Anyone with a .30-30 tube magazine carbine will know that it shoots higher with the faster 150 gn ammo than with 170 gn.
I have found that even with the same bullet, my 1949 Marlin 336A .30-30 rifle (24” barrel and more sensible magazine hardware) also has a predictable POI response to velocity. With 2208/Varget and LVR powders I have compared group velocities and POI, and individual shot velocities and POI. The same responses have been observed both ways.
In the past I have said that this rifle at 50 meters shoots about 2” higher (nearly 4 moa) for every extra 100 fps of velocity - with variation of course. This applies to groups and to individual shots, and has been a reliable predictor of what happens when I deliberately increase or decrease velocity. It has also been a useful pointer to what to do about vertical stringing within the group – find a less variable load.
There will of course be other explanations for stringing such as bedding, tight forends, loose buttstocks, bench technique etc, and some will be valid. Plotting shot by shot POI against velocity may however save a lot of fruitless beating about the bush chasing invalid explanations.
I find vertical stringing is exacerbated with cast bullet LAS Ram loads, where velocity is up there, but not all the way, and the loads used are on the lower edge of where the rifle powders want to burn.
Cold barrel high shots are another issue, which likewise follow the POI/velocity relationship. An extra 50 or 100 fps puts on another 4” – 8” of elevation on the Ram, requiring a first shot belly line hold – if I remember.
My Savage 99 .30-30 exhibits about half the POI effect of the Marlin when velocity is changed, and with the heavier hunting loads has no problem with stringing. The Savage of course is heavier and lacks the magazine tube.
My even heavier Ruger 77V .308 with jacketed bullets puts just about everything from 130 to 180 gns, with velocities of 2600 to 3000+ fps, in the same 3” group at 100 m, and zippy cast bullet loads are close enough for field use also. Within that 3” of course, individual loads do much better.
