No two issues are entirely the same or we’d call them the same issue. But while you make some good points, seems to me they are in opposite directions in the first and second paragraphs. In the first you seem to say that ‘moving the ball’ politically on abortion at all has a real effect because some women won’t get an abortion who ‘should’ have, whereas it’s implied gun restrictions (as they would be, realistically) couldn’t prevent any person legitimately defending themselves. But then in second paragraph you admit that private charity can/does provide a around at least some real world abortion restrictions, but no similar mechanism helps people prevented by gun restrictions from defending themselves.
And you are cherry picking one example of a gun restriction (a magazine size limit), somebody else could cherry pick just one example of an abortion restriction.
Where I live for example owners of convenience stores in bad neighborhoods have a heck of time legally having a gun in the store. Not federal law, and totally different in most US states. But likewise significant abortion restrictions by and large apply to the most anti-abortion states, I live in one of the most anti-gun states. You could argue to the store owner the gun doesn’t really make him any safer…from your crime safe neighborhood and job (the general you, not you personally), but might not convince him. And it may be an objective fact he suffers fear and stress because he has choose between being strictly law abiding or having a gun at hand.
In any case I think the position of very pro-abortion rights and very pro-gun rights people is more similar than different on ‘slippery slope’. They don’t want to give ground mainly because they think it will embolden their opponents to seek further advances, not because the effect of a little given ground is that categorically different in the two cases.