Thank you for your clarification. Yes I was aware of the distinction in ammunition loading for the unicorn. The precise loading I had forgotten. The later shell guns of the 1820-40s fired hollow shell as in the French and British.
It is interesting that the 1/2 Pud Unicorn was only slightly shorter than the Russian M1805 Light 12-pdr. The use of fixed ammunition as you kindly pointed out means that the role of the Unicorn was certain in the mind of the Russian Artillery establishment as a field piece rather than a hybrid mortar as still practiced with the Gribeauval Howitzer that took various powder charges and higher elevation. This is what I was trying to elude to.
The Russian came to the conclusion that longer howitzers were better for range and accuracy back in the 1750s. It was not until 1803 did France realise this which was rather late and soon all other countries followed suit over the next decade or so.
1. Saxon M1766 4-pdr Granadstuck that was 9 calibres long.
2. Bavarian M1800 7-pdr Howitzer of 7 calibress
3. French AnXI 24-pdr Howitzer of 7 calibres 
4. Wurtemberg M1809 7-pdr Howitzer of 8 calibres
5. Royal Horse Artillery from its inception in the 1780s used the Light 5.5in Howitzer on the block trail but still only 5 calibres. Its elevation was only about 16 degrees so accuracy was poor. Only with the introduction of the Heavy 5.5in howitzer of 7 calibres was there a considerable improvement but this was about 1810 for the block trail. This had been used by the Foot Artillery since the 1780s on a bracket trail. Only in 1824 with the introduction of the Millar Howizitzers that were 10 calibres long do we get a British howitzer as accurate as the equivalent Russian Unicorn.