Honestly just tell her how you feel (WITHOUT the boyfriend around so he cant deck you

) Save yourself time and trouble and just put it out there--shes going to feel one way or the other and you will probably be able to tell without her having to tell you just by her reaction--then you will know how to proceed---no use trying to woo a girl that has no interest---just saying
A. Probably picked the worst time in history to want to buy a gun, wait for the dumbness to die down, research.
B. She has a bf already? If you respect her, Wait. Homewreckers suck. Besides a chic usually knows you like her before you've even figured that out for yourself.
Definitely can be tough to break that ice. I'm not a huge fan of interfering with people who are already in a relationship, but honesty is always the best policy. Try to put both of you in as comfortable a setting as possible, if you're going to tell her. Don't try to rig things. It's actually okay to say it awkwardly, as long as you don't come off as a creeper. You can't fear rejection, because either it works out for you, or else she says no, in which case you know that it wouldn't have worked out anyways. Either way it works out for the best for you.
The thing that would worry me is that, if you aren't already friends, and she doesn't really know you, you're in no position to butt in on a current relationship. If you're already close friends, however, (and you'll have to forgive me for forgetting if that's the case), then that's about as ideal of a position as you can be in (timing aside on both counts, as Skystorm points out). Either way, I've always felt that you should be close friends with someone if you're looking for a lasting relationship. If you aren't compatible enough to be friends, there's no way you're compatible enough to be more than friends. Physical attraction only takes a relationship so far.
We're already really good friends (which may or may not be contributing to my problem, I don't quite know yet). And the worst part, possibly the reason I haven't told her yet, is I understand what Skystorm's saying... Homewreckers do suck.
And speaking of her boyfriend (not meant the way it sounds, I'm not trying to call him a homewrecker), it doesn't seem like he's on the verge of anything other than more kissy faces on FB. Which I'm all but infinitely sure translates to a major road block for me.
A little background on the situation:
We've been really close friends since about November 2010, so over 3 years.
She used to date my best friend (they are each others only ex that they're still on speaking terms with, it was mutual), and minus about 30 lbs and plus about 3" we're almost twins (separated by 6 months difference in birthdays and about 1500 miles, and the fact that we have no blood relations) interests, (lack of) intelligence, and generally being unshaven 90% of the time.
When we met I was literally only capable of talking to 4 other people at youth group (which is where we met), I started off as the fat guy that sits in the back looking down and twiddling his thumbs. Now I'm the fat guy that tends to wear a lot of black, plays with knives, fire, and guns, and can now yell at a group of 40 or so middle school kids for not listening, and act like nothing happened afterwards. Oddly enough, our interests line up almost perfectly, knives, fire, guns, and yelling at little kids for disrespecting someone.
Reasons I really don't want to wait much longer to tell her:
My first steady job is likely to include an M16, 90lb pack, and cammies.
Her current boyfriend has lasted longer than any boyfriend she's had since I've known her (she dated my best friend back when they were 14), and may not end anytime soon.
Her job cut out the 3 hours a week we used to hang out regularly, leaving me to Facebook and texting for contact with her. Granted that time was at youth group, where me saying anything about such a topic could land me in a rather awkward talk with the youth minister.
My chances only seem to be getting slimmer as time goes on.
As far as the timing for the SKS goes, I noticed, but I literally turned 18 less than 12 hours ago... Not much of a choice for me up till now.
Research into the Yugo SKS resulted in a quote of $375, and some mods that might turn the SKS into an "assault weapon" (composite body/stock, pistol grip and adjustable stock, and lets not forget that it's big, black and goes bang).
Edit: Missed your post RB. Very nice setup. Have you tried boiling water on the stove yet?