MOA differences between guns.

[tillroot1]

I have a question that I may already have the answer to, two rifles, same chamber, 6BRA. both 28" long, both shooting Lapua brass, both shooting 103 grain bullets, one loaded with 32.xx of varget, one loaded with 31.xx of varget. Dial up from 100 to 600 yards on the heavier load was 14 MOA. Dial up on the lighter load was only 11MOA from 100 to 600.
If the scopes are mounted at different heights would that be enough to make 3MOA difference?


It just seems strange that a grain of powder in identical set ups that there would be this kind of results.

 

[NZ_Fclass]

It's not rocket science. ...oh...wait...


Do all the measurements, including scope height, speed etc and feed the data into a ballistic programme.

 

[Jeff Porter]

Yes scope height matters.

As does the actual velocity of the loads. Different barrels will give different velocities with the same powder charges.

 

[wwbrown]

I was curious so I used JBM Ballistics to make a couple of estimates using a Berger 105 g VLD with a muzzle velocity of 2,900 fps one with a scope height of 1.5" above the bore the other 2.5" above the bore.

With a 100 yard zero the come ups to 1,000 yards was 28.9 MOA for 1.5" above and 27.8 MOA for the 2.5" above. So yes scopes mounted at different heights will need different come ups at range. For the example given the differences in come up for the two examples given were not significant and the difference in scope height was significant.

With these numbers in mind I don't think your difference in 3 MOA for 600 yards can be fully attributed to a difference in scope heights.

Maybe this helps, maybe it doesn't...

 

[tillroot1]

It's not rocket science. ...oh...wait...


Do all the measurements, including scope height, speed etc and feed the data into a ballistic programme.

I get all that, but it just seemed a bit extreme to be a full 3 MOA at 600 yards with the stiffer load taking that much more elevation.

 

[NZ_Fclass]

I suggest, given the previous reply from wwbrown, that there is a combination of events.

Some time ago, a friend and I had 2 280AI hunting rifles built, using "new - old stock" 7x57 barrels. The shorter 22in always produced more velocity than the 24in, using the same ammo.

It could also be the fitting of the barrel - one pointing down, one correctly up - they are never completely straight, added to scope height.

I assume that the scope rings and rail have been checked (is the "20 Moa" rail really 20 on both rifles?)

And does it really matter? Once the zero is set for the rifle... That me out of ideas!