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Cylinder Throat Reaming

This is a discussion on Cylinder Throat Reaming within the Gunsmithing forums, part of the Firearm Forum category; nawagner, Nearly all 45 cal jacketed bullets are .451" in diameter so yes, .451" jacketed bullets can be used without degrading velocity or accuracy. I've ...


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Old April 30th, 2017, 07:37 AM   #16
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nawagner, Nearly all 45 cal jacketed bullets are .451" in diameter so yes, .451" jacketed bullets can be used without degrading velocity or accuracy. I've done many "before and after" range tests on revolvers where I did throat reaming. In nearly all cases, accuracy was better, never worse. Chronograph testing showed more uniform velocities with both jacketed and lead bullets. The fact that all reamed throats are the same diameter helps eliminate flyers with both types of bullets and as I noted above, sometimes throat diameters are well below jacketed bullet diameters. The standard 45 cal pilot for a Manson throat reamer is .448" and I've seen a few cylinders where the pilot would not fit in the throat. That's at least .003" smaller than jacketed bullet diameter and .005" smaller than optimum diameter. It's no wonder why some revolvers end up with excessive endshake with a low round count.

I will say, when the proper hardness and diameter lead bullets are used, accuracy will be at its best. This is true for virtually all revolvers, no matter what caliber. Why? Jacketed bullets are too hard to obturate so they never expand to full bore diameter. This means the seal between the bullet and bore is not tight and will allow expanding gasses to vent around the bullet. This "less than perfect" condition will allow good accuracy .... just not quite as good as with lead bullets.

With 45 Colt revolvers, there's no logical reason to use jacketed bullets unless you are shooting high pressure "Ruger Only" loads. Here's a situation where uniform throats at .4525" is even more important with jacketed bullets because tight throats will increase chamber pressure .... possibly high enough to cause cylinder or frame damage and for sure will cause endshake to increase.



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Old April 30th, 2017, 08:33 AM   #17
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DHD,
Quote:
Personally, I'd rather have the throats too small than too large.
I have seen this comment before but believe me, it is NOT what you want. Experience is a good teacher. In my 31 years of gunsmithing, virtually every time I got a revolver in with excessive endshake, the cylinder throats were too tight. It's not unusual to see excessive endshake with less than 500 rounds fired when tight throats are involved. It is also common to see endshake well within factory specs with 10's of thousands of rounds fired when cylinder throats are sized properly.

As for accuracy with tight throats, bullets get their diameter sized smaller when being forced through a tight throat. This means the bullet will likely be too small to develop a good seal in the bore. This is especially true if bullets are just a tad too hard to obturate properly. So .... now you have an undersized bullet traveling down the bore and allowing expanding gasses to vent past the poorly sealed bullet. This will cause the circumference of the bullet to erode and turn into lead vapor .... some of which will remain in the bore and harden to form lead deposits. Any time you have lead deposits in the bore, accuracy will suffer.

In case you aren't aware .... endshake is the fore and aft movement of the cylinder. In a Blackhawk, the optimum endshake is .002". That gives you just enough room to remove/replace the cylinder when cleaning or to convert to a different cartridge but not enough space to cause future issues, assuming the throats are the proper diameter. When endshake exceeds .005", some strange things start to happen. The cylinder may "unlatch" when fired. This can be a very dangerous situation if the cylinder happens to get nudged out of alignment with the bore. Excess endshake will also cause excessive headspace which in turn causes primer detonation issues. When the firing pin has to expend more energy pushing the cartridge to its stop, there is less energy available to detonate the primer. In other words, you get a lot of duds when the actual ammo is just fine.

So .... not only is there an accuracy issue with tight throats, there's also a notable wear issue. It boils down to a simple concept .... trying to force a larger bullet into a smaller hole is bound to increase chamber pressure. When a bullet is moving through a tight cylinder throat, the cylinder tries to move forward with the bullet until the gas ring slams into the frame. As this action is repeated many times, the cylinder's gas ring and the firearm's frame will get peened, which in turn causes endshake to increase at a much faster rate. The cylinder will bounce back and cause the ratchet column to strike the recoil shield. This will also cause the recoil shield to get peened and again open up endshake. Damage to the front of the cylinder can usually be repaired with a shim washer but damage to the recoil shield is not repairable short of replacing a frame. You can't shim the rear ratchet column to recoil shield surface because of the slot for the hand will get covered .... thus no cylinder rotation. When cylinder throats are too tight, the higher the chamber pressure, the harder the cylinder will slam into the frame, thus the faster the wear. It's amazing how long a Ruger SA will hold up when cylinder throats are sized properly .... 10's of thousands of rounds sure beats hundreds of rounds when throats are too tight.

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Old April 30th, 2017, 05:35 PM   #18
 
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Originally Posted by Iowegan View Post
With 45 Colt revolvers, there's no logical reason to use jacketed bullets unless you are shooting high pressure "Ruger Only" loads. Here's a situation where uniform throats at .4525" is even more important with jacketed bullets because tight throats will increase chamber pressure .... possibly high enough to cause cylinder or frame damage and for sure will cause endshake to increase.
Is this only the case if you are matching bullet hardness to pressure? By your previous info using jacketed doesn't make sense because you can have better accuracy with appropriately loaded lead bullets, correct?

Thanks for all the info! Trying to pick up as much as I can.
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Old April 30th, 2017, 09:03 PM   #19
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nawagner, I'm not sure I understand your post. Here's the deal .... no matter what caliber revolver you have, the potential for accuracy is always better with lead bullets .... especially when bullet hardness matches chamber pressure. Accuracy with jacketed bullets is OK but just not as good as lead bullets. That said, there is a limit where lead bullets may not work well .... typically with magnum velocities when loaded to high chamber pressures. The formula for optimum bullet hardness is: chamber pressure divided by 1400. That means a 357 Mag loaded to the upper chamber pressure limits (35k psi) will need a BHN 25 bullet. Typically, BHN 22 is the hardest commercial cast lead bullet you can find. Here's where a jacketed bullet would be a better choice. Another option would be to reduce the powder charge where it produces about 30k psi .... then a BHN 22 bullet can be used with exceptionally good accuracy potential.
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