What is the caliber of your M1891 Mauser? The original 7.65 Argentine/Turkish Mauser? Or has it been re-chambered or re-barrelled
I've encountered a (thankfully) very few rifles with the problem you described. All were caused by a bad combination of minimum chamber and maximum sizing die dimensions.
Please describe your cartridge and load COMPLETELY:
caliber, bullet size, weight, diameter, powder weight and type, overall length,