Hello,
It's been my experience that these old air rifles prefer lighter pellets. Having tested quite a few, I finally settled on H&N Pistol Match. These are light, flat head pellets which seem to suit all variations of the BSA Improved Mod D. For me, they are also the most consistent both through the Chronograph and in terms of group size. They are relatively expensive though, but in my opinion well worth the extra cost.
Regards
Brian