Calculating CBTO

[Gila]

Ah, haven’t considered this. For a 6.5 PRC with expected barrel life around 1,000 rounds, how quickly do you think it would change? Every 100 rounds? 200? 500?

For the 6.5 x 284 the throat dimension will increase an average of about .004 per 100 rds. That cartridge is usually run hot and is very overbored though. For the 6.5 PRC it is less than that possibly around .002. For a typical hunting rifle in 6.5 PRC the numbers are coming in for barrel life around 1500-1600 rds. The first 2 inches of the rifling will go to shit before the throat wears out anyway.

 

[ID_Matt]

For the 6.5 x 284 the throat dimension will increase an average of about .004 per 100 rds. That cartridge is usually run hot and is very overbored though. For the 6.5 PRC it is less than that possibly around .002. For a typical hunting rifle in 6.5 PRC the numbers are coming in for barrel life around 1500-1600 rds. The first 2 inches of the rifling will go to shit before the throat wears out anyway.

Don't the 6.5-284 and 6.5 PRC run about the same amount of powder? How is one of them very overbore and one not?

 

[bmart2622]

Its not

 

[Gila]

Don't the 6.5-284 and 6.5 PRC run about the same amount of powder? How is one of them very overbore and one not?

Case design and how each are typically loaded. Open that case up to 7mm and problem solved. Barrel life increases by quite a bit. I went through this issue when I shot out my .270 win barrel and decided that I wanted to shoot VLD bullets. I have a bolt face of .473 which fits the rebated rim of the 284 case. The .284 Win (7mm) barrel was the logical choice.

 

[Gila]

Oh, what is laughable is to think that if you put the same amount of powder into two different cartridge cases and expect the pressure to be the same. Getting back to CBTO, it is pressure that erodes the throat. However that type of throat erosion isn’t enough to change CBTO for the life of the barrel, even in over-bored cartridges.

 

[bmart2622]

Shoot 90+ grains of N570 out of a 30 cal magnum pushing 230gr bullets and tell me the throat doesn't erode!!!! But that would require you to actually have first hand experience instead of parroting worthless Google searches

 

[Gila]

Never said there wasn’t throat erosion…I said it is minimal and doesn’t require increasing the CBTO over the life of the barrel. Maybe you should do some google searches for yourself? With my 300 win mag i am limited to 3.37” COAL because of the DBM. I could give a rat’s ass what’s the CBTO, what the throat erosion is, or what the jump is for that matter. So how many barrels have you monitored for throat erosion over the life of a barrel!!!! LOOOOL….

 

[Mike D Texas]

For the 6.5 x 284 the throat dimension will increase an average of about .004 per 100 rds. That cartridge is usually run hot and is very overbored though. For the 6.5 PRC it is less than that possibly around .002. For a typical hunting rifle in 6.5 PRC the numbers are coming in for barrel life around 1500-1600 rds. The first 2 inches of the rifling will go to shit before the throat wears out anyway.

How is a 6.5-284 “very overbore” but a 6.5 PRC is not?


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[Gila]

Mostly it is powder and bullet choice. Also as mentioned it is the case dimensions. i used the 6.5 x 284 Norma as a comparison to the PRC because there isn’t much difference in performance between the two at different ranges. But the 6.5 x 284 is over bore which will have a greater throat erosion over time than the 6.5 PRC. The two cases are quite different in dimensions. The 284 case has a shorter neck and slightly different shoulder angle which affects pressure (flame cutting) at the case mouth. The 284 has 6 gr more powder capacity, yet the PRC case has a higher max pressure:

PRC - 65,000 psi (max) powder cap. - 62 gr (H2O)

284 - 58,000 psi (max) powder cap - 68.3 gr (H20)

Op’s question was how much throat erosion occurs per 100s of rds. The answer is the PRC has much less. However, even the 284 doesn’t have enough throat erosion over the life of the barrel to necessitate moving the CBTO measurement out toward the lands before the barrel is spent.

 

[bmart2622]

Never said there wasn’t throat erosion…I said it is minimal and doesn’t require increasing the CBTO over the life of the barrel. Maybe you should do some google searches for yourself? With my 300 win mag i am limited to 3.37” COAL because of the DBM. I could give a rat’s ass what’s the CBTO, what the throat erosion is, or what the jump is for that matter. So how many barrels have you monitored for throat erosion over the life of a barrel!!!! LOOOOL….

Yet again your ignorance shines through. I dont need to do Google searches like you because I actually shoot and reload, you spew nonsense. My boys 300 win mag loads 215 Bergers to 2.895 base to ogive which comes out to be .040 jump to the lands on that particular gun. My ultra mag I load 230 Bergers at 2.995 which is .040 jump to the lands. The throat grows .004-.006 every 100 rds which requires attention and tending to. And Im about to rebarrel my ultra mag with the 3rd barrel. So tell me you dont know what you are talking about without telling you dont know!!!!

 

[Mike D Texas]

Man, threads like this always make me wonder why people use that piece of garbage when working up a load. It's SO much more accurate to just do this with an actual cartridge than use that stupid gauge. I played around with that thing when I first got into reloading and there is just so much room for error in that thing it is totally worthless in my opinion.

1: Remove firing pin and ejector from your bolt so your bolt will drop with zero effort...just gravity.

2. Figure out headspace: Take a fired piece of brass, put it in your press and adjust the die so it just BARELY pushes in the shoulders. See if this case allows the bolt handle to drop (it won't), adjust die a tiny bit to push shoulders in a bit further, size the case, put in chamber....lather, rinse, repeat until bolt drops with no resistance. You now know EXACTLY what your headspace is in your chamber and you have your die adjusted perfectly for your rifle.

3. Figure out your CBTO. Do the same as above, just with your seating die. Put the bullet in and seat it just a bit, see if you can drop the handle (you won't), adjust die to push bullet in a bit further, lather, rinse, repeat until the bolt drops. Measure CBTO. You now know your EXACT distance to the lands. Adjust your the die to push the bullet in to whatever distance off the lands you want, you are done.

4. Reassemble bolt. You are now done.

I make my own modified cases and I get very repeatable measurement with the Hornady LnL tool. I don’t see how you couldn’t.

I’ve found much less consistency using the shove the bullet in the case method.


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[Gila]

Yet again your ignorance shines through. I dont need to do Google searches like you because I actually shoot and reload, you spew nonsense. My boys 300 win mag loads 215 Bergers to 2.895 base to ogive which comes out to be .040 jump to the lands on that particular gun. My ultra mag I load 230 Bergers at 2.995 which is .040 jump to the lands. The throat grows .004-.006 every 100 rds which requires attention and tending to. And Im about to rebarrel my ultra mag with the 3rd barrel. So tell me you dont know what you are talking about without telling you dont know!!!!

You are nothing but a condescending fool…or village idiot take your pick! You like to argue with people and insult them,,,,I have more to do then respond to your crap…buzz off!