Cylinder and Bore Dimensions - Pistolsmith

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Old 12-20-2013, 01:02 AM   #1
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Join Date: Aug 2013
Location: Tampa, FL
Posts: 5
Cylinder and Bore Dimensions

1) Do you WANT the cylinder throats to be a bit larger than the bore? I know you definitely don't want the chamber undersized.

After reading about Ruger Blackhawks and thread choke I decided to sit down and do some measuring. I have a Lipsey's/Talo SS convertible 45 Colt/ 45 acp. I used feeler gauges, pin gauges. dial calipers, and a micrometer, as well as slugging the bore. To say I was amazed is an understatement. First, I checked cylinder gaps with feeler gauges. The 45 Colt cylinder was .0015 and the gauge had a little resistance. Second, All the chambers in BOTH cylinders were the same .451 using the pin gauges. Then I slugged the bore with soft lead. I had groove measurements of .451 and lands of .443. Then I wanted to check for thread choke. The largest pin gauge that would fit was the .442 gauge. It slid all the way down with no thread choke. This is a production gun. A Belt Mountain base pin is the only part switched out on this gun. Uniformity is awesome! But, my real question is DO I WANT the cylinder throats to be the same size as the bore? Or, should it be .0005 to .001 larger?

I have not had the chance to run this gun through all the paces yet as I have been using my 500 JRH this hunting season.

Last edited by Mike44; 12-20-2013 at 01:03 AM. Reason: typo
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Old 12-21-2013, 05:17 AM   #2
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Join Date: May 2006
Location: near Huntsville, Ala.
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A .443" pin gauge is not likely to fit a bore with .443" land diameter unless forced. To slide through there has to be some clearance and if a .442" pin gauge goes through easily, then you can't have thread choke. Oh wait, did the slug go the length of the bore with even, or fairly even, pressure? If so, you should have a uniform bore. If not, do it again to find where the tight and/or loose spots are.

As to chamber throat diameter vs groove diameter, think of it this way: When the primer fires the charge, the gas/heat pressure is literally pushing in every possible direction. The main path of least resistance is the bullet which is what we want. However as the base of the bullet is initially pressured, the rest of it still has to put into motion.

So as the base can't quite instantly push the rest of the bullet forward, the base expands at least slightly, first to max out inside the case mouth diameter which has expanded to fill the interior diameter of that portion of the chamber.

Then as it clears the case, the base will expand to fill the throat if the powder charge is adequate for proper obduration. As the front of the bullet begins to contact the lands, it slows slightly while the rear is still moving at the same speed. So as the base clears the front edge of the cylinder, the base will expand slightly again.

Then of course the bullet diameter will become a mix of groove and land diameter as you saw with your measuring slug.

There is a lot of debate about what is optimum chamber throat diameter vs a given groove diameter. Elmer Keith preferred chamber throats .001" less than groove diameter IIRC. He was shooting somewhat hard lead bullets with medium to possibly more than factory pressure powder charges. He wanted the minimum squeezing of bullet diameter (from larger chamber throats) back down to groove diameter in order to minimize bullet distortion from all the expanding and reducing. He did a lot of experimenting with this. In no case did he want chamber throats greater than .001" over groove diameter. Of course there were instances where he could only compensate for such.

Of course jacket bullets may not expand like lead bullets. Hardened lead bullets don't expand like softer ones. Bullet diameter is also a factor. Then there is how much pressure does the powder charge generate, over what time interval, and how fast? Faster vs slower powders, charge weight, primer brisance, etc.

The forcing cone, cylinder throats, and crown must be smooth and concentric. Cylinder face and barrel rear face must be flat and true, although the latter is usually more about avoiding cylinder bind as the gun heats from firing. There are some other factors, but they usually are insignificant if your gun guages the way you say. I.e. for mere mechanical accuracy.

For practical accuracy, the trigger pull, sights, grip, ammunition, light (sun usually), and of course the operator all matter. Again not a complete list, just the highlights.

I expect your gun is fine. Since you went to so much effort, maybe you want to get a range rod or better yet get a machinist to make one for you at .442" diameter. The latter if straight and true would tell whether the bore axis and axis for each chamber are all PERFECTLY true. They might well be.

Actually if a range rod from Brownell's is cheaper, try that first. If the gun ranges on all chambers with that, it would probably be a waste of time and $ for a custom rod.

Revolvers are quite difficult to perfect due mainly to the multiple chambers and same not being integral with the barrel. However unless you are a dedicated benchrester, if everything is within proper spec's, very small imperfections are insignificant due to the other factors outside of mechanical accuracy.

Hope I helped you. Now let's see if a real gunsmith will weigh in and explain anything else pertinent or a mistake I made.
Old 12-22-2013, 04:54 AM   #3
Junior Member
Join Date: Aug 2013
Location: Tampa, FL
Posts: 5
That was a great reply. I really appreciate your time. I will be shooting it coming up soon as Christmas is past. I will report back then. Thanks again.

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