Yes,
Ray B, your question is indeed different from the one
IvW is answering
To be specific, the 75% rule quoted by
IvW applies to attempts to shoot lighter bullets TO THE SAME POINT OF IMPACT and IN THE SAME COMPOSITE RIGHT & LEFT BARRELS GROUP SIZE, i.e. WITH THE SAME REGULATION as original loads. It indeed works well in some rifles in some calibers, and less so in other rifles in other calibers.
Your question
Ray B, if I understand it well, is whether loads with various bullets weights and various velocities can be designed to use the right barrel as a single shot "without regard for where the same load may impact when fired from the left barrel."
The answer is generally "yes", within the same limitations as the usual load development for single barrel rifles. For example, bullet weight and powder charge must more or less corelate; bullet length and rifling twist must be compatible; etc. Within reasonable parameters, and unless extreme velocities (high or low) are pursued, there is no issue with getting accuracy from the individual barrels of a double, regardless of load. For example a 500 gr .458 TSX will generally group as well in a .450 NE double as a 450 gr .458 TSX, although at a different point of impact, but this may or may not be the case with a 250 gr .458 TSX.
Please note however that precautions must be taken (such as using a filler) with light loads.
To a point, this discussion is deeply connected, although in reverse, to the usual discussion about regulation, where the issue is the grouping together of the right barrel group with the left barrel group.
I believe that the following pics will help illustrate:
1) Here, with this Hornady 500 gr DGS load, my Krieghoff .470 NE right barrel groups very well, and the left barrel also groups very well, but the two groups are not together. Each barrel shoots well, but this particular load does not regulate very well in this rifle because the two barrels are a little over 4" apart at 50 yards. This load is still well within "minute of Buffalo or Elephant body shot", but would not be ideal for an elephant frontal brain shot, although still marginally acceptable for a side brain shot. However, each barrel, used individually, would be deadly on Duiker at 200 yards with a properly sighted scope on either barrel. This example likely illustrates
Ray B's question.
This load with the Barnes 500 gr Banded Solid behaves similarly.
2) Here, with this Woodleigh 500 gr FMJ ("solid") load, my Krieghoff .470 NE right barrel groups well, and the left barrel also groups well, and the two groups are reasonably together (2") at 50 yards. Each barrel is accurate, and the load is quite acceptable for this rifle for any DG hunting out to 75 yards, but could be limited for small PG at 150 yards.
3) Here, with this Woodleigh 500 gr RNSN ("soft") load, my Krieghoff .470 NE right barrel and left barrel group very well together at 50 yards, but each barrel individually behaves differently. The right barrel strings vertically, but the left barrel strings horizontally. Each barrel is accurate, and the load is acceptable for this rifle, but in fact not really better than the "solid" load above.
Note: all three loads were subsequently centered by moving the composite groups 3" left by clicking 3 clicks on the Leica Tempus red dot.
4) Here, with this 500 gr Trophy Bonded Sledgehammer Solid load, my Krieghoff .470 NE right barrel and left barrel group well individually and together at 50 yards, although the left barrel shoots higher than the right barrel, and although both barrels shoot to a different point of impact compared to both Hornady DGS and Woodleigh FMJ or RNSN. This bullet/load is likely to disperse vertically significantly at 100 yards, hence is not ideal for this rifle.
This load with the Barnes 500 gr TSX behaves somewhat similarly, but shoots at roughly the same point of impact as Hornady DGS, Woodleigh FMJ or RNSN and Barnes Banded Solid.
In summary, if the double rifle is to be used as a single shot rifle (which is typically done for longer range shooting when it is scoped), it is generally easy to develop a load that groups well in one individual barrel, typically the right one because it fires with the first trigger on most side by side doubles (designed for right handed shooters), with whatever bullet of choice (within reasonable parameters), and to sight that specific barrel to the scope point of aim.
I hope this answers the question
