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Old February 4, 2013, 08:57 PM   #1
RANGER94
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Help with measuring revolver throat

Hello - can you double-check me here to make sure I am on track measuring the chamber throats on a .45 Colt Ruger New Model Blackhawk? I removed the cylinder, placed a cast 250 grain bullet into the throat, and then used a dowel to wack it through the cylinder. I used a micrometer to measure the bullet, and all 6 came out at .452.

Does that look normal?

thanks!

Ranger94
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Old February 4, 2013, 09:19 PM   #2
jepp2
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It certainly looks in the range I would expect. I don't know the resolution of your micrometer, has it been properly calibrated, or your skill in using it. Not criticizing, just identifying what I don't know.

If you slug the bore, then take that slug and see if it will pass through each throat with only finger pressure, it will give you the relationship between the barrel groove diameter and your throats. If you use pure lead for slugging (muzzleloader balls are swaged pure lead) it will result in better accuracy.

I use pin gages to measure throats, but since the throats are not always round, I wind up measuring the minor diameter, not the major diameter.
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Old February 4, 2013, 09:19 PM   #3
SL1
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That is the right way to do it, except that you did not mention a couple of points. First, I hope you lubed the cylinder throats first. Second, I hope that you started with a bullet that was originally larger than 0.452" in diameter. Because you wrote "wacked" I suspect that it was large enough for your cylinder throats. However, if you are going to do the same thing with the barrel, remember that you may still need to wack a bullet to engrave the rifling, even if the bullet's diamter is not large enough to get completely to the bottoms of the grooves, so you could get an undersized indication that way. Finally, you did say that you used a micrometer, so I think you have an acceptably accurate measurement. Sometimes folks use a caliper that has a precision of only 0.001" and do not realize that it is not ACCURATE to 0.001". For instance, mine reads about 0.001" to large in the range of 0.500".

As for "normal," your reading is probably about right for good accuracy in a Ruger barrel, which typically runs around 0.452". But, it is more usual for Ruger cylinder throats on their recent guns to be a little undersized. Mine were 0.450" when new and opened to accept a 0.051" plug gauge after about 100 rounds of .45 ACP ammo.

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Old February 4, 2013, 09:31 PM   #4
RANGER94
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Thanks!

I did lube each cylinder first. I put some oil on a q-tip and made sure it was heavily lubricated. The cast bullets I used were sized at .452. I put a bullet into the throat of the cylinder, (upside down, so the tip of the bullet would be backward and help get it started) most of the bullet did not fit into the cylinder. I then placed a wooden dowel onto the bullet, and then wacked the dowel with a brass hammer. It was very tight, and took me a while for each bullet. I did use both a micrometer and a dial caliper to double-check. All of the bullets came out .452.
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Old February 10, 2013, 09:01 PM   #5
RANGER94
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I just slugged the barrel, and the bullet came out at .4505.

If my throats measure .452, and the barrel is at .4505 - does that look normal?

Thanks!
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Old February 11, 2013, 12:04 AM   #6
slowrider
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The bore should be .451. It's fairly common for the Ruger cylinder throats to be undersize for shooting .452 cast bullets. Many have them reamed to .4525. Your measurements are pretty close to both of those so you are probably good to go...at the very least with jacketed bullets. If you want to shoot .452 cast bullets I'd first see how it shoots for accuracy and leading with them. If you are not satisfied I'd have the throats measured accurately with pin gauges and reamed to .4525 if needed. There is a lot of good info specific to this issue over at rugerforum.com. If you are going to shoot and reload for Rugers you really ought to join that forum.
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Old February 11, 2013, 09:17 PM   #7
SL1
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Ranger94,

When you slugged your barrel, did you push the bullet all of the way through and then measure it? If so, then what you measured is the SMALLEST diameter in the bore. Often, that is located where the barrel is screwed into the frame. There is a little compression there from the tight fit in the frame, and it showes up most on large bores like the .45s. Beyond there (towards the barrel) the bore may be larger. But, you don't want that, especially if you shoot lead bullets, because the tight spot swages the bullet down to be smaller than the bore beyond, so gas leaks by the bullet after it passes the tight spot and that causes leading. (Sometimes that leading can be avoided with gas checked bullets.)

So, what you really want to do is push a bullet just part way into the bore from the muzzle end, then pull it back out and measure it. Then, push a bullet all of the way through and measure that. If the muzzle measurement is the same or smaller than the all-the-way-through measurement, then you do not have a constriction to worry about. But, if there is a constriction, then you either need to fire-lap to remove it or stick to jacketed (or maybe gas-checked lead) bullets.

Once you have the bore staying the same or getting smaller from the forcing cone all the way to the muzzle, then you want to make sure that your cylinder throats are at least as big as the bore near the cylinder, and it would be better if they were about 0.001" larger. That way, the bullet si always getting squeezed-down slightly, and gases never get the chance to leak by.

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