40 S&W Strange Results

T

Thomasq

Inactive
I just started working up some loads for my SIG 229 in 40 S&W. The results have me totally puzzled.
I am using Longshot with 165 Gr Jacketed HTP and 155 Gr Hornady XTP.
I started with 7.3 gr and stepped up to 7.7 with the 165's and 7.5 to 9.1 with the 155's. None of the loads showed any pressure problems on the spent cases.
My problem is with the velocities I am reading with my old Pact Model 1 Chrono.
With the 165's the velocities started at 2118 with each increase in powder went down until they got to a low of 1921, The 155's were similar in that they started at 1750 and stepped down to 1612 with each increase in powder. I know that it is impossible to get that velocity from a 40 S&W so I am looking for what could be causing the erroneous readings. I first thought I was getting muzzle blast so I moved the screens to about 25 feet and got similar results. Thinking it was the Chrono I fired some 22LR and got readings within 1 % of the published speed for the round.
It was a bright sunny day both times which I know is not ideal.
Any thoughts?
Thanks.
Tom
 
22LR read correctly yet 40 didn't. Shot at same distance, same location thru the chrono, lighting, units selection, same power source, etc. Will be interesting to see what comes of it. If no clue, check with the chrono's tech supt.
 
How far away from you was the Chrono ?

You might need to move it farther away.

Your chrono might be trying to read the muzzle blast.
 
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