What is the impact on having a prochrono too close to the muzzle? I had my chrono set about 6’ away and was getting a wide spread (50 fps) while testing .308 loads. Other than not turning necks, I was very careful with prep on these loads. I weighed each charge using both a balance beam and digital scale making sure the weight was exact between both scales. I later read that the chrono should be 10-15’ away. I wasn’t really doing an ES/SD test, just getting a velocity to include in Quickload. Each shot got faster.
Proper Chronograph Set Up
- Thread starter Old Tanker
- Start date
Thanks, I suspected muzzle blast. The rifle is a Tikka T3 w/20” sporter barrel on a Boyd’s Laminate stock. I have some decent IMR4064 loads but a buddy gave me some 3031 so I thought I would experiment with it and some Hornady 165 Interbonds. Shot two groups of 5, one at 40.3g and the other at 40.6g. Groups were OK, both under 1”. The 40.6 was more of a horizontal spread, which leads me to believe they weren’t as erratic as the numbers indicated. 1. 2556; 2. 2570; 3. 2526; 4. 2579; 5. 2586. Fed GM Match cases on second firing. Charge wt within .1. With a 20” barrel hunting rifle, I want to be a little above 2600 fps. Tough to get 2700 on a 20” barrel and I limit my mule deer and elk shots to <300 yards.
So, my next question for all you responders. How long between each shot? It was obvious that velocity was increasing as the barrel warmed and the cold shot is critical for a hunting rifle.
When developing a load I shoot my three or five shot group as fast as I can. With a hunting rifle I do not let the barrel get hot, but a properly built rifle will usually shoot to the same POI hot or cold.
My experience is that first shot from a clean bore may well be low, but if the bore is fouled, but it usually doesn't matter whether the barrel is cold or warm as to the POI of the first shot or subsequent group size.
A new barrel may speed up after a few rounds are fired, and this can affect the load as well.
When I have my load settled, then I like to verify zero from a dead cold and fouled barrel, just like I would see when hunting. I'll shoot two shots, then let it get completely cold again.
Others do this differently. I do not try to get target rifle accuracy and consistency from a hunting rifle
My experience is that first shot from a clean bore may well be low, but if the bore is fouled, but it usually doesn't matter whether the barrel is cold or warm as to the POI of the first shot or subsequent group size.
A new barrel may speed up after a few rounds are fired, and this can affect the load as well.
When I have my load settled, then I like to verify zero from a dead cold and fouled barrel, just like I would see when hunting. I'll shoot two shots, then let it get completely cold again.
Others do this differently. I do not try to get target rifle accuracy and consistency from a hunting rifle
SSL
Gold $$ Contributor
6 feet is a little too close, but you may be experiencing shot reading issues. I can't explain just exactly why, but I found that placing a strip of clear packing tape over the sensor slots seemed to produce more consistent/accurate readings. I think it does a better job of reducing occasional glare from strong sunlight. Works for me anyway.
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