cylinder throat size

[castmiester]

I ran an oversized ~.35975 cast through my .357 mag and had to drive them pretty hard to get them through. The driving bands took a beating to make me think if .001 over the groove might be excessive. I found a ~.3577-.35785 cast and it took just finger pressure to get it through with no flattened bands. The bands on the larger bullet weren't completely flattened but the amount of hammering with a 4 ounce ball peen got me thinking. Maybe .357 in a .357 groove will be good enough. BHN is 10

[Tatume]

Barrel groove diameter is not the parameter with which you should be concerned. Also, it sounds like you may be driving a relatively hard cast bullet through your barrel. When you slug a barrel or a chamber throat you should use pure lead, which is dead soft.

Chamber throat diameter is the measurement you should take. You can drive a pure lead slug through each of the throats and you can use them to determine which is the largest. You need a micrometer, as the precision and accuracy of a caliper are insufficient. However, there are better ways of making the measurement. Some use pin gauges, and just try them until you find the largest one that will enter all throats.

I use a Starrett Small Hole Gage. It measures in only one direction, and so I can determine if a throat is round or oblong.



Cast bullets should fit the largest throat.

[DougGuy]

What make is the gun? S&W 357 has 5 lands and grooves making it quite difficult to measure the slug with even a micrometer. slug the barrel again with dead soft lead, now see if the slug will go through the cylinder throats from the front. Poor man's pin gage but it will let you see how much the cylinder is shaving the slug if it's undersize.

[Hickok]

Size the boolits to .358" and give them a try.

After sizing, boolits will sometimes spring back slightly depending on the alloy, .0003"-.0005". I bet sizing .358" will work,...it always has for me in various .357 magnum revolvers, mostly Smith & Wessons.

My loads have been 900-1300fps with 50/50 ACWW/lead, or simply ACWW, probably about 10 bhn to 12 bhn, just guessing.

[BD]

As mentioned above, the throat dimension is what determines the correct boolit diameter. The trouble comes if the throat dimension is under the bore dimension.

[Tatume]

As mentioned above, the throat dimension is what determines the correct boolit diameter. The trouble comes if the throat dimension is under the bore dimension.

Actually, that is the ideal. Just send the cylinder to DougGuy and it will be returned perfectly dimensioned.

[castmiester]

Barrel groove diameter is not the parameter with which you should be concerned. Also, it sounds like you may be driving a relatively hard cast bullet through your barrel. When you slug a barrel or a chamber throat you should use pure lead, which is dead soft.

Chamber throat diameter is the measurement you should take. You can drive a pure lead slug through each of the throats and you can use them to determine which is the largest. You need a micrometer, as the precision and accuracy of a caliper are insufficient. However, there are better ways of making the measurement. Some use pin gauges, and just try them until you find the largest one that will enter all throats.

I use a Starrett Small Hole Gage. It measures in only one direction, and so I can determine if a throat is round or oblong.



Cast bullets should fit the largest throat.

Fishing weights to slug barrel to check the barrel, .357. I don't consider 10 BHN, which 9 is pure lead, too hard to slug the throat.

Doug...

My Taurus is 7 lands 6 grooves and the grooves are opposite of each other EASY to mic. The slug I used fits nicely through the throat.

Hickok... the left bullet is the one through the throat. They varied from throat to throat. Some casts didn't make complete contact, just a couple, the rest varied all the way around with contact. So not full contact tells me they are oblong.

Using a .358 with very flattened bands concerns me with not enough lube especially with soft casts.

[Tatume]

Use a 0.432" sizer die and your bullets should be a perfect fit for this gun.

You keep saying "throat." This is a revolver we're talking about, isn't it? You should be checking all the throats.

Also, you said you were working on a 357 Magnum, but now it looks like a 44 Magnum. Which is it?

[castmiester]

Use a 0.432" sizer die and your bullets should be a perfect fit for this gun.

Sorry mixed in my 44..... this thread is only about the Taurus 66 .357

[Tatume]

The bands on the larger bullet weren't completely flattened but the amount of hammering with a 4 ounce ball peen got me thinking. ... BHN is 10

Another point: when pushing slugs a soft blow with a heavy hammer is better than a sharp blow with a light hammer. I missed the "10 BHN" statement.

[castmiester]

Another point: when pushing slugs a soft blow with a heavy hammer is better than a sharp blow with a light hammer. I missed the "10 BHN" statement.

I used a 4 oz ball peen with light taps but others required a little harder, but not hard blows.

BTW l did check all the throats, they’re all ~.358, .35872 some a little less but an even .358 cast fit good with a light drag with a wooden dowel finger pressure.