Author Topic: Tight Brno/CZ barrels  (Read 3413 times)

Offline ZG47

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Tight Brno/CZ barrels
« on: July 04, 2011, 09:47:57 PM »
Six years ago I bought myself a barely-used ZG47 (imported from Sweden) in 30-06. I bought  six packets of PMP (South African) 150gr SP ammo (not currently available) and started shooting.
The ammunition was good quality but velocities were at 1906 levels. I had the same issue when I started reloading the cases, using Hdy #3031 150gr SP bullets and a medium charge of AR2209 (H4350 in the U.S.A.) but one day I decided to load Sierra 150 GK (boat-tail) bullets with the same powder and bullet seater setting.
Average velocity increased from 2539 to 2679 fps, range (E.S.) reduced from 98 to 58 and the coefficient of variation (s.d. divided by a.v.) reduced to .84%. Accuracy appeared to be the same.
I decided to slug the barrel and came up with bore diameter of .30" and groove diameter of .30815". I believe that with traditional Mauser style, i.e. equal width and groove rifling, a groove diameter of .309 to .310 would be more appropriate. If you read Hatcher you will see that the U.S. Army specified a wide groove diameter for the 1903 rifle barrels.
NB  I tested Federal Powr-Shok boat-tail ammo and was only a few per cent off the factory test velocity. I also checked the PMP website and discovered that the factory test velocity was 860m/s (2822fps) @5m.
I now use Sierra 165gr HPBT bullets, partly because they seem to work well in my rifle and partly because I recalled that all the other guys in my club who used Brno rifles for deer strongly recommended Sierra boat tails. Now I know why!
I would like to know if any other people in the forum have had similar learning experiences with their Brno/CZ rifles.
« Last Edit: August 12, 2011, 07:14:33 PM by ZG47 »
A good shot at close range beats a 'hit' at a longer range.