Also you need to be careful which PDI Bull Barrel spacers you buy, as the
NEW PDI 6.01 barrel has a 10mm outer diameter, and the old ones were 8mm. Ask me how I know.

I needed to remove the o-rings from the inner hole of the spacer, and machine the hole to fit the outer diameter of the barrel. If memory serves me correctly, an X drill bit was my drill bit of choice. I drilled it out using a lathe to keep the bit properly centered.
What a pain, as PDI hadn't released their new spacers before the released the new barrel.
Someone wasn't thinking at PDI.