By golly if you are going to try out the diabolical '5mm' then anything could happen! Diabolos is one name for Satan because the Lord of Darkness works his evil through confusion. Is it a .177 with flat shooting? no ... is it a .22 with easy loading and good energy retention? no.... it is a .20 which was only invented to keep the Sheridan airgun company in profit. Star breeches will be the least of your troubles.
Get an FAC and use your HW80 in .22 at 20 fpe as God intended.