Pressure, Bullet Jump & Lands..

[Sammy]

A question for you paper punching people....us less experienced accuracy seeking hunters often stay away from engaging the lands with our projectiles, for reasons of safety and function in our repeaters...

I am looking with interest at the 6.5 x 47 Lapua as a hunting project, and of course all of the information is coming from the benchrest community, from rifles of 28" barrels with VLD bullets sticking into the lands 0.010 - 0.025. 120g pills doing 3200fps+ max and 140g pills doing 3100+max.

What happens when I jump my bullets 0.010. Presumably I get less pressure for the same loading? Does my velocity increase or decrease? Can I load stronger and get increased velocity?

I realise that the answers to these may not strictly relate to the 6.5 x 47 Lapua as not many hunters in that cartridge may yet be out there, but any info about the principles etc would be greatly appreciated..

Cheers

 

[jb1000br]

Using a 284 win, there was not a noticable velocity difference from 10thou off to 25thou in.

BUT, 10thou OFF the lands shot HORRIBLY,2-3x the vertical as in the lands) with VLD's whereas 15/20/25thou IN the lands shot very similarly. and shared POI at 1000yds

I should hopefully soon run a similar test on my 6.5x47 and 130 Berger VLD's

as always though, YMMV,

JB

 

[Sammy]

Cheers JB...

Did you notice anything happening with the pressure in the load? If it was less pressure, presumably you could have loaded it up some more?? Maybe then more velocity??

Sam

 

[Shelley]

I've never noticed where "in the lands" or "off the lands" really changed the pressure signs. I have noticed that when I shortened the leade by .050" the pressure went way up due to having a smaller combustion chamber.

Shelley

 

[jb1000br]

being the same load and same velocity, I'd assume pressure follopwed suit...

but no, no vast noticable difference as I recall.

JB

 

[Forum Boss]

Sammy,

This is one of those situations where you'll just have to try and see. In my 6BR,a shorter, smaller case), when I went from .020 into the lands to .015 away from the lands,.020 jump), my pressures INCREASED, due to the effectively reduced space in the case. I had to knock down the load about 0.2 grain to avoid pressure issues. I lost velocity compared to seating long.

Now the 6.5x47 is a larger case with considerably more capacity to start with. Still, however, I predict that moving your bullet 0.25-.030 further down in the case WILL increase your pressures. I'd reduce your load half a grain and see where you are.

Also, those numbers you quoted are on the very high side with BR actions and 30" barrels. Expect to max out about 2830-2850 with the 140s, 2950 with the 130s, and 3000 with the 120s. And those are stout loads.

 

[Sammy]

Thanks guys..yes they were stout loads...I have seen them stated in various places as absolute maxs....and all were dropping them back.....I will try the gun and see when it is built

Cheers

 

[BarryL]

Just finished going from a .025" jam in mine with 139Lapua's to .025" jump. As soon as I came out of the rifling the pressure and velocity dropped. Decided on .015" jump then picked up the charge .3gr. to get the velocity back up.