Author: **jimmy101** » Fri Jun 13, 2008 1:59 pm

A larger chamber for a fixed barrel (and ammo) will usually increase performance. The optimal CB, and how much performance you might gain, depend on many factors.

For very light weight and low friction ammo the optimal CB for a particular barrel may be less the 0.8. For this type of ammo the optimal CB is a pretty sharp peak, if you are off by a small amount in the chmaber volume the performance suffers significantly.

For intermediate weight and friction ammo (say a tennis ball), the optimal CB is often pretty close to 0.8.

For extremely heavy and/or high friction ammo (like spuds or tightly wadded bb's, marbles etc.) the optimal CB may well be 3 or 10 or ...

For very large optimal CBs you get into the range of diminishing returns, the difference between a practical CB of 2 and the optimal CB of 10 (or whatever) might be minimal, as in a couple percent in muzzle velocity.

HGDT will help you answer this question. I wouldn't consider HGDT to be completely verified yet but it appear to be getting all the trends correct and gives reasonable answers. Enter you barrel and ammo and chamber diameter then do a "sensitivity study" on the chamber length. You'll find that if the ammo is very low mass the optimal CB will be less than 0.8 and small errors in the chamber volume have a large affect on the muzzle velocity. For heavy and high friction ammo the optimal CB is much larger than 0.8 and the muzzle velocity is much less sensitive to small changes in the chamber volume.