Learn English – What does ” expand our reach” mean in this context

[. . .] a bullet that will transfer its energy quickly onto its target. This is where soft-point and hollow-point projectiles come into their own. They are designed to "mushroom" or expand when they hit any resistance ([e.g.] animal flesh). This expansion makes the diameter of the bullet increase. This is done for two reasons: When travelling through flesh, this expansion slows the bullet down quicker than a bullet that doesn't mushroom ([e.g.] full-metal-jacketed bullet), and causes the energy to transfer in a shorter time span, causing greater shock. And the other reason is that when the bullet increases in diameter, it leaves a wider wound channel in the animal, causing greater damage and a quicker kill.


_{Source: huntingclubshoalhaven.org, Which Bullets?}

Basically, some ammunition is designed so that it deforms throughout impact, rather than retaining its point and passing cleanly through. This is actually designed to make the bullet go slow in the body, stopping sooner (imparting more energy at once) and harmfully widening the impact channel.

This is the sense intended:

mushroom
^VERB
2 Form a shape resembling that of a mushroom:
the grenade mushroomed into red fire as it hit the hillside

      →2.1 (Of a bullet) expand and flatten on reaching its target:
         these are high-performance bullets which mushroom upon impact

_{Source: Oxford Dictionaries definition of mushroom}

This is more of an informal way of speaking. To "burst" implies (in this context) :

to be full as if to the bursting point with pride.

This simply means that the speaker is super proud of his hardworking girlfriend and his feeling of pride towards her is very high, and for the same he has used "burst" to describe the level of pride he has towards her. This feeling fills him up and he feels like he may explode (metaphorically, of course).

"To marry the sh*t out of her..." simply means to marry her and keep her very happy. This is a common usage, especially in American English. Similar sentences would be:

I'm going to scare the sh*t out of them this Halloween.

This is just to create an emphatic impact. There is specific meaning and sh*t has got nothing to do with the actual concept.

Another usage is to replace 'sh*t' with 'hell' or 'bejesus' :

I'm going to scare the hell/ bejesus out of them this Halloween.

This simply means : 'to make one feel extremely frightened'.

These are highly informal and never to be used formally. It may come off as very rude or offensive to many people.

NOTE: 'scare the sh*t out of someone' is commonly used. 'Marry the sh*t out of her' is an analogy to the same, but isn't used widely (it is content-specific). There are many ways you can recreate it, based on the context.