Tuning I don't have wandering zero anymore....but I DO have wandering velocity

[Khornet]

After establishing a good tune on my Impact MkII PP 700 mm .22, at 912 fps with ES 9-10, I found my velocity dribbling down to low 890s. Regulator reading unchanged at 95, valve adjuster and hammer preload screw unchanged (though my hammer screw DOES wander if I don't loctite it), pellet probe adjustment in spec, and breech O-ring replaced. Cleaned the bore well in case it's leading, since I'm shooting H&N 23 gr slugs in a pellet liner. No difference. I can see a drop in POI at 70 yd but no change in group size. All slugs are sorted by weight, and all are 23.2 gr, unchanged, same lot.

Does velocity "wander" from time time, just a fact of life, or is something out of whack?

 

[JohnnyPDX]

912 to low 890's. That's 20 fps change on the highest side. Doesn't seem like much to worry about.

 

[elh0102]

Environmental conditions will cause some velocity variation, but not 20 fps. I think something has changed slightly. Rifles with a lot of adjustments have a lot of places for error. I don't know if FX has outrun their headlights, but they've outrun me. Sorry, I shouldn't editorialize. Good luck with your diagnosis. 

 

[nervoustrig]

If the regulated pressure is staying consistent at 95 bar as indicated, the behavior points to some small change in hammer strike. 

Or it could simply be the nature of the system breaking in. For example, if the poppet takes a bit of compression set after hammering off and on the valve seat, it may need a slight more hammer strike now. My suggestion would be to dial the HST back up to your original velocity and monitor its behavior.

 

[DemWitt]

I would expect an inch or two of drop from a 50 yd. zero at 70 yds.. Your ballistics program should verify that. I’d say your gun is shooting well.

@elh0102: I agree, with so many variables, it gets to be a challenge to sort out which one has been maladjusted.

 

[Guest]

There is many a reason I avoid FX. I know their impact has more places of failure than any other gun I can think of. Isn't there like 138 o-rings or more? Sorry to hear your troubles 

 

[nervoustrig]

More adjustments + more features = more opportunity for greatness or more opportunity for frustration and disappointment

 

[Vetmx]

Just keep monitoring your potential issue. Usually a trend will reveal itself. Sometimes jumping online looking for a quick fix sends us down a path of twisting knobs and taking stuff apart. Then we wind up chasing our tails. Be patient and see if it gets worse, if it’s temperature related, after fill related or if it happens only in a certain pressure range on your gauges. Once you have more data, then maybe do something.

 

[Khornet]

It has been a trend over several days. I did try increasing HST in small increments, which did give higher average speed but still with a lot of variability. I had trouble replacing the breech seal, and thinking I might have damaged it I removed it, cleaned out the groove, and carefully replaced it. Immediately got 930-935 fps. So now I'm gradually dialing back and getting some strings with ES of 5, one string with a spread of 3. So I guess it was the breech seal. 

Of note, those tight ES don't necessarily mean tight groups. Gradually closing valve adjuster in tiny increments has tightened them up though.

 

[Vetmx]

You failed to mention that you were into the gun recently. That changes things seeing how we, the users induce 50% of our problems.

 

[Odoyle]

Always make it a point to shoot at least 500 break in shots before adjusting anything.

 

[Khornet]

Erm.....what I described is the things I did AFTER the velocity loss. That includes replacing the breech O-ring, but as I said, I thought I might have damaged the new ring trying to install it, so I re-replaced it. As for 500 break-in shots, that mark was passed long ago. Now I'm back to my settings from before the velocity loss, and groups seem to be back to baseline at 70 yd.