need help

[xpilot]

Air Arms TX200,.177, H&N target trophy, 25 yds. The problem:  will group 2-3 shots in close to one hole then a shot 1-1/2 inches high or low (usually high) the back to same original hole. The rifle is hold sensitive, but I am quite sure of the placement, resting point, trigger pull etc. All dispersion is vertical.  Scope is proven & checked. Screws tight etc. All the normal stuff I can think of is OK. Normally will set on rest with poa a little high, slide rear bag a slight amount to lower poa to center, light pressure and squeeze. (also tried the reverse,low to high),With no apparent difference in hold etc, the vertical will show up. (1out of 4-5 shots).
Any thoughts..  (Have shot some 22 rf benchrest with success)

Thanks,    Jim

[xpilot]

Have tried hand on bag but cannot seem to obtain a steady hold. The rifle has at least 1000 shots by me, previous unknown.
No chrony.

[johninthecamper]

someone else posted about group analysis, tried to find its thread.the site is rifleman org uk small bore target analysis. tried to post a link,unable to figure out how.if somebody smarter than me sends me a email,I can reply with a link,then they can post a link.when I first saw it I read it three days in a row

[Roadworthy]

I suspect seals - either piston or breech.  If you have never replaced the breech seals (there are two) it may be time.  With the lever in the "park" position they are constantly being compressed.  They will eventually "remember" that shape.  As I said, the piston seal could be a culprit as well.

[dennis-baker]

The vertical grouping suggests variations in velocity. Have you tried a couple of drops of chamber oil? When the groups  from our spring piston guns begin to open up, a couple of drops of chamber oil will tighten them back up. Make sure you use the right kind of oil.  Dennis Baker, Baker Arguns

[xpilot]

How much of a velocity change would cause 1-1 1/2" vertical at 25yards?  If I recall, rifle was at about 900fps.

Could seals cause a high AND low dispersion?

[Bullit]

Justa question...   Are you shooting your groups quickly?  Maybe more time, between shot/re-cocking, would help?  At that speed, (900fps), perhaps a different heavier weight pellet? Maybe it got "speed tuned" ?
Edit: Sorry to hear that you are having hold sensitivity issues with that specific rifle. Bummer.

[xpilot]

Shooting at a pace for the benchrest competition. 25 shots in approx 20-21 minutes.

[Pappy]

Wow, could be a lot of things, from inconsistent pellets, your hold, or internal issues.  I think you should find a chrony and run a few strings .... that will indicate any issues more accurately.

[racvt]

I suspect seals - either piston or breech.  If you have never replaced the breech seals (there are two) it may be time.  With the lever in the "park" position they are constantly being compressed.  They will eventually "remember" that shape.  As I said, the piston seal could be a culprit as well.

Does this mean we should store the airgun with the barrel open?  Makes sense I guess, we do not leave the springers cocked........

[Bullit]

Shooting at a pace for the benchrest competition. 25 shots in approx 20-21 minutes.

This is a great point made Xpilot!  Letting it settle after the shot, before "recocking", (justa few seconds),  does make a difference in cooling ( to me), for the seal. It's polymer, and collects heat faster, than it dissipates it.
It only takes a little time to get rid of it.  In short, review the shot before you re-cock it, if you're doing that.
The TX 200 should be an easier shooter tho... JMO.

[xpilot]

Thanks for the info....I'll let it cool for a bit, see if it helps.