anaconda 44 Magnum, a Ruger Bisley 41 magnum and a Ruger OM Vaquero 44-40.

The Anaconda has a .429 groove barrel and .429 cylinder throats. I size the bullets .429.

The Ruger Bisley has a .410 groove barrel and .410 cylinder throats.

The Ruger OM vaquero has a .429 groove barrel and .429 cylinder throats. I size the bullets .429.

All three of those revolvers shot extremely well, better than I can any more, "out of the box" with nothing needing to be done to "fix" anything.....

Conversely I have other revolvers with cylinder throat diameters .001 - .002.5" over the groove diameter. Sizing cast bullets at .001 - .002 over groove diameter is what I use regardless of whether or not it is of cylinder throat diameter in those revolvers. I also have a couple revolvers with throats .003 - .004" over barrel groove diameter. Having thoroughly tested cast bullets sized to throat diameter in the larger cylinder throats I have found bullets sized .003 + over barrel groove diameter do not really improve accuracy over those sized .002 over groove diameter. What I did find was the hardness of the alloy along with the use of a soft lube had a greater affect on accuracy than did the sizing to throat diameter.

As to leading I've found using a correct alloy, a proper lube and the choice of powder to be more the solution to leading than is sizing to fit the cylinder throats.....as long as the bullet is not smaller thean the barrel groove diameter to begin with.

I realize the above is heresy on this forum these days but it is what I found. To quote the other Larry from Midway...…"and that's the way it is"......

[Bass Ackward]

This is a first. I agree with Larry on everything except the part about “hardness & lube” being the limiting factor. Bullet design also plays a part. The WORST mix to requiring sizing from cylinder to groove diameter is ANY water dropped mix that age softens to ANY hardness. My “guess” is mix uniformity resulting in bullet balance issues. The best? ANY pure lead / tin mix that fulfills load bullet design requirements. That may get you to .004 before load flexibility / limitations develop. And don’t refer to hardness with L /T mixes cause that’s not why they work to prevent leading. Performance continues to improve long after hardness peaks.

[Shuz]

I just started powder coating boolits for my .44 revolvers so what I am about to say should be taken with several grains of salt.
I have 3 Smith .44's that have .4285" throats as best I can measure without pin gauges. In all 3 revolvers I used to get a slight lead wash in the forcing cone area when using Saeco 7 to 9, .429 sized boolits, WFFL, and Green Dot or Unique powders. It has always bothered me some, altho accuracy did not seem to suffer, but then again I did not let the leading build up to the point where it would matter. I simply fired a few, usually 5 gas checked 429215's sized .429 to .431 and 19.7g of WC 820 and the leading wash disappeared. Now enter powder coating.......MP 432-423HP's coated with Smokes Jet Black and sized to .429 after coating, shot with 8g of Green Dot in all three revolvers have shown absolutely no lead wash! Accuracy was every bit as good as I've experienced down thru the years. More testing with my other .44 mag revolvers and rifles to follow.YMMV.

[DougGuy]

The Anaconda has a .429 groove barrel and .429 cylinder throats. I size the bullets .429.

The Ruger Bisley has a .410 groove barrel and .410 cylinder throats.

The Ruger OM vaquero has a .429 groove barrel and .429 cylinder throats. I size the bullets .429.

All three of those revolvers shot extremely well, better than I can any more, "out of the box" with nothing needing to be done to "fix" anything.....

I realize the above is heresy on this forum these days but it is what I found.

I wouldn't consider any of that heresy.. Except maybe the bullet word that might be heresy, I dunno, have to thunk on that one... :roll:

The three revolvers you have listed there all but the 44-40 run with enough pressure that .429" is probably swaged into the rifling equally as firm as a .430" or .431" would be, so that may be somewhat of a moot point considering the pressure, and the 44-40 having .429" throats is either luck or someone worked that cylinder over, possibly at Ruger if you bought it new. All of those I have seen will barely take a .427" pin and many will only let a .425" pin through the throats.

My only question would be what do you do to address age hardening which generally promotes growth in diameter by a few tenths? Boolits sized to exact throat diameter then assembled into loaded ammo often will not fully chamber after sitting a few months. I ran into this problem with the Lee C430-310-RF boolit sized .432" it is quite long and must enter the throat to chamber in the cylinder, half of mine wouldn't until I went from .432" to .4325" with the throats.

[Larry Gibson]

Come on Bass.....we have agreed before.....mostly agreed to disagree......:drinks:

[Larry Gibson]

I wouldn't consider any of that heresy.. Except maybe the bullet word that might be heresy, I dunno, have to thunk on that one... :roll:

The three revolvers you have listed there all but the 44-40 run with enough pressure that .429" is probably swaged into the rifling equally as firm as a .430" or .431" would be, so that may be somewhat of a moot point considering the pressure, and the 44-40 having .429" throats is either luck or someone worked that cylinder over, possibly at Ruger if you bought it new. All of those I have seen will barely take a .427" pin and many will only let a .425" pin through the throats.

My only question would be what do you do to address age hardening which generally promotes growth in diameter by a few tenths? Boolits sized to exact throat diameter then assembled into loaded ammo often will not fully chamber after sitting a few months...…..

I've really not been affected by that problem. I cast the bullets in quantity and size/lube the bullets just before loading. Once loaded I've not seen any "growth" even after a year of two that might hinder chambering.

[Good Cheer]

As always, what works for one isn't what does for another.

By the way, I have a .41 that .412 diameter boolits will create an interference fit, the brass hanging up in the chambers.

[megasupermagnum]

My model 57-1 is an odd duck. It has around .4105" throats, but the groove diameter is about .408". So I've been sizing to .410"

[salvadore]

Back in the early 80s I owned a SAA Colt in .44 spec. Groove dia. of .427, throats @ .434+. Casting to the throats gave me some leading, but anythng less than .433 was a nightmare. Since then I cast to the throats, seldom check groove dia. except for rifles. Btw, I still own that shooter but is now a .45 Colt. Also I woud not argue with Doug Guy concerning this issue.

[murf205]

There really is no right or wrong way until you shoot the gun and let it tell you what it wants. As Doug says, a perfect world would have a boolit .002-.003 larger than groove diameter. Larry Gibson sizes his most accurate revolvers to match throat and groove diameters. I have a friend who has a 7 1/2" SRH Ruger with .451 throats and a .453 barrel. We size his boolits to .451 (might as well) and they don't lead at all and it is very accurate. The mix is range lead + 2% tin and he can't spell "starting loads", he likes 'em hot.

[megasupermagnum]

There really is no right or wrong way until you shoot the gun and let it tell you what it wants. As Doug says, a perfect world would have a boolit .002-.003 larger than groove diameter. Larry Gibson sizes his most accurate revolvers to match throat and groove diameters. I have a friend who has a 7 1/2" SRH Ruger with .451 throats and a .453 barrel. We size his boolits to .451 (might as well) and they don't lead at all and it is very accurate. The mix is range lead + 2% tin and he can't spell "starting loads", he likes 'em hot.

Soft lead with a stout load is likely the key. If they don't fit, squish them to fit.

[DougGuy]

I have a friend who has a 7 1/2" SRH Ruger with .451 throats and a .453 barrel. We size his boolits to .451 (might as well) and they don't lead at all and it is very accurate. The mix is range lead + 2% tin and he can't spell "starting loads", he likes 'em hot.

Two things that physics dictate.. One, you cannot shoot a larger boolit through a smaller throat and expect it to come out the same size you loaded it, it will come out throat diameter. Two, a soft enough alloy will obturate to throat diameter if it is smaller, i.e. a .430" boolit fired over a full house load of 2400 into a .4315" throat, it will definitely leave the front of the cylinder at throat diameter. The SAME occurs when you fire that softer alloy through a tight throat and there is STILL enough pressure to obturate it to the bore once it clears the forcing cone. You could in theory load a .451" cast in lets say 50/50+2%, fire it though a .451" throat, and once it clears the throat it is still being subjected to enough pressure to bump it up to seal in the .453" bore. Without leading. Accurate. J words will do the very same thing.

I had a 4 5/8" Vaquero that had a horrible thread choke, down to .449" best I could measure, throats were horribly tight as well. I used a 340gr SSK truncated cone hard cast boolit over 23gr W296, it shot several inches higher than the 300gr XTP boolit loaded over the same charge of 296. The interesting thing was after I reamed the cylinder throats and Taylor throated the choke out of the barrel, the XTP loads retained the SAME point of impact as they did before the changes, and the SSK cast boolits dropped 4" in point of impact at 25 yards.

That tells me that the J words were clearing the throats and the thread choke, and were then sealing in the barrel once past the choke, this is the only explanation for them shooting to the same point of impact. The SSK changed point of impact because the extreme resistance from the throats and the choke was removed, and they spent much less dwell time in the barrel.


Soft lead with a stout load is likely the key. If they don't fit, squish them to fit.

LOL we typed the same thing at the same time, more or less...

[bigted]

That surprises me that your jacketed bullets would obturate down barrel past a thread constriction. Must be a pretty stout load to cause this to take place.

[onelight]

There really is no right or wrong way until you shoot the gun and let it tell you what it wants. As Doug says, a perfect world would have a boolit .002-.003 larger than groove diameter. Larry Gibson sizes his most accurate revolvers to match throat and groove diameters. I have a friend who has a 7 1/2" SRH Ruger with .451 throats and a .453 barrel. We size his boolits to .451 (might as well) and they don't lead at all and it is very accurate. The mix is range lead + 2% tin and he can't spell "starting loads", he likes 'em hot.
I am with murf205 ....find out if you have a problem before trying to fix it.

[murf205]

Like megasuper and smoke said, if you boot 'em hard enough, they will squish to fit. In the case of the 454 Cassull, enough= 60k pounds of pressure. That will obturate a LOT of stuff! Now does that mean it will be accurate just because you kicked it hard enough to not lead? Good question, that's why you have to shoot first and adjust later. My friend is just lucky that his SRH is accurate with the measurments he has in his gun.