It is highly relevant, because the question asks explanation of a real distinction between the positions. Distinguishing between positions without referencing (let alone defining) either is practically a contradiction in terms. If one asks how A and B differ no satisfactory answer is possible by referencing neither A NOR B.

Fair enough so far as it goes but, in my admittedly limited subjective experience, those who recognize a host of valid justifications for taking a life with a gun recognize few if any for taking one with abortion. That is a contradiction, inconsistency; a cognitive dissonance in the dual position all life is supremely sacrosanct and can only be taken to save that of another—except when it is NOT.

Were exceptions superfluous as justifiable homicide, abortion ban authors would not VOLUNTEER to state rape prevents pregnancy, and that abortion bans therefore require no rape exception. Obviously pregnancy from rape does not, in itself, endanger life or limb, which threats I personally consider necessary justification for abortion. That is tangential to the real problem though: Non-explicit legal exemptions—whatever their nature—have a dismal record of efficacy. We can say blanket abortion bans assume exceptions to save a pregnant womans life, "legitimate rape," or anything else, but unless bans EXPLICITLY state such exceptions prosecutors could (and would) counter that, unlike (other) homicide laws, recently proposed and enacted abortion bans state NO exceptions.

In other words, defense of ones own or anothers life legally justifies homicide BECAUSE LAWS AGAINST HOMICIDE SAY SO, explicitly, not because law invariably bows to unspoken common sense and decency (it quite frequently and flagrantly thumbs its nose at both, actually.) Legal exceptions are always absent where absent from law (that is a tautology, after all.) Virtually NOTHING can (or at least, should) be taken for granted within a courtroom, or a legislative chamber. Most conservatives are keenly aware of that each and every time any gun regulation is proposed (often even in reevaluating existing ones,) but tend to overlook it when considering or even crafting abortion regulations (or outright bans.)

I comprehend the philosophy; as Aristotle noted, grasping a position does not require accepting it. However, and once again, the issue is not many "pro-lifers" broadly, fiercely, opposing abortion access and supporting gun access on "a principle that says that you do not take life without a good cause." The issue is parsing (or denying) "good cause" for one but not the other. I dare say that is so for nearly everyone, regardless of issue or stance; as others have noted, many pro-choice anti-gun advocates play the same game in reverse, demanding guns have strict narrow laws yet abortion little or none. Very few on either side of guns or abortion oppose OR support them under ALL circumstances. Virtually everyone recognizes there is sometimes is and sometimes is not "good cause(s)" to take a life via guns AND abortion; the usual sticking point is specifying that cause.

Hence, while the rule of thumb is "it's legal if there's no law agin' it," when there IS a law agin' it, it's ALWAYS illegal unless that law specifies some exception.

In a discussion where one side assumes the consequent (i.e. all the unborn are people) and implies a monopoly on just cause, my civility is near its limit. In terms of "hyperbolic invention," VERY few pro-choice people support casually killing fetuses, or even blastocysts, whether or not they believe they are or might be people, so that was a rather extreme strawman. Likewise, very few anti-gun people would casually watch their familys slaughter rather than pick up a gun and defend them. We all recognize "a principle that says that you do not take life without a good cause;" the OP implicitly recognizes that principle. It is unique to no one side and thus grants none any moral ascendancy. The questions are what qualifies, who decides and for whom.