A Muhrim should not carry weapons with himself/herself even if they are cold weapons or firearms, but it is a precaution not to use defensive weapons like shield etc. But in the cases of necessity, danger and fear from thieves, ferocious animals and enemies, using any necessary weapon is permitted.
The thing, which is Haraam, is to carry the weapon like having a sword on the waist or having a gun on the shoulder or carrying it by hand. But if these are placed in the house, tent or trunk of a car of a Muhrim then it does not harm his/her Ihram although it is a mustahab precaution to abstain from that except in the case of necessity.
Whenever a Muhrim carries a weapon deliberately, then the atonement of this act is one sheep as a precaution.