Does anyone have a clue whats wrong with my air rifle?

[jedman]

  I have a Mendoza RM 600 air rifle in .177 cal. I have owned it for about 3 years now and just getting to the point of shooting it enough to find it really erratic and doesn't group worth beans.
  I have a 4 X scope on it and solid rings and do not feel that the scope is the cause.
 I have shot 5 different brands of pellets thru this gun and it doesn't like any of them ?
  I shot it thru my PACT  chronograph and it has a extreme spread of over 500 FPS shooting just about any brand of pellet.  I have had some shots as low as 593 fps and one as high as 1145 FPS ?
    When I fire it , It sounds the same and has the same type of feel / recoil but shoots groups of 2 + inches at 40 feet ?
 The only thing I have noticed is the pellets seem to fit the bore loosely, sometimes they fall out when I tip the barrel up to close the gun.
  Has anyone else had a problem like this with a break action rifle ? Its just about useless the way it shoots.             Jedman   

[BUGEYE]

sounds like blowby to me if the pellets are loose. but I'm no expert.

[Sensai]

I'm no air gun expert, by any means.  I have a few, and work on some for family members.  My guess is dry seals.  Check your users manual, or get one online, to see what to use to lube it.  DO NOT USE REGULAR OIL!!!  Most use compressor/air tool oil (mineral oil).  It's important to know where to put it, and not to put it, too.  Since you said "break action" this is probably a spring chamber gun.  I wouldn't shoot it too much until you get it lubed, they're notorious for throwing "O"rings.

[jedman]

   I bought this gun new but it was a display or out of box gun at a discount and it did not have a manual.  I have lubed it thru a hole on the side of the spring chamber with mineral oil , that was before the chrono testing ?          Jed

[Victor3]

If you're getting that wide of a spread, you're almost certainly experiencing 'dieseling.' That's when oil in the compression chamber burns upon firing, making the piston/cylinder work similar to a diesel engine. This gives inconsistent & higher velocities until the excess oil burns off.

[STUMPJMPR]

The extremely fast velocities are most likely dieseling effects.  The low velocities could be damaged seals due to dry firing.  Shoot it more and see if it settles down.

[keith44]

dieseling and blow-by seem to be the top culpurits here.  Clean all the oil you can out of it.  put a high grade moly grease on the spring.  As for the blow-by there are actually two different sized .177 pellets.  German brand guns and pellets work best together, as do other "european" guns and pellets.  but if you mix euro guns and german pellets or vice versa they will not fit well together.  One is larger than the other (I forget which is the larger, but I'll look it up)  Try crossman premier 7.9 grain domed pellets and see if that helps.  They are available from pyramidair.com

Good luck, hope this helps.

Keith

[jedman]

  Thanks for all of the ideas and help,  I'm thinking  I need to get the proper lube and try again.
  The larger pellet idea has a lot of promise also,  I really think the bore in this rifle is oversize.
  I own several other break barrel rifles and they all have a tighter breech, when you push the pellet in there is a small amount of resistance felt, with this gun none at all?
 I have tried taking the pointed nose of one pellet and reaming / expanding the skirt or base of the pellet I am loading but that has not improved anything.
  I may try to push a soft lead split shot thru the bore to get a accurate measurement of the bore size.
  I shoot allot of centerfire rifles and know the bullet must be at or a tad bigger than the bore to get any accuracy.        Thanks,   Jed