You should check the valve stems. Many of the 4.5mm and .177 version only have the 2 port version whereas the 6mm had the 3 port version. You can either drill out the extra third port on the 2 stem or just purchase a 3 port version and substitute for better fps results.
I’m presently modifying my M29 with a full length, 6.5”, rifled barrel and reworked valve stem.