As tried to find an old article I discovered an 1994 Wild und Hund ( German quality shooting magazine) in wich there is an intresting article. They used a 98 action with a 7x64 Heym barrel. The 7x64 is considered as not ideal for a short barrel. I know RWS load them with slow powders, magnum or just 1 step down.
First the selected the ammo that shot well in the Rifle . MEN / Starkmantel ( no longer available) 10 grams ( factory ).
The test is done out of a shooting device to eliminate all shooters error. Barrel cleaned chemicaly after each shortening. Multiple 5 shot groups are shot with every barrel length and the numbers we get are averages from all the groups.
65 cm barrel // velocity : 880 m/sec // 5 shot groups = 3,5cm at 100 m
60 cm barrel // velocity : 869 m/sec // 5 shot groups = 3,5cm at 100 m
52 cm barrel // velocity : 840 m/sec // 5 shot groups = 2,8cm at 100 m
From approx 26" to 20" less than 5% velocity loss
Accuracy improves with shortening from 65cm to 52cm, as you get less swing in a shorter sporter weight barrel.