Personally, I use a table and chair. Supporting my arm, I place a number of cushions in front. I do not lean on the cushions as this seems to produce a slightly different point of impact. For the Mk3, I have sighted it in at 25yds but intend to increase this distance and take detailed notes when I find time and the weather permits. That might now be next year. I do not use a scope for the Mk3 because of recoil creep and the poor scope rail. I merely use the existing open sights.
If a scope could be used, and has been securely fixed, this would produce a better indication of accuracy.
Rgds
A