Criminal Code "careless storage" charge
OK, I will try to help out here.
The two main recent Canadian cases are the well-known ones of Mr. Montague in Dryden and Mr. Knight in Victoria. These cases had extreme facts, including full-autos and ground-off serial numbers, so I think that a judge looking at a "simple" ammunition storage charge would be entitled to go back to square one and interpret the statute carefully.
To repeat, the law says:
Every person commits an offence who, without lawful excuse, ... stores ... any ammunition ... in a careless manner or without reasonable precautions for the safety of other persons.
It is very important when considering criminal charges that every "element" of the offence be shown. For example, the Crown would have to prove that the rounds in question were not blanks (no projectile), because the item must have a projectile to be "ammunition" under the definition found in the Criminal Code.
More interestingly, the law uses the verb "stores" to describe the active element of the offence. There is no definition of "stores" in either the general or firearms-related definitions listed in the Code. Dictionary definition will have to do: "to place or leave in a location for preservation or later use or disposal" (Merriam-Webster).
Note the deliberate factor here. To store is to do something for a reason. Accidents don't count. So the Crown would have to prove that the person charged decided to place the ammunition at the spot with intent.
Suppose a mail order package of ammunition is left on the front step. Did the accused person deliberately store it there? I would say "no" and the charge would fail.
Then there is the problem of the latch that did not latch. I think that if the accused person testified that he intended to store the ammunition with a lock, but that he accidentally did not push the latch all the way home, the Crown's case would be in trouble.
Next, look at the word "carelessly". I think that the Crown would have to show that the accused showed no care at all to make this version of the offence stick. (Remember, the burden is on the government to prove lack of care, not on the accused to prove that he acted carefully.)
Because of the use of the word "or", the Crown gets two kicks at showing the accused failed to meet the standard required: carelessness and the subjective test of "without reasonable precautions for the safety of other persons". Subjective, yes, but not simply an arbitrary reaction by a police officer. The Crown must be able to articulate a reasonable precaution that was not taken and be able to show that the absence of that reasonable precaution put the safety of other persons at risk. This may be harder than it looks.
Say the ammunition was stored in its factory packaging on an open shelf. So the accused did not take the precaution of using an ammo locker. But let's say the shelf was in a shed with a functioning padlock. The accused did not take every precaution, but he did take some precaution. Since he did not act "without precaution"; the charge fails.
What is meant by "reasonable" is always dictated by the circumstances. A reasonable lockup in the city might be different from that in the country. In the Knight case, the judge seemed to think it was relevant that Mr. Knight lived in an area where burglaries were frequent.
Don't forget that the safety of "other persons" must be at risk. So again, if the accused is living twenty miles from the next house, the Crown is in trouble. Proximity of matching firearms is probably relevant here too.
Finally, we have to look at the escape clause: "without lawful excuse". This gives the accused the opportunity to put forward an affirmative defence. For example, if a certain type of storage proposed by the Crown would tend to damage the ammunition, the accused could argue that it would be unsafe, and therefore unlawful, to take that precaution. (This would also fit into the "reasonableness" argument.)
Regarding the specific question of buying a big safe for cases of surplus ammunition in the garage, I would say probably not. First, the case itself may be secure by way of the metal seals. Next, the garage may be sufficiently locked. If you are the only person within miles who has a 7.62x54 firearm, and that one is in your gunsafe, then what other persons are at risk?
As a responsible gun owner, you will, I trust, be asking yourself that question on a regular basis: is anyone being put at risk by my actions? If the answer is "no", then you should not be worried about criminal charges.