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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; OK, been reading a lot about cylinder throats in 45's and I might be ahead of myself at this point but have a question. I ...

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Old April 16th, 2017, 08:40 AM   #1
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Cylinder Throat Reaming

OK, been reading a lot about cylinder throats in 45's and I might be ahead of myself at this point but have a question.

I bought a used BH 45 colt a few weeks ago and have been out to shoot it once. I did not shoot at close range though since I went to primarily sight my new AR 556. Shooting the BH at 50 yards I was able to hit a 17x22 target 11 out of 12 times (4 pages of 8 1/2 x11 targets set up on same target).

I did not shoot at closer than 50 yds so have no idea of the grouping at closer range and plan on checking it soon.

I did slug the barrel and the slug will not pass through the cylinder throat with finger pressure. I have not measured with a dial caliper and I only have a slide caliper currently that does not provide readings to a thousandth. The slide caliper shows the slug at .45" and the throats at slightly less.

The question is given what I know at this point, the barrel slug won't pass through the cylinder throat, does it make sense to ream the throats out to .4525?


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Old April 16th, 2017, 08:58 AM   #2
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Hi nawagner,

It made an enormous difference in mine. I went from 4, 5, 6" groups to 2" at 25 yards.
My cylinder throats went from .450 to .451 and were all cut to .4525. I am more than happy with the results. I hope this helps.

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Old April 16th, 2017, 12:29 PM   #3
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Ruger cylinder throats are notoriously tight. That doesn't men they all are small but it certainly is common to see tight throats on a Ruger.

With jacketed bullets, an undersized throat is a little easier to live with than with cast bullets, however accuracy generally improves when the throats are sized properly with either type of bullet.
Guns that are set up for jacketed bullets can tolerate throats that are a bit too small and still shoot well.

In a perfect world, the throats should be slightly larger than the groove diameter of the barrel. And in a really perfect world, the barrel should have a slight groove diameter taper near the muzzle.
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Old April 16th, 2017, 01:43 PM   #4
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I have also been reading about all this "cylinder throat" diameter stuff for awhile.

As you can see in my signature below, I have already collected a few un-converted Old Model 3-Screw Blackhawks in various calibers, barrel lengths and years of manufacture.

I am about to buy an un-converted 1971 OM BH 3-Screw in .45 Colt to add to my collection.

Does this cylinder throat size issue mainly deal with the .45's?

If so, why is that?

Should I slug the barrel and measure the cylinder throats on the .45 or just not worry about it and take her out and shoot the hell outta her?

Any information will be appreciated...

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Old April 16th, 2017, 02:15 PM   #5
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My older 45 convertible has .454 throats and a .451 slug. I am looking to get a new, tighter cylinder, but for now have been reloading .454 plain base bullets, which seems to mitigate leading pretty well. My dies don't like that fat bullet though. I get lead shaving with even slight crimp. Workin' on it.

Many of my other guns have been reamed, namely .357 to .3575 and 45 Colt and 45 ACP to .4525. I don't recall messing with those for any other calibers/cartridges, only where I had the most trouble with leading and where a lead bullet could not be readily coaxed through the throats. I do have one 38, a 36-1 Chief's Special, that is too tight for lead.
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Old April 16th, 2017, 03:05 PM   #6
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Reaming has to do with promoting obturation with lead bullets. Basically, you want the lead .001 to .0015 larger than the cylinder throats, and the cylinder throats .001 to .0015 larger than the bore.

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Old April 16th, 2017, 03:40 PM   #7
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Doug over at cylinderhone.net says that the most important thing cylinder reaming does is make all of throats the same size. On 45 Colt I believe he reams the throats to 4525 so that you can pass a .452 bullet through it. He does everyone's cylinders at castboolits.com
and I have heard nothing but good things.

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Old April 16th, 2017, 03:50 PM   #8
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Originally Posted by Yellowsevenpot View Post
Reaming has to do with promoting obturation with lead bullets. Basically, you want the lead .001 to .0015 larger than the cylinder throats, and the cylinder throats .001 to .0015 larger than the bore.

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Not quite what I aim for. The chamber, throat, forcing cone, bore, and and rifling all reduce diameter in sequence, fitting the bullet to the bore and rifling. The bullet is the largest or no smaller than the throat. Seems to work, but the final ingredient is adequate lube.
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Old April 16th, 2017, 04:41 PM   #9
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Basically, you want the lead .001 to .0015 larger than the cylinder throats, and the cylinder throats .001 to .0015 larger than the bore.
Yep. since the bore of Ruger .45 Colt is .451, you want your throats to be .452ish. The reamer is usually .4525. That said if your throats are .454 (as some older Rugers are) then you want to bump up your bullet size to .454. The bullet will be squeezed down when entering the bore for a good seal which is key.

My original Vaquero was driving me crazy ... until I reamed out the throats (was .449ish, very tight) to .452. Cut my group size in half and leading went away. From then on if a .452 bullet did not pass into the throats of a newly acquired .45 Colt revolver with finger pressure, the throats got reamed.

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Old April 16th, 2017, 05:16 PM   #10
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Yellowsevenpot, Not true. You never want the bullet diameter to be larger than the cylinder throat diameter. Why? Poking a larger bullet in a smaller hole raises chamber pressure .... often dramatically. This will cause excessive wear and will open up cylinder endshake. Further, the excess lead has to go somewhere when it is stripped off the bullet .... and last, the stripped off lead will end up fouling the bore which in turn diminishes accuracy. The very purpose of obturation (lead expanding under pressure) is to force the bullet to expand to the limits of the cylinder throat. If the bullet is already too large, there can be no expansion.

What we are dealing with is an old issue that was solved more than 100 years ago but somehow got lost in the shuffle with modern ammunition. Back in the black powder days, very soft lead was used for bullets. Why? Soft lead would expand (obturate) at lower chamber pressures that were common with black powder. Seems when we transitioned to smokeless gunpowder, people lost the ability to deal with pressure versus bullet hardness. There is a formula that will determine the optimum bullet hardness for a specific load ..... Chamber pressure divided by 1400 = bullet hardness (rated in BHN, bullet hardness number). A perfect example is a 45 Colt where max chamber pressure is 14,000 psi. 14,000/1400=BHN 10. You will find if you load with a medium burn rate powder such as Unique, Power Pistol, or Universal, a 255gr lead bullet driven at 850~860 fps will develop a chamber pressure very close to 14k psi, thus a perfect match for a BHN 10 bullet. I typically load with 8.5gr of Unique and BHN 10 Hornady 255gr "Cowboy" bullets. Accuracy is superb and bore fouling is almost nonexistent. Hornady "cowboy" bullets are swaged with a .454" knurled body that reduces to .452" when the bullet is seated.

As for cylinder throat diameters .... with Ruger revolvers, 45 cal cylinders tend to be the tightest and have the most variation. It is not uncommon to find throats as tight as .448" and it is also common to see all six throats at a different diameter. Why? When Ruger manufacturers cylinders, they drill pilot holes for each chamber, after which a finish reamer is used to complete the chamber then another reamer is used to finish the throats. Herein lies the problem. The pilot holes are too small which means the reamers have to remove too much material to meet optimum specs. As such, throat reamers get dull and/or wear out pretty fast, leaving the throat too small (worn reamer) and possibly oval (dull reamer). This has been an ongoing issue with Ruger revolvers since the very first 45 cal Blackhawks rolled off the assembly lines back in 1971 and still continues to this day.

Back in 1955, S&W began making the Model 1955 in both a 45 ACP and 45 Colt. The model numbers were later changed to Mod 25-2 for a 45 ACP and 45-5 for a 45 Colt. At the time, the only gun manufacturer that made 45 Colt revolvers was Colt and these revolvers were made in very small numbers. As such, S&W petitioned SAAMI to standardize all 45 Cal handgun bore diameters to .451" .... the same as a 1911 in 45 ACP. This allowed S&W to use the same barrels on both 45 cal models so only the cylinder was different. To this day, the bores for all 45 cal handguns .... pistols or revolvers made after the mid-50's is .451".

Ammo manufacturers settled on .452" lead bullets, which were the most accurate and developed the least amount of bore fouling. Taking a lesson from the past, factory loaded 45 Colt ammo was produced with 250~255gr BHN 10 bullets with a muzzle velocity of 860 fps. The primary differences being .... older 45 Colt ammo had .454" bullets.

The optimum throat diameter for a modern US made 45 Colt in any revolver is .4525". This will allow a .452" bullet to pass through with minimal friction and will deliver a bullet to the forcing cone measuring about .4525". The forcing cone will reshape the soft BHN 10 bullet without skinning the circumference .... making it a tight fit from the barrel face to the muzzle. This will prevent gas "blow-by" that erodes the bullet's circumference and leaves deposits of lead fouling in the bore. When the proper chamber/barrel pressure is applied, the bullet will keep trying to expand until it finally leaves the muzzle. This concept of constant obturation is what keeps the bore from getting fouled and keeps accuracy at its best. Don't bet on foreign made Colt clones to have .451" bore or .4525" cylinder throats .... they might have the old .454" bore and .455" throats. That said, I bought a new Beretta Stampede in 45 Colt (made in Italy). All the throats were a perfect .4525" and the bore was a perfect .451". This gun shot remarkably tight groups right out of the box.

With Ruger Blackhawks, the only chambering that is commonly under optimum throat specs is a 45 cal. Occasionally you will see a 41 cal or a 357 cal that is too tight but most of these revolvers are shipped with the proper throat diameters. The only other exception is 357 Mag GP100s where tight throats are also common.

The simple way to test any revolver throat is to try to push a factory lead bullet (.452" for a 45 cal, .358" for a 357 Mag) through the throats with the bullet nose first into the front of the cylinder (check each hole). If any of the throats are so tight where you can't push a bullet through with just finger pressure, it will need to get reamed. Reaming does three things .... it rounds out oval holes, it makes all throats the same uniform diameter, and it optimizes the revolver for lead bullets. Jacketed bullets can still be used with no degrading of velocity or accuracy. On paper, you will see tighter groups with no flyers (assuming a good marksman) and the groups will be well centered. I have been reaming Blackhawk throats for several decades and never found one single case where accuracy was worse after reaming. In fact nearly all revolvers improved dramatically .... the tighter the throats prior to reaming, the better the improvement.

There's a lot more you can learn about optimum bullet hardness, bullet diameter, bore diameter, and throat diameter in a document I posted in the Forum Library. It is titled "Lead Bullets and Revolvers". Here's a link for those with more than 10 posts: http://rugerforum.net/library/19869-...revolvers.html
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Old April 17th, 2017, 07:03 AM   #11
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I've reamed a couple, 45 colt, 357 and a little 380 revolver. I shoot my own cast boolets so this was a must for good accuracy. I purchased my own reamers along with the bushings. After a couple times they've paid for there self.
I've been well pleased with the results.
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Old April 17th, 2017, 07:12 AM   #12
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Everything Iowegan said . . . In spades!

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Old April 17th, 2017, 12:33 PM   #13
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The bad news is, the .45 Colt and .45 ACP cylinders of the NMBH tend to be tight to very tight.

The good news is, a .4525" reamer will usually clean up the imperfections of the factory job, things like slightly tapered chamber, oval, S shaped, all kinds of things, that fortunately cleaned up nicely with the reamer.
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Old April 29th, 2017, 06:56 PM   #14
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Originally Posted by Iowegan View Post
Jacketed bullets can still be used with no degrading of velocity or accuracy.
Does this include jacketed .451 diameter bullets?
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Old April 30th, 2017, 07:04 AM   #15
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I bought my 45 Colt Vaquero through a gunsmith and he reamed the throats before I ever picked it up many years back. I wasn't aware of any of the undersized throat issues then and since I bought my cast bullets from a commercial caster, didn't really care. Fast forward about 12 years, last year I started casting bullets for this revolver and started caring about bullet fit. All of the throats were not the same and had some marks from the reaming done to it. Not too long story made short, DougGuy honed my throats to the same size and a decent shooting revolver is now a great shooting revolver.

As I now have a somewhat educated guess of what to look for, when I started having these things called Blackhawk's show up at my house like weeds in the yard, checking throats is one of the first things I do. Dumb or smart, if I find undersized throats on a new Blackhawk, I will typically send it to DougGuy before it's even fired. Yes, it may have shot fine just the way it came from the box, but I know it will shoot with the throats uniformed to the same size. FWIW, I only shoot cast bullets I cast myself now and I can control everything involved.

2 45 Colt's needed honing, 1 357 needed honing, but 2 44 calibers were great out of the box. The point of my last sentence was you just never know the size of the throats unless you measure. Personally, I'd rather have the throats too small than too large.
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