A few considerations:

- the chamber might be pitted. Having the chamber cleaned means that all of the fouling and brass that filled these pits are gone and now ehn you fire the rifle, the brass flows into the pits, locking it into the chamber. It will get better with more shooting as brass is rubbed off into the sharp edges of the pits slicking them up a little.

-Ammo- European ammo makers often load well beyond SAAMI specs, especially S&B. Sticking with milder handloads may reduce the problem.

-excessive headspace- did the gunsmith pass the headspace with the proper no-go guage or with a longer field guage? Did he use the correct headspace guages? there are two different types of 8x57 headspace guages with different shoulder angles. Is there a shiny ring on the brass near the case head? Its not always a sign of excessive headspace, but atleast a sign of increased headspace. Bend a straightened paperclip into an ' L' shape and use it to feel the inside of 10 or so fired cases. if you feel a groove in any of the cases at the casehead, do not fire the rifle until you take it to a competant gunsmith to have it checked- I' m not nocking Gander Mountain, but I' ve seen a variety of of levels of talent and knowledge when it come to gunsmiths who work in their stores.
You can sometimes fix headspace problems on turks by buying a few differenrt bolts and checking the headspace with them in the receiver- You can find bolts on Ebay or at milsurpshooter.net for under $10.
-if you fail to find any problems, don' t clean the chamber for awhile and see if the condition improves, otherwise shoot milder handloads.