Even with no choke, most can shoot decent with the right pellets. First, the barrel can be selectively lapped a little from the rear to loosen the tight area near the breechblock by pushing three or four of the big Dae Sung Korean pellets that are well-charged with JB borepaste back and forth (trapped between two rods) ... swell em a bit once in awhile by giving the front rod a whack while supporting the rear one. Make sure to use rods with a good fit, such as at least .170"-.175 for the .177 cal and at least .210".215"- for the .22 cal to avoid the rod flexing too much. Make sure to not get paste on the rods, keep them clean. Never push the charged pellets out the muzzle.
Lap the crown with a roundhead brass woodscrew charged with JB ... don't spin it too fast and don't use heavy pressure ,,, the right amount of lapping is when all the rifling is chamfered evenly and theres is at least a .005" or so chamfer beyond up into the face. A nice trick I learned from Russ Best yrs ago is to check for burrs with a Q-tip by pushing it slowly in and retractinng it while inspecting with high magnification .... burrs will make some fibers hang up and you can see em sticking. Clean the barrel really well from the rear and then slowly push pellets from the breech thru the muzzle .... they should'nt hang up or try to pop to one side on the corner of the crown, as this also indicates a burr.