Reloading .303 British : Effect of Primer Selection on Muzzle Velocity

Two different load fired to obtain different mean velocities:

Load 1 - 40.0 gr Varget,  MV = 2332 fps

Load 2 - 41.5 gr Varget,  MV = 2451 fps

From Strelok, bullet drop predictions at 200 yards between these two loads is 1.3 inches, 0.65 MOA.

I shot each load at 200 yards with the scope removed, sighting with the Mk I aperture sight with the following measured results at the target: 

Load 1 Elevation MPI = - 2.0 inches from target vertical center

Load 2 Elevation MPI = +2.0 inches from target vertical center

Elevation rise from 118 fps mean velocity increase = 4.0 inches (2.0 MOA)

The ballistic bullet drop difference between these loads accounts for 0.65 MOA of the above 2 MOA with a net difference of 1.35 MOA. Therefore, the jump is affected and the rifle without the scope has negative compensation. 

I then shot each load at 200 yards with the No. 32 scope fitted with the following measured results at the target:

Load 1 Elevation MPI = + 0.50 inches from target vertical center

Load 2 Elevation MPI = + 1.75 inches from target vertical center

Elevation rise from 113 fps mean velocity increase = 1.25 inches (0.62 MOA)

The ballistic bullet drop difference between these loads accounts for the 0.62 MOA elevation rise. So, with the scope, there is no change in angle of jump as bullet velocity increases. 

CONCLUSIONS:

1) This rifle does not exhibit positive “compensation”; without the scope, faster bullets leave the muzzle at a higher angle of departure, not lower.  With the scope, the angle of jump is unchanged by bullet velocity changes. 

2) The No. 32 Scope and mount alters action body stiffness such that it mitigates the effect of bullet velocity variations at short range. 

3) This rifle has a barrel bearing between the sling swivel band and chamber reinforce which must affect barrel vibration and is the likely reason it does not possess positive compensation that the standard stocked No. 1 and No. 4 rifles possessed. 

While “compensation” is highly desirable for long range, it comes at a cost of larger vertical spreads at short range. Mick may know the answer to this, but I believe competitors were permitted to use different rifles at different ranges, perhaps stocked up differently, in Bisley SR(b) matches.