reaming chamber throats

[Black Jaque Janaviac]

How do you ream chamber throats?

I have a .357 Blackhawk that has small throats, I think. Is there a special tool for this? Or is this something best done by a 'smithy?

The reason I say "I think" is because it depends on what caliper I use. I own one of those cheap RCBS fiberglass dial calipers and according to that the throats are something like .358". But when I measured with a friends steel RCBS caliper the throats were .356"! I don't get it because a copper jacketed bullet will push through fairly easily. At any rate, the groove diameter is .358", so I think the throats would tolerate a bit of work.

I also noticed that the supposedly ultra-low pressure of the .357 Lil' Gun powder aren't so low-pressure in my Blackhawk. As in, take out the cylinder and pound out the cases. Perhaps it's because the throats are too tight?

[swheeler]

http://www.cylindersmith.com/ Check this link out, it will enlighten you some.. The tools are avaiable from several places, Brownells has them for about 80.00 for reamer and bushings, other sized bushings are available for another 25 or so. A caliper is not accurate enough to take your measurements with, I was taught the best are good for +- .003", use a mic good to .0000 places. To measure your throats you will need to slug them, you'll have to bump up a slug in each throat, you need to slug the barrel to and ntake all your measurements with a mic.

[Black Jaque Janaviac]

A caliper is not accurate enough to take your measurements with, I was taught the best are good for +- .003", use a mic good to .0000 places

Wow I had no idea that calipers were that lousy. I do have a mic though, so I'll have to slug the throats.

I made a casting of the forcing cone and first part of the barrel. It turned out only so-so. That's how I measured the groove diameter although I used the caliper for that too.

Got any tips for using cerrosafe? My castings come out wrinkly which casts some doubt on the subsequent measurements (pun not intended). I also found it a real pain trying to cast the chamber of my Rossi '92. Pouring it in through the action was a pain & getting it out was a pain.

[leftiye]

Use pin gauges to measure your chamber mouths or slug and mike.

I use a fixture that I made to hold the cylinder parallel to the axis of my mill. I've seen others just chuck it in a vise ( I'd pay close attention to getting it very vertical if you do this) on a drill press table. You could use a centering scope to get zeroed on each chamber mouth, but there's a lot of flex in a chucking reamer. I zero the first one, then make a mark on the fixture for the flutes to meet when I rotate the cylinder. You must check the reamre to make sure it's centered on each cylinder. Then I ream to minus one thou. from final diameter and finish off with a sanding rod in a hand drill ( I don't have a Sunnen hone!). Use a pin gauge to see when final diameter is reached.


Chucking reamers cost 12 to 30 dollars at MSC.

[mtgrs737]

Pin gauges are the most accurate way for me with my lack of skill in measuring.

[MtGun44]

Do a search in the gunsmithing section for an old post of mine showing
how I did the throats in my Blackhawk 45 convertible.

+1 on pin gages. Go to ENCO and they are cheap.

http://www.use-enco.com/CGI/INSRIT?PARTPG=INSRAR2&PMAKA=616-8131&PMPXNO=951510

Bill

[Char-Gar]

I agree that calipers are not the way to go. I also am not to fond of slugs and how you do it, will effect the results you get. The pin gages are great things, but I don't have any. I have an old set of Starett adjustable hole gages that I have had for 30 years. they work just fine for me, but it takes a little practice to get the right feel. Once you have that feel, you can get accurate and repeatable measurments to .0001".

Reaming a hole is not a difficult task with the right tools and lubricant and a little know how. If you don't have those things, better to pay somebody to do it.

[Whitespider]

The easiest way to tell if the throats are under-sized is slug the barrel and than drop the barrel slug through the cylinder throats. If the slug falls through with little or no help the throats aren’t under-sized, but this won’t tell you the dimensions of barrel or throats. Getting accurate dimensions requires slugging the barrel and each separate cylinder throat, than taking careful measurements with a micrometer capable of repeatable accuracy to the forth decimal place (.0001).

I have opened cylinder throats by honing with a wooden dowel, or brass rod, split at the end to hold fine emery cloth wrapped around it, and chucked in a drill. Work slowly, using lots of cutting oil, slugging often. Surprisingly, a very uniform, accurate job can be done if you don’t get in a hurry and slug often. I’ve done this with three revolvers, all shoot very well, much better than before the honing.