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POI moves lower with higher charge?


ozzyj

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I'm doing some ladder testing with TAC in 308 from an AR10 (Windham SRC 308 - 16'5 barrel, 1/10 twist) and seeing something I didn't expect.

 

I started working up from a the suggested starting charges and worked up to about only the mid-way point to the highest average charge from 3 different sources.  It seems as though my POI started moving quite a bit lower as I got towards the middle end of the range.  I started at 40gr of TAC with a 168gr Hornady BTHP and used 0.3gr increments in each load.  Moving from about 40.8gr of TAC to 41.5gr moved the POI about 3-4 inches lower at 100 yards.  I wasn't expecting this and not sure if its normal as all of the other charges leading up to 40.8 were pretty close in POA/POI.  Only once 41gr was exceeded that the POI seemed to shift suddenly.

 

Does this make sense to anyone?  The cases didn't seem to have any significant pressure signs as best I could tell (although I am still a bit of a novice reloader)

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2 reasons. 1 is the load hit a different spot in the harmonics of the barrel and is exiting muzzle when barrel is on a downward oscillation. 2nd bullet is moving faster and exiting before muzzle can rise as much do to recoil. likely a combination of the two.

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