Clark makes barrels using Lothar Walther blanks; the barrels without the rear extension and cut for optics reduce extraction/ejection problems to the vanishing point. They also give you a low sight mount.
The only other barrel maker that I know of made Bully Barrels they seem to be out of production, great barrels and sought after now.
Clark does trigger jobs although frankly I think the M41 trigger needs very little in the way of improvement.
I shoot mostly CCI-SV & Green Tag these two fit my requirements. For the best precision I have had good results with SK Jagd Pistol Match and the similar RWs Pistol Match.
I have not had good results with Eley Pistol Match but have not given it a fair shot it is so expensive that I cannot bring myself to shoot it.

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Ipsis Rebus Dictantitbus