Help Me Understand

[Pawnee Bill]

Been working up some loads for my custom Savage 110LA with a Bartlein 1-10 barrel in 30/06 at 100 yds.
Was trying 4064, 4350, 4320 and 4831SC pushing a 175 grain SMK that was meplat trimmed and pointed.

48.0 grs of 4064 gave me a good group at around 2650 fps at POA. 4350 & 4320 not so good.

56.0 grains of 4831SC at just over 2600 fps gave me a good group, but it was 2" lower than POA.

So, I went up to 57.0 gr of 4831SC, same good group at around 2650 fps, but this time it was 2.5 in lower than POA.

Ballistically, what causes that to happen?

 

[Ggmac]

Harmonics would be my first guess , wind , sun , shooter , scope .
Anything after harmonics is not in order .
Were you using a rest and rear bag ? Sun or clouds change ? , could you reshoot and get to orig POA? If you couldn't get back to verify the load that shot POA, I would chech action screws , scope , scope base ,and barrel tightness .

 

[Pawnee Bill]

Harmonics would be my first guess , wind , sun , shooter , scope .
Anything after harmonics is not in order .
Were you using a rest and rear bag ? Sun or clouds change ? , could you reshoot and get to orig POA? If you couldn't get back to verify the load that shot POA, I would chech action screws , scope , scope base ,and barrel tightness .

Was using Targetshooting.com Model 1000LP rest. Repeated the load that shot POA (48.0 gr. 4064) next day. All screws & barrel tight. I don't think it was sun or clouds. Maybe the just the harmonics of a different load. I never thought about that.

 

[Ggmac]

That's why loads shoot and some don't . Powders have a lot to do with accuracy even though some can duplicate the velocity, the harmonics CAN be different

 

[mac86951]

Others said it best.

;-) try Imr or H 4895.

Not only are harmonics going on; pressure waves are bouncing between the back of the bullet and the case and each-other; all whole the barrel is twisting, bowing, stirring, etc...

Load workup techniques such as ladder testing are in my opinion really looking for at what powder load ranges are the barrel harmonics similar. External velocity and SD can be used guess if a load is good; but I know of lots of 100-200y bench resters that don't even use a scale when measuring powder. Volume only, meaning when measured the standard deviation (SD) of velocity of their loads isn't the only driving factor nor the major determining factor for short range BR.

-Mac

 

[4xforfun]

The way I understand it (could be wrong), the BBL vibrates up and down. Up and down because of the way the action is mounted to the stock. Maybe if it were attached at the sides of the action it would vibrate side to side.....Idunno . Anyway, an accuracy node happens when the bullet leaves the bbl the instant the bbl is either at the top or bottom of the vibration cycle. At that moment the bbl stops moving...the instant the movement goes from up to down (or visa versa) the bbl stops moving. THAT is where the accuracy node is. If you have a few ft per second difference in MV (bbl time) the gun still shoots good because the bbl movement is stopped or very slow and the bbl is pointed at pretty much the same place. If you have a load where the bullet is leaving mid stroke in the cycle, where the bbl movement is at it's fastest, any difference in MV would result in the bbl aiming at a much different point. It is all about bbl time. Now, what you are seeing IMO is one bullet/powder may be leaving at the top of the cycle while another combination may be leaving at the bottom of the vibration cycle. thus one groups higher than the other even though the speed is the same. Again (BBL time)

That is how I understand it. I could be WAY off!!! To me it's all voodoo. i just know that actual POI vs point of aim doesn't matter...especially when you are trying different bullets/powder combinations. Find a load that shoots, find your zero, and kick everyones butt!!!

Tod

 

[Pawnee Bill]

The way I understand it (could be wrong), the BBL vibrates up and down. Up and down because of the way the action is mounted to the stock. If it were attached at the sides of the action it would vibrate side to side. Anyway, an accuracy node happens when the bullet leaves the bbl the instant the bbl is either at the top or bottom of the vibration cycle. At that moment thee bbl stops moving...the instant the movement goes from up to down (or visa versa) the bbl stops moving. THAT is where the accuracy node is. If you have a few ft per second difference in MV (bbl time) the gun still shoots good because the bbl movement is stopped or very slow and the bbl is pointed at pretty much the same place. If you have a load where the bullet is leaving mid stroke in the cycle, where the bbl movement is at it's fastest, any difference in MV would result in the bbl aiming at a much different point. It is all about bbl time. Now, what you are seeing IMO is one bullet/powder may be leaving at the top of the cycle while another combination may be leaving at the bottom of the vibration cycle. thus one groups higher than the other even though the speed is the same. Again (BBL time)

That is how I understand it. I could be WAY off!!! To me it's all voodoo. i just know that actual POI vs point of aim doesn't matter...especially when you are trying different bullets/powder combinations. Find a load that shoots, find your zero, and kick everyones butt!!!

Tod

I believe you are right. This was a first for me to see it first hand, where 2 different loads group good with same POA but different POI.

 

[Webster]

Been working up some loads for my custom Savage 110LA with a Bartlein 1-10 barrel in 30/06 at 100 yds.
Was trying 4064, 4350, 4320 and 4831SC pushing a 175 grain SMK that was meplat trimmed and pointed.

48.0 grs of 4064 gave me a good group at around 2650 fps at POA. 4350 & 4320 not so good.

56.0 grains of 4831SC at just over 2600 fps gave me a good group, but it was 2" lower than POA.

So, I went up to 57.0 gr of 4831SC, same good group at around 2650 fps, but this time it was 2.5 in lower than POA.

Ballistically, what causes that to happen?

Is the barrel touching the forearm?

 

[Pawnee Bill]

Is the barrel touching the forearm?

No, it is free floated all the way.