pellet fit

just a quick question ....i have a viper non express model after recieving it back from getting a tune and trigger from cdt i started the sight in proccess the gun is very accurate however it seems to only shoot that way with gamo match pells or any pellet that seems to fit loose into the breech ...i group beautifuly with the match then i put a beeman crow mag or a gamo hunter pell that fit tighter than the match do and the groups go south on me....now i know wieght makes a big differance but its not just shooting low or high....its off left and right and i mean by a huge margine....i can sit down fire a 10 shot group the first 4 using match ...hit the center then on the 5th shot use a tighter pell.....and inches left or right....i dont want to be stuck using just match pells ...before i have always used crow mags and did well with them....i have put about 250 rounds through it since i got it back from cdt and have only used match pells until yesterday i took a shot at a starling that should have been a slam dunk kill and missed!! using a crow mag pell....so i tested it on paper and sure enough the groups were troubeling..but on the positive note the are consistant on where they hit....they just very from pellet to pellet do i need to allow more break in time for it.....i know that certain guns shoot well with certain pellets....any suggestions?? by the way ....this gun rocks and thanks a million bob ....oh ya one more thing before i use to use a pellet seater on my crow mags ....now it just likes loose fitting match pells

[SDale]

From what it sounds like, ( "They're consistent on where they hit." ) you're groups may be just fine. You may just be experiencing POI shift.  That's what the post reads like anyway.

If you sight the rifle in with one pellet, then switch to another and the POI will shift to a different spot but the pellets will still Group Together consistently. That's just simple Point of Impact Shift and you should adjust your scope accordingly.

If that's the case, then just sight the rifle in with the pellets you wanna use and call it done.  
I'd get a pellet sampler and start shooting to see which pellets it likes best and/or keeps to the same (or close) POI and stick with those. That way there's less headach with having to sight the rifle in every time you wanna change ammo brands or shape.