My .172 cal is but one of four calibers (.172, .224, .243 and .257) I can interchange on two PCPs I built from scratch, one regulated and one not (good for up to 4500 psi).... Here is a photo of the regulated one....The unregulated one has a full length reservoir with no bottle underneath.... Here is a photo of the .172 cal slugs I tried, four weights of Varmint Knockers and the cast NOE 24.7 gr., which was the best.... The longest VN was unstable in the 9" twist, and the next longest one was marginal....Here is a photo of the NOE HP in a block of soap at 60 FPE (at 2800 psi)....Lots of fun.... Bob