In our CAR78a carbine version of the QB78, the barrel is shortened by 7-inches from the standard factory length - down to 14-inches. This reduction in length lowers muzzle velocity by about 10%, according to our tests. This applies to both calibres when tested at the same ambient temperatures.
So, as you have made a similar reduction in length, you would have up to a 10% reduction in muzzle velocity compounding the effect of the calibre reduction.
Stephen Archer
Archer Airguns Inc.
http://www.archerairguns.com