Need clarification on chamber vs. bore vs. groove diameters.

[BBQLS1]

I need some clarification.

I was trying to figure out what diameter cast bullet I need for my 25-13 to maximize accuracy. I slugged the barrel using a lead weight. The best measurement I could obtain was .455-.456 using dial calipers. I wasn't really happy with how well I was taking the measurement. I decided to take that slug and see how well it fit through the chambers and it would not easily push through.... as a matter of fact, I stopped and pushed it back out because it was very hard to do.

Now, I have some hard cast bullets that measure .452 that push through the chambers with hand pressure. It's snug, but it's not too bad.


All this tells me is that the major diameter of the barrel is bigger than the chamber.



Does this sound right? I read some stuff and it sounded like the chambers should be a little larger than the bore, but I need some clarification.

 

[USBP SW]

Ideally, the chambers would measure approximately .452, the groove diameter ( I believe the largest diameter of the barrel) would measure .451 or .4515. It is supposed to be less than the chamber diameter in order to engrave the bullet properly and shoot accurately.

I hope this answers your question and good luck,
Steve

 

[BBQLS1]

Thanks. Sounds like I'm going to call S&W.

 

[BBQLS1]

Called S&W a few times today. No answer. Something going on I don't know?

 

[Pardue10]

I got back into casting a year or so ago. It is near impossible to measure a slug with dial calipers. The best way to measure the cylinder is with pin gauges. It dont hurt anything to load a bullet that is a couple of thousands larger than cylinder. I would shoot the 452s and see if they lead the bore. You need to size to the cylinder throats. Hopefully the cylinder throats are larger than the forcing cone. I just sent 2 686s off to have the cylinders opened up to 358. They were 356.

 

[GCF]

... I just sent 2 686s off to have the cylinders opened up to 358. They were 356.

Mind if I ask where you sent them? Thanks in advance!

 

[BBQLS1]

See, I don't think the cylinder throats are larger than the forcing cone.

 

[USBP SW]

Think of it this way, if the bullet is swaged down too small by its trip through the cylinder, it will not thoroughly engage the rifling and more or less rattle down the bore. This condition is detrimental to accuracy, because it does not fully impart a stabilizing spin on the projectile and also allows gases to flow around the bullet. Additionally, these problems are exacerbated by the accompanying lead deposits left in the bore by the aforementioned skidding and incomplete sealing of the undersized bullet. Incidentally, it also makes it more difficult to clean.

I hope this explains things a little better,
good luck, Steve

 

[USBP SW]

I don't recall if you attempted to run the "lead weight" you used to slug the barrel, through the cylinder. As someone mentioned, it is difficult to get an accurate measurement of the bore on a Smith & Wesson (due to land and groove position). However, if you cannot drop the "lead weight" you used to slug the barrel through the cylinder, you have a problem.

Keep us informed of your progress,
Steve