AFAIK I have not found any references anywhere to any of the Warlords having acquired anti tank rifles or specialist anti tank guns of any sort. AFVs were quite rare in the Chinese Civil War, as were actual field artillery pieces.
The usual solution to an armored vehicle IIRC was to either turn a field gun or mountain gun on the offending AFV or armored train, or get the Hell out of its way!
Wu Pei Fu and his great rival Chiang Tso-lin each had a small inventory of AFVs, the former Citroen-Kegresse halftrack armored cars, the later Renault FT-17s, but to date I haven't come across any accounts of encounters between them at any time.
Armored trains and armored vehicles like field artillery were prestige weapons, so the warlords were often quite reluctant to actually risk their prized toys in direct combat, preferring instead to "engage with the ordinary…" as Sun Tzu advised, and "…win with the extraordinary" with their best troop units rather than risk losing on elf their prized armored cars or field guns.
I do know that a small number of Boys ATRs were supplied to the KMT for use against Japan, but as to their actual numbers and actual deployment in battle, I haven't found any accounts to date.
I do know that the 88th Division (German trained and equipped) used a small number of Pak 36 during the fighting for Shanghai in 1937, but again, very few, and well after the Warlord era had come to an end.
Hope this helps!
Leland R. Erickson
Metal Express
metal-express.net