Throats at .431, barrel at .416 - What am I missing here?!?

[Fergie]

I'm in the process of measuring the throats and barrel on my Ruger RH .44 magnum. I'm using a new set of pin gauges that are ZZ minus.

On the cylinder throats, the .431" gauge slides in with no play, on all cylinders. On the barrel, starting at the crown end, a .417 initially fits, but with a restriction about 1" in, and a .416 will slide all the way through.

Since I am new to this, and this is the first use of pin gauges, with the intent of firelapping the barrel to increase accuracy, the difference in the pin gauges is confusing to me.

In reading Fermin Garza's write-up on the topic, I was under the impression that the throats should only be slightly larger than the barrel at .001". Mine are .015" larger.

So, I'm of the impression that I am missing something in my understanding of the process and would greatly appreciate if you kind folks could get me squared away in my understanding.

Let me know if I need to provide more info, or if I need to correct my methods.

Thank you.

G

[R.M.]

You are measuring the bore diameter, not the groove diameter. You'll need to slug the barrel to get the groove dimension.

[Fergie]

You are measuring the bore diameter, not the groove diameter. You'll need to slug the barrel to get the groove dimension.

*smacks head*....I did not even consider that and had the lead ball for slugging it right in front of me.

Thank you.

[Fergie]

I slugged the barrel and it measure out to .4306 and .4307 the two times I did it.

With the cylinder throats at .431, that seems to be good, correct?

[R.M.]

I'd say so.

[EDG]

Using pins for the cylinder does not account for the .000X in the 4th decimal place. You pistol seems to be ok.
But does it have a tight spot at the threads ?

I slugged the barrel and it measure out to .4306 and .4307 the two times I did it.

With the cylinder throats at .431, that seems to be good, correct?

[Fergie]

Using pins for the cylinder does not account for the .000X in the 4th decimal place. You pistol seems to be ok.
But does it have a tight spot at the threads ?

I do think that there is a tight spot, about 1" in from the crown end of the barrel.

[DougGuy]

As long as the boolits you shoot are A.) larger than groove diameter, and B.) slide into the cylinder throats with minimal or no resistance, you are good to go as far as cylinder throat to bore diameter dimensions are concerned.

As far as the thread choke, normally, the .44 cal Rugers have less thread choke than the .45 cal Rugers because as the OD of the barrel is the same, the .44 has a slightly thicker barrel than the .45 and distorts less when screwed into the frame. That does NOT exempt them from thread choke.

Since most pin gages are too long to put into the barrel from the forcing cone, you could shorten one and use it to measure the bore diameter in front of the forcing cone, this will tell you if there is any thread choke because the same gage that you used at the muzzle, will fit in the other end if there is no thread choke. 50/50 chance that will happen. I do not recommend pushing a hardened steel pin gage down the bore from the muzzle, in theory it should work fine, but what if it gets stuck in the middle behind the rollmarked warning?

Another couple of ways you can look for thread choke, with a really clean dry barrel, remove the cylinder and hold a piece of white paper at the recoil shield under a very bright light. Look down the bore, and you can notice a couple things. First, on the sides of the bore you can see where the rollmarked warning actually impressed metal inwards when it was applied, and you can see bumps in the rifling that coincide with words on the outside of the barrel. Looking farther down the bore, at the area the barrel joins the frame, you can see a hazy shadowy distorted ring all the way around the barrel in the inside, about an inch up from the end of the barrel. This is the thread choke. It will be visible in this light and you can see it like a sore thumb if it's very bad.

Another way, and my favorite way, of determining thread choke, does not actually measure in mathematical terms how much choke there is, but it lets you "feel" exactly what the boolit will be subjected to upon firing, with a cleaning rod. Use a plastic or brass cleaning jag, patch it VERY tightly into a clean dry bore, and shove it down the bore. Observe how much effort it takes to keep the jag moving. When you get it to the area the threaded part of the barrel screws into the frame, what does it do? If it stays constant and has very little change to how much you are pushing and it falls out the end of the bore, you are VERY LUCKY, because you have NO THREAD CHOKE!

If it changes a little bit, (and some older barrels from the 70s will actually have a loose spot just in front of the threads), and gets just a slight bit tighter, you are feeling the thread choke. THIS amount of choke, where you can barely feel the difference on the jag, is easy to firelap out, and even easier if it is a blued steel barrel as opposed to a stainless barrel.

If it changes a LOT, as in you REALLY have to shove on it to get it through, you have a severe thread choke that can be as much as .003" to .004" below bore diameter. In this case, there is TOO MUCH choke to firelap out, you will damage the rifling in the rest of the barrel before you will EVER lap out that much choke, and the barrel is well below SAAMI specs (which Ruger does adhere to!) and you could contact their Customer Service and ask them to address the issue. They should issue an RMA and repair the gun for thread choke that is this severe.

The only other alternative to sending it back in is having a smith set the barrel back one turn and recut the end of it, in hopes that they won't have to torque it down hard enough to choke it, or Taylor throat the barrel using a special reamer which cuts the choke and the rifling completely out for about 3/4" into the barrel, it makes a long tapered freebore much like Weatherby rifles are done from the factory. And it does work.