Measuring revolver cylinder throats question

[What Cheer]

I understand that the best way to measure cylinder throats is with pin gauges.

My question is how accurately can the throats be measured using vernier calipers. Has anyone tried this and checked the results of measuring with a caliper by then using pin gauges to determine the relationship.

If so, I would like to know the outcome.

Thanks, and best regards,

What Cheer

[5Shot]

Take an egg sinker that is a bit over or just under the throat size (if under, smash it a bit to upset it and make it bigger), then tap it through the throat and use a dowel to drive it back out. Then use a Micrometer to measure it.

Calipers won't work.

[str8wal]

Calipers won't work.

Calipers work just fine, when measuring the OD of a lead slug. Not always on the money for measuring I/S diameters though.

[5Shot]

They'll work...but they won't be spot on. Calipers are made for length and rough OD Dimensions. Micrometers are going to give you a more accurate OD Dimension on something like this.

[DougGuy]

You can also use a boolit for a gage. If the boolit won't go through the throat with finger pressure, and it is the boolit you have chosen to shoot, then the throat is too small.

Also, if you slug your bore and the boolit won't then go into the front of the cylinder, it is safe to say the throats need reaming.

If you slug the bore and the slug gets VERY hard to drive it through the area of the barrel that is threaded into the frame, then the thread choke will size your slug down below what the rest of the bore is, and you cannot rely on this method to give accurate results.

It is also not uncommon to find both tight cylinder throats and a thread choke on some revolvers.

http://i1202.photobucket.com/albums/bb374/DougGuy/44%20Magnum/1e067e33-3760-4368-961e-e7305cac71d2_zps75482635.jpg (http://s1202.photobucket.com/user/DougGuy/media/44%20Magnum/1e067e33-3760-4368-961e-e7305cac71d2_zps75482635.jpg.html)

[5Shot]

If only we knew someone who could fix the throats...

[5Shot]

BTW - I usually have my bullets sized to pass through the throat with just the slightest bit of slip, mainly because with hard alloys you can't chamber the rounds if they are even the slightest bit too big. My bullets run maybe 0.0005 smaller than the throat. If you drop them through the cylinder they won't fall out the other end, but stop just flush with the end of the cylinder.

So the question is...is that too small? I usually run an alloy that will obturate with the pressure I am running.

[C. Latch]

It is also not uncommon to find both tight cylinder throats and a thread choke on some revolvers.


I'm sitting here wondering whether you were laughing when you posted that.

[DougGuy]

If only we knew someone who could fix the throats...

I bet he was laughing when he posted that one! :bigsmyl2:

[DougGuy]

BTW - I usually have my bullets sized to pass through the throat with just the slightest bit of slip, mainly because with hard alloys you can't chamber the rounds if they are even the slightest bit too big. My bullets run maybe 0.0005 smaller than the throat. If you drop them through the cylinder they won't fall out the other end, but stop just flush with the end of the cylinder.

So the question is...is that too small? I usually run an alloy that will obturate with the pressure I am running.

Have you slugged the bore with a dead soft ball and pushed it into the throats like the photo of my .44 cylinder? That, without any tools other than the lead ball, will tell you how the throats relate to the bore size wise. It will tell you if the throats are smaller than groove diameter if it won't go through the bore and then go through the throats. Of course this test is only accurate if you don't have any thread choke which will swage your measuring ball down smaller than the rest of the bore.

The boolit is supposed to pass through the throat, that is correct. To do this it must be some degree smaller than the throat, we generally accept half a thousandth .0005" as the prescribed difference between the two, and this works. Now, the question remains, is the boolit larger than the bore? At least the same size?

I often have said in a perfect world the boolit would be .001" to .002" larger than bore diameter, and the throats should be .0005" to .001" larger than boolit diameter. Most of us here on castboolits size .001" or .002" over bore.

Another user on this forum, Cbrick sizes his throats the same as the bore, and sizes boolits the same as the throats. This works too. The bottom line is fitment is KING. There are more than one way to achieve this. His may not push through with finger pressure but ignition pressure will certainly push them through, and with throats the same size as the bore you can at least be assured they aren't downsizing the boolits as they are fired. The downside of this method is that you would need near perfect cylinder-to-bore alignment when in lockup, and with the way current revolvers are made, this is at best a 50/50 chance of how well it will align. I stick to my guns on recommending boolits .001" to .002" over groove diameter, and then size the throats to fit the boolit with a light drag fit. This way, I -know- the boolit fits the bore once it is fired.

I have also found it very important to have each throat the same size as throats have a direct affect on pressure, which affects recoil in the shooter's hands, which ultimately affects point of impact of the boolit. Large and small throats on the same cylinder will open up groups from this. The way Ruger used to ream cylinders virtually insured the gun left the factory with three pairs of cylinder throats all different diameters.

They used a Hitachi machine with 3 reamers to punch 3 holes at a time, index over one hole and punch 3 more, and as the reamers wore, they would replace them but not all at once, rather they replaced them one at a time, at random, so the new reamer made larger holes, the other two were in different states of wear so they punched smaller holes. Very common to find a SBH .44 caliber cylinder with two throats measuring .4322" two more measuring .4315" and two more measuring .430" those are the exact measurements of the cylinder in the photo, before reaming and honing to .4325" for all 6. In these cases, it is best to size all the throats to the largest one, then size the boolit accordingly.

Alloys.. The softer they are, the more you can get away with in an ill fitting gun. As pressure increases, the more it will obturate a boolit fired from an ill fitting gun, and the less the shooter will notice things like thread choke and tight cylinder throats. On the other hand when you address the problems with the cylinder and make all the throats even in size, and you cut the forcing cone to 11° and polish it, it tends to improve groups regardless of what alloy or boolit style you use, as this is the best way to 'smith a revolver for shooting cast boolits.

[Larry Gibson]

I understand that the best way to measure cylinder throats is with pin gauges.

My question is how accurately can the throats be measured using vernier calipers. Has anyone tried this and checked the results of measuring with a caliper by then using pin gauges to determine the relationship.

If so, I would like to know the outcome.

Thanks, and best regards,

What Cheer

I have compared the differences between pin gauge measurement, slug measurement and measuring with calipers.

A couple things to consider; first are the cylinder throats tapered or straight? Second are the calipers accurate?

The worst offender and most often encounter problem is with calipers that are not accurate. I have checked numerous calipers over the years and have found many to not give accurate measurements. I have found them to be as much as .004 off when measuring .30 - 45 caliber. If accurate and the throats are straight and the cylinder is held rock solid in a viseyou can be quite accurate, within .001, with a calibrated caliper. That is usually .001 on the small side. With a tapered throat it is real easy to miss measure by .001 - .003.

With a single slug per throat it will only give the smallest dimension and then one must consider "spring back" of the alloy the slug is made of. The larger the diameter the greater the spring back. With tapered throats it is easy to get a cockeyed slug and get a miss measurement depending on the amount of throat taper and method used to push the slug through. Also the skill of the individual at measuring the slug and the accuracy of the micrometer or caliper used is variable, sometimes by a large amount. This is all why we see many "slugged" measurements way outside of the factory's own +/- specifications.

The final method of measurement is the pins gauges. With them you can easily measure both ends of a tapered throat to get a bullet that fits the larger end of the taper. They are the most accurate. However the downside is they will only measure through a hole they fit through. If the throat is not round the pin will slip through only the smallest diameter. Precision pin gauges also are excellent for checking the accuracy of calipers and micrometers.

I almost always use pin gauges these days and seldom slug and revolver throats any more. Also I will add that an absolute fit of the bullet to the throat is no guarantee of improved accuracy or non leading. Other factors come into play, especially the difference between the forcing cone and throat diameter and the groove diameter of the barrel. Using a bullet too large for those can actually degrade accuracy. I know that goes against "fit is king" but I have found it all too often to discount it.

Larry Gibson

[5Shot]

Have you slugged the bore with a dead soft ball and pushed it into the throats like the photo of my .44 cylinder?

I didn't bother to slug the bore. This is a recent run Ruger Alaskan, and the throats are, well...huge. They measure 0.4555 (at least they are all the same). I sized the bullets to 0.455" after honing out an RCBS Die to give me the correct fit. I have no doubt they are quite a bit larger than the bore. No way to thread choke an Alaskan, so that shouldn't be an issue.

Sorry for the drift...

[BCRider]

On the topic of calibrating calipers? I've always checked mine for accuracy by first checking the zero then using a 1" standard from a Mitutoyo 1-2" mic I've got. So far I've yet to find a set which is more than a half thou out at 1". And most are within a line's width on the dial scale.

Because of issues with centering and side cocking when measuring internal holes I check them with some outward tension on the thumbwheel and look for the maximum reading while slightly moving the calipers around to find the spot where the fingers are truly axial and as far apart as possible.

Mind you pin gauges would sure be nice too.