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Sudden groups scatter - looking for the cause

tstanek

Private
Minuteman
May 21, 2021
49
26
Czech republic
Hi guys,
yesterday I went shooting my 300 PRC B14 HMR Bergara for final tweaks of the load with tuner brake....it was the very same indoor range as always, same distance, same temp., logically no wind, same shooting position, scope, optics...I'm quite confident that like nothing regarding rifle and environment has changed. I was shooting batch of handloads with exactly the same specs as the load before. I compared those two batches and everything seems to be the same (CBTO/COAL, primer depth, whole procedure totally similar, same powder and batch of bullets, primers), but didn't have the time to try previous load again. To my surprise, some fine tuning with EC tuner brake (which previously gave me good results) wasn't at the table as the groups totally scattered from I'd say decent ones to this failure. I put pics of the previous (working) load target and this failure. Any ideas what could have cause this? I'm even more nervous about it as I have some local comp. next week:(
Thanks a lot in advace and have a nice day.
Best,
T.
 

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Are you saying that the only thing that changed, that you know of, was the tuner setting?
 
Oh I forgot to mention I tightened 2 action screws, but according to recommended torque from Bergara and discussions on the internet, and they really weren't loose.
P.S. even same tuner settings I have, for example those marked with "6 on the pics are the same (EC tuner brake setting)
 
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Well honestly, it wouldn’t be the first time that action screw torque made a difference in group size. I can’t say that is your issue, but if it were me, I’d at least try backing those torque values back down to where you had them and see if that makes a difference.

It especially can matter if your stock or chassis is not bedded. That’s not to say that all stocks/chassis need to be bedded at all by the way. But some do benefit.
 
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Did you leave powder in a hopper? (basically somewhere outside of the powder bottle? )
 
Ya, there’s a lot of things that could be an issue.

-parallax
-shooter
-equipment
-loading/powder issue
-long term sample dispersion

Etc etc.

You’re just going to get everyone’s guess in a single they they feel strongly about or most recently had an issue with.
 
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Shooter: stress, hydration, anxiety, caffeine or other drugs, time of day, sleep/fatigue, flinch/anticipation, focus/concentration, expectations,

Equipment: YOU MESSED WITH THE ACTION SCREWS, parallax, YOU MESSED WITH THE ACTION SCREWS, barrel heat, did you clean it, is there a screw loose somewhere else (base, rings, etc).

Load: I’ll assume your loading process is “good enough” but you indicate that maybe the load changed? The powder left in the hopper idea is worth exploring. Maybe.

PFM/Statistical Analysis: it’s theoretically possible that you shot the three best groups the rifle will ever shoot followed by the four worst groups it will ever shoot but it’s extraordinarily unlikely. Feniks can fill us in on the numbers that describe the likelihood of that scenario.

I have never tuned a rifle using torque values on the action screws but if it was me, that’s the first thing I would ”undo”. Next on my list would be checking the other screws…no need to “retorque”. Just put a tool on them and see if they are loose. Finally, make sure you have parallax dialed out completely. After that, I’d assume it was me Having a terrible shooting day. It happens.
 
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Brand new barrel? MV speeding up w breakin? You clocking the rounds?