She had been intending to sell it but the sale had not happened, which was why the gun was in her car.
Defence lawyer Grant Vosseler said Jones pleaded guilty to the drugs charge to "get it out of the way rather than go through the drama of a defended hearing".
Mr Vosseler said his client did not know she had the bag in her shoe and hadn't deliberately taken it to prison.
"She certainly didn't take it there to slip to her son," he said.
Mr Vosseler said Jones lived in a "remote outback Taranaki location" and had owned the gun for many years. He added that she was on a sickness benefit.
Judge Dugald Matheson convicted her on possession of drugs and possession of an offensive weapon. Jones will be sentenced on April 28.