Andrew Leigh
Member
I have agonised over jump. I read of people adjusting their COL with remarkable results, I have tried but simply cannot replicate their experience. I also want my groups to dramatically close up by simply seating differently.
As a Hunter my groups are perfectly acceptable, but that does not stop me from wanting smaller groups to prove better reloading technique and shooting ability.
So my question is then twofold. Is the altering of the jump due to mechanical consideration? Is the jump merely affecting pressure and changing barrel time bring one onto an accuracy node?
Mechanically.
-I understand that as your approach the lands that pressure will experience a non linear increase due to the inertia required to overcome the rifling. As you move away from the land their is a point at which the pressure will increase again.
- I also understand that if you bullet is not seated 100% concentric to the barrel that a longer jump could have an effect on how true the bullet enters the rifling.
- You have the effect of the free bore which is essentially an extension on the jump.
- Then you have the angle at which the leade is cut.
- You have the hardness of the projectile.
- There is the angle of the ogive to be considered.
Pressure
Now of the above there is only one factor that to my mind that is purely mechanical and that is concentricity, the rest ALL result in a pressure change. The pressure change in turn will affect speed and barrel time.
So I have asked if I am not fooling myself. If I am on an OBT node I am in tune with the harmonics of the barrel and therefore any change in jump would require a change in load.
Conversely, If I do not possess ballistic prediction software or a chrony am I simply not unknowingly bringing my rifle onto an accuracy node but altering the jump (sic the pressure and thus the barrel time).
Searching these and other forum you get the school that says jump makes a difference and those that say it does not. Surely we as loaders can get closer to the truth?
By way of example. Although not a great shot I can get 0.6MOA with my 6.5mm. I have changed the jump with out changing the load. I have changed the jump compensating for the load and my groups do NOT change. Neither do they get smaller nor do they get any bigger.
Conversely my .375 has a jump of 0.28". I cannot get any closer as the magazine restricts me. I shoot 1" generally but have managed my best group ever of 0.27MOA? So how important is jump really? My shooting buddy has a 9.3mm and is 0.43" off the lands and shoots sub 0.5MOA, in fact all his rifles are 0.5MOA and better with SAAMI COL's.
Now while I cannot get the jump to have any affect in my shooting I am no way claiming that it does NOT have an effect. In fact I would love it to be true, seat a little more in and groups go from 1MOA to 0.5MOA, hey presto.
I am really keen to get to the bottom of this so anecdotal evidence will be of little value. Has anyone got real data and pic's showing a progression of a load where the change in COL closes and then opens a group?
I can find lots of people that tell me that jump make a difference but have yet to find any proof.
As a Hunter my groups are perfectly acceptable, but that does not stop me from wanting smaller groups to prove better reloading technique and shooting ability.
So my question is then twofold. Is the altering of the jump due to mechanical consideration? Is the jump merely affecting pressure and changing barrel time bring one onto an accuracy node?
Mechanically.
-I understand that as your approach the lands that pressure will experience a non linear increase due to the inertia required to overcome the rifling. As you move away from the land their is a point at which the pressure will increase again.
- I also understand that if you bullet is not seated 100% concentric to the barrel that a longer jump could have an effect on how true the bullet enters the rifling.
- You have the effect of the free bore which is essentially an extension on the jump.
- Then you have the angle at which the leade is cut.
- You have the hardness of the projectile.
- There is the angle of the ogive to be considered.
Pressure
Now of the above there is only one factor that to my mind that is purely mechanical and that is concentricity, the rest ALL result in a pressure change. The pressure change in turn will affect speed and barrel time.
So I have asked if I am not fooling myself. If I am on an OBT node I am in tune with the harmonics of the barrel and therefore any change in jump would require a change in load.
Conversely, If I do not possess ballistic prediction software or a chrony am I simply not unknowingly bringing my rifle onto an accuracy node but altering the jump (sic the pressure and thus the barrel time).
Searching these and other forum you get the school that says jump makes a difference and those that say it does not. Surely we as loaders can get closer to the truth?
By way of example. Although not a great shot I can get 0.6MOA with my 6.5mm. I have changed the jump with out changing the load. I have changed the jump compensating for the load and my groups do NOT change. Neither do they get smaller nor do they get any bigger.
Conversely my .375 has a jump of 0.28". I cannot get any closer as the magazine restricts me. I shoot 1" generally but have managed my best group ever of 0.27MOA? So how important is jump really? My shooting buddy has a 9.3mm and is 0.43" off the lands and shoots sub 0.5MOA, in fact all his rifles are 0.5MOA and better with SAAMI COL's.
Now while I cannot get the jump to have any affect in my shooting I am no way claiming that it does NOT have an effect. In fact I would love it to be true, seat a little more in and groups go from 1MOA to 0.5MOA, hey presto.
I am really keen to get to the bottom of this so anecdotal evidence will be of little value. Has anyone got real data and pic's showing a progression of a load where the change in COL closes and then opens a group?
I can find lots of people that tell me that jump make a difference but have yet to find any proof.