Tinker,

When I get any new double rifle, after getting it back from the gunsmith
who does all the checks I like done (I've explained this in previous posts),
I load up a range of Standard loads for that cartridge in sets of 4.
So 5 sets of 4 cartridges, generally starting well below normal load and
the last set of 4 at or slightly above normal load.

I then go down the range and see how it shoots.

Apart from 1 or 2 doubles, all of the ones I have shot like this (30+)
have shot reasonably well.

Once I decide on the best load, I then reload all the cases with this load
BUT I may play around with the seating depth of the bullets to see if this
has any affect on accuracy / regulation.

I sometimes find that it does have an effect on how close the shots fall on the target.

In regards to any recommendation on the OAL being longer than shorter than normal,
I have not done a comparison but nothing has ever jumped out at me as to one being
better than the other.

In regards to my Merkel, it just seemed to have quite an effect - the gun was accurate
but I just decided to play around a bit and it seemed to make a difference.

500 Nitro