The cylinders are intended to be lined with steel or aluminum barrels to contain the chamber pressure.
Walls between barrels are very thin. The cylinder structure *requires* the barrels to be epoxied into the
cylinders to provide proper strength for the entire structure. I recommend a slow-setting JB Weld to allow 
enough working time and the ability to adjust headspacing before it sets. 

I recommend using .305" or 5/16" OD 22 Rifled Steel Barrel Liners.
To account for plastic shrinkage or differing barrel diameters, you may need to print alternate cylinders. 
Regardless of cylinder used, you may require manual finishing with a drill bit.

Cylinder_Smallbore.stl : 7.8mm diameter.
Cylinder_Standard.stl : 8mm diameter.
Cylinder_Largebore.stl : 8.2mm diameter.
Cylinder_Grenade.stl : Plastic-only version. Will likely break, but may be viable with carbon filament and 22bb or 22cb.

The cylinder is designed to ride on a 5mm bolt. Previous cylinders were oversized, with 5.25mm axle,
which proved to be too loose on some printers. It's now dimensioned at 5.1mm, and may require drilling.