What's causing very high extreme spread ?

[Shepherd672]

My Hatsan was shooting fine, Then was planning on getting chrony numbers for 7 different pellets. 5 shot strings. First 2 strings do well. The next 3 strings gave me well over 100 fps extreme spread. The highest  was 132fps Ext.Sp. i reshot some of the first pellets that did well to confirm a problem   They had went from 14 to 117 ex. Spr.    I put in a new breech seal thinking this would solve the problem. It only helped a little. Normally my gun shoots those pellets at around 942fps. Most are much slower now with only about 1 in 10 shooting normal speed.  Is there anything else besides the breech seal that could be causing the high Ex. Spr. ?  The rifle has a gas piston.

[Big Bore Bart]

  Clean the barrel first, if that doesn't help, check the piston seal.   Also check the piston for binding.     If you are still getting some shots at normal velocity, the ram should be good, but something is dragging.

[David.Soliman]

Had a similar problem with a Chinese rifle at one case the piston was binding in another case the piston seal was torn not from the edge as they usually do but from the center. This I think caused the pressure build to sometimes close the tear and other times to open it.

[Lee]

I second the possible piston seal damage. Maybe you could try small amount of silicone oil first before opening the rifle.

[Shepherd672]

I had just got my Hatsan back from Hatsanusa a week ago, they had to replace the leaking vortex piston, So I figured the problem was in there. I already have a supply of JM piston seals & would like to fix this myself rather than send it back to them again, They had it for a month. How can I determine if the piston is binding ?  If I need to go in I plan to do a deburr & moly lube. Hatsan said the replaced the piston seal with the ram,  and I've had it only a week with only about 60 pellets through, Is it possible the seal could get damaged that quickly. I thought it was not recommended to use chamber oil in a gas Piston chamber.

[Roadworthy]

If there are burrs or rough spots it is possible for a piston to be damaged on installation.  In any case, no you should not put oil into the compression chamber.  Quality wise the factory seals are not at the quality level of the Maccari seals.

[Shepherd672]

Ok this is weird after 2 days testing
Thursday.  Ok a little stumped now.  5 shot strings. The vortex express average was 972.8 good ex. Spread. Then the crosman, 83.2 ex. Spread.  The superdomes & polymags that had high ex. Spr. Before were good now.  Put the crosman's through again, and now they seem to be normal now. 9.8 ex.sp.   So the last 15 pellets from 3 different ones that were shooting bad before suddenly seem to be ok now.   Maybe not seal & things are starting to smooth out, tomorrow will shoot many pellets & see what happens, or if problem comes back.

Friday
After shooting pellets well yesterday, they are all shooting very slow today. 40-100 fps slower.  Something to note is that accuracy has not. Changed at all,  still get nice tight groups so thinking piston seal must be fine.  So why has everything changed again after 1 day?  I also rechecked the chrony with my other gun, its fine.