SD vs shoulder pressure?

[Subwrx300]

Has anyone here tested/noticed SDs change with varying hold and/or shoulder pressure?

I'm going to rerun a test in a few or two with more rounds, but today I noticed that with firm to heavy pressure into rifle (semi auto 224V) my SD was about 7-8 over three 5 shot strings, 15 shots total. When I relaxed a bit with lighter pressure into shoulder, my SD jumped to about 20fps (only had 8 shots using this hold though so small sample).

It seems reasonable that if loose into shoulder on a lighter rifle, SD would grow and vice versa, all else being equal. Curious if anyone else has noticed this. Specifically gas guns thought; I have not noticed this on my bolt guns but they are quite heavy by comparison.

 

[mdmp5]

On target, yes. But never on a chrony, just bc I never tried it. Sounds crazy

 

[308pirate]

On target, yes.

Yep, we all have seen how changing the forces we exert on the rifle changes POI. Which is the same as saying that changing the forces we apply to the rifle changes the bullet trajectory.

If the OP is using a photoelectric (traditional) chrono, changes in bullet trajectory over the sensors will yield changes in measured (not actual) MV due to the slight changes in time of travel between sensors caused by the different trajectories.

That's my theory anyway, based on how I know traditional chronos work

 

[Subwrx300]

Using LabRadar for Chrono. My poi was the same for each string and these where shot at 100 yards. I'm talking about changes in pressure that are enough for experienced shooter to noticed the difference but not enough to distort precision on target.

And I can't recall exactly but I don't remember seeing the same issue quite as much with my magneto speed, which actually backs up the theory a bit more: because Magneto is attached to the barrel, some of the movement would be "canceled" out. The magneto speed would be traveling the same speed reward during recoil as bullet would lose so it will read "the same" +-.

Think about playing catch with a baseball down the middle of moving train. Both people on the train and a 50mph ball is still perceived as 50mph because your both on the train. However, throw a 50mph baseball to someone off the train while train is moving at 50mph too and the other person would see it as either 0 or 100mph depending on trains direction of travel.

 

[Subwrx300]

Yep it's real: found this with some extra Google Fu...
Should have been following @Lowlight 's training series on the Tube. Good stuff! While I was shooting prone in all my practice yesterday, I did change how I loaded system and only noticed the increased SD (much lower velocity shots) when I relaxed pressure.



When in doubt, search Frank's channel for the answer. He's probably covered it and then some! Lol!

 

[308pirate]

Think about playing catch with a baseball down the middle of moving train. Both people on the train and a 50mph ball is still perceived as 50mph because your both on the train. However, throw a 50mph baseball to someone off the train while train is moving at 50mph too and the other person would see it as either 0 or 100mph depending on trains direction of travel.

FML the last time I had to think about that was way back in HS physics learning about inertial reference frames.....LOL

 

[RTH1800]

It’s true. No question. There is an art to shooting over a chrono.