Joe asked:

I have heard that there is an offence of “careless storage of ammunition”. Where is this offence defined in the Criminal Code? What is considered “careless storage” of ammunition?

Section 86 (1) of the Criminal Code provides as follows:

86. (1) Every person commits an offence who, without lawful excuse, uses, carries, handles, ships, transports or stores a firearm, a prohibited weapon, a restricted weapon, a prohibited device or any ammunition or prohibited ammunition in a careless manner or without reasonable precautions for the safety of other persons.

You will notice that the offence contains numerous constituent elements. In addition, each of these elements is phrased in the alternative, meaning that any combination of the three will suffice to constitute an offence under s. 86 (1).
First of all, there are six possible prohibited actions involved in s. 86 (1):

(1) uses, (2) carries, (3) handles, (4) ships, (5) transports or (6) stores

Similarly, there are six prohibited objects which those actions may apply to:

(1) firearm, (2) a prohibited weapon, (3) a restricted weapon, (4) a prohibited device or (5) any ammunition or (6) prohibited ammunition

Finally, there are two alternative conditions that must be satisfied if the accused is to be found criminally liable:

(1) in a careless manner or (2) without reasonable precautions for the safety of other persons

Some simple math (6 x 6 x 2) results in s. 86 (1) containing some 72 distinct criminal offences!
However, for the purposes of Joe’s questions, let us deal with the one offence that was asked about:

Every person commits an offence who … stores … any ammunition … in a careless manner …

First of all , what is considered “storing” ammunition? Second, what is the legal definition of a “careless manner” for the purposes of s. 86 (1)?
In other sections of the Criminal Code, the jurisprudence distinguishes between the storage and use of firearms. In other words, a firearm does not need to be “stored” legally if it is in use by an authorized individual. For example, if a gun is being cleaned or shown to others, it is not subject to the safe storage requirements outlined in the Code or the Regulations.
The same, it would appear, is true with regards to ammunition. If it can be shown that the ammunition is “in use”, for whatever lawful purpose, it will not be subject to s. 86 (1).
The more difficult question regarding the careless storage of ammunition pertains to the meaning of “careless” in this context. Unlike s. 86 (2), which refers to the storage, transportation, etc. of firearms and makes explicit reference to the Regulations, s. 86 (1) does not.
Moreover, in any event, the Regulations are of little assistance when it comes to the storage of ammunition. The only pertinent reference to ammo storage is in regard to the storage of firearms, where the Regulations provides that a firearm may only be stored if:

it is not readily accessible to ammunition, unless the ammunition is stored, together with or separately from the firearm, in a container or receptacle that is kept securely locked and that is constructed so that it cannot readily be broken open or into.

In terms of criminal legislation, this is the extent of the statutory requirements with regards to storing ammunition. It should be noted, by the way, that storage is not deemed careless simply because it contravenes the Regulations. In any event, the case law, however, has developed a broader (although not unlimited) definition of “careless storage” regarding ammunition.
First of all, it should be noted that the majority of these rulings are from provincial trial courts (such as Ontario’s Court of Justice) and are therefore not only not binding on other provinces, but not even technically binding on  trial courts in that same province. The result, unfortunately, is major uncertainty.
In regards to this provision, “careless” means “conduct showing a marked departure from the standard of care of a reasonably prudent person in the circumstances”. Note that a marked departure is required, meaning that not every departure from the standard of care will result in criminal liability.
It should be stated at the outset that every careless storage of ammunition case somehow involves the presence of a firearm. Ammunition on its own, therefore, is unlikely to be subject to any independent storage requirements for the purposes of s. 86 (1).
Courts have found that the Crown, when prosecuting a s. 86 (1) charge in relation to ammunition, have an obligation to lead evidence about the legal requirements of ammunition storage. Specifically, where existing regulations dealing with the storage of ammunition are not relevant, the Crown must lead expert evidence to establish that the storage of the ammunition was careless.
Finally, it should be noted that s. 86 (1) only creates an offence where a person is acting “without lawful excuse”. Courts have found that the following can constitute a lawful excuse for the purposes of s. 86 (1):

(1)  Self-defence or the defence of others
(2)  Defence of property

This list is, of course, non-exhaustive.
The end result of this survey reveals that this is a relatively uncertain area of the law. Without firm statutory guidelines regarding the storage of ammunition, the essential elements of this charge remain somewhat ambiguous. As always, experienced and knowledgeable counsel (and sometimes, with the support of qualified experts) will be required to ensure that the court is clear on the legal requirements of section 86 (1).
(Note: Like all of the material on this site, this post does not constitute legal advice. Consult counsel with the specifics of your situation for advice. This material is presented as legal information only.)