284 seating depth question

[Keith Glasscock]

I'm just trying to make certain I have a good set of numbers here - I'm having a hard time determining when the bullet contacts the lands.

Straight 284 with a .218 freebore. What kind of OAL (the only thing we can reasonably compare) with a 180 hybrid should I be seeing just touching the lands?

Thanks,
Keith

 

[Erik Cortina]

Make a couple of dummy rounds with no powder.
Seat bullet .020" long
Apply die wax to bullet.
Chamber round and close bolt on gun.
Snap bolt open, extract round, and measure.
That is your jam length, seat shorter than that or you will stick a bullet in the lands when ejecting a live round.

 

[dkhunt14]

Make a couple of dummy rounds with no powder.
Seat bullet .020" long
Apply die wax to bullet.
Chamber round and close bolt on gun.
Snap bolt open, extract round, and measure.
That is your jam length, seat shorter than that or you will stick a bullet in the lands when ejecting a live round.

Once i find that i load a bullet in a case and steel wool the bullet I would make it about 10 thous. shorter and chamber and take out and look at the marks on the bullet. If you keep steelwooling and move the bullet 5 thous at a time it will soon disappear and when it does that is real close to touching. Most of my guns shoot best at 1000 yards when they are 10 in.

 

[Erik Cortina]

Make a couple of dummy rounds with no powder.
Seat bullet .020" long
Apply die wax to bullet.
Chamber round and close bolt on gun.
Snap bolt open, extract round, and measure.
That is your jam length, seat shorter than that or you will stick a bullet in the lands when ejecting a live round.

Once i find that i load a bullet in a case and steel wool the bullet I would make it about 10 thous. shorter and chamber and take out and look at the marks on the bullet. If you keep steelwooling and move the bullet 5 thous at a time it will soon disappear and when it does that is real close to touching. Most of my guns shoot best at 1000 yards when they are 10 in.

I don't see the point of finding touch. All you are looking for is a reference point, jam is the best reference point I have found. I reference all my seating depth as jam - .xxx"

 

[Keith Glasscock]

Erik,

I understand where you are coming from. I am going to try to run this thing with a jump. I'm interested in where to stop going longer in order to avoid a pressure spike if that makes any sense.

 

[Erik Cortina]

Erik,

I understand where you are coming from. I am going to try to run this thing with a jump. I'm interested in where to stop going longer in order to avoid a pressure spike if that makes any sense.

I just told you where to stop to avoid "pressure spike". Don't go past jam and you have nothing to worry about.

 

[dkhunt14]

ERIK Because once I find the touch then I will know far I am in and how much I need to move them to jump or to put in 10. Jam length can be anywhere from 15 to 40 in, depending on how much tension and other factors. In benchrest 5 thous. difference can mean the difference between 3 inch groups and 6 inch groups at 1000 yards.

 

[dkhunt14]

The question was how to find touch.

 

[Keith Glasscock]

Erik,

Do you not see an increase in pressure between jumping and touching the lands? I suspect that this thing is going to want to run on the warm side with the powder I have available.

 

[Erik Cortina]

.010" in or .010" out doesn't matter, what matters is where the gun shoots best. To find where the gun shoots best, you need to find a repeatable reference point. Jam is exactly that.

I know .005" makes a difference in seating depth, but I'm not arguing that.

 

[Keith Glasscock]

Thanks Erik,

I haven't shot this barrel yet, and my bullet left in the case neck is only .170 when I think I'm touching. I will run a jam check when I get home.

Do you have a number for jam oal that I can compare to?

 

[Erik Cortina]

Thanks Erik,

I haven't shot this barrel yet, and my bullet left in the case neck is only .170 when I think I'm touching. I will run a jam check when I get home.

Do you have a number for jam oal that I can compare to?

No, jam is relative to neck tension, leade angle, bore diameter, etc. It is unique to your rifle and reloading technique. Luckily, it's easy to find if you follow my instructions above.

I have not found much difference in pressure between jumping and in the lands, it's when you jam that pressure shoots up.

 

[falconpilot]

Using Lapua 6.5x284 brass, my freebore of .212, 3.175 +/- is just touching using a 180 Berger Hybrid