Rhiannon,
First my disclaimer: take everything I write here with a grain of salt, including my suggestions about stance that I wrote last night. That little label to the left of here may say "senior member", but that's only because I've posted a lot in the two months I've been on this great forum. I'm really a novice. Even though my shotgun was my main weapon when I was a teen, I haven't owned one in years. I'm really on THR to learn from the folks who know what they're doing.
I wrote my suggestions based on what I
think to be correct, knowing that if I'm too far from wrong, others will kindly correct me.
As for patterning, you'll get the idea better by reading some threads on the topic, especially Patterning 101. The basic idea is that until you do that procedure with your
specific shotgun (yes, the one in your hand, not just the same model), with several different types of shells (e.g., 00 v. #1 buck or #7 birdshot), and makers (e.g., Remington v. Federal), for various distances (say 5, 15 & 25 yds...), you won't know what kind of pattern the projectiles (e.g., buckshot or birdshot) will produce at any given yardage.
Patterning involves shooting your shottie under controlled conditions at a stationary "target" (like a large piece of paper with a dot on it for target) so that you can see what pattern is produced. Once you find the pattern that works best for you, stick with that specific shell by that specific maker so you know what it'll do. Once I find the 'one', i'll probably buy a case.
When I was a kid, I never patterned my shottie. No wonder I missed so many rabbits & birds.
Even when I joined THR a while back, I might have scoffed at the need to pattern. But after reading a bunch of posts and threads about it, now I'm a believer. That's the first thing I'll do when I get my 870.
_____
As for why I'm buying an 870P (as opposed to, say, an eXpress), the 870P is just a
little "better" weapon, at least in the minds of some (including mine after MUCH reading). Not that the eXpress isn't a fine gun. It is. But just like auto makers produce several different models at different price points, Remington makes several 870's for slightly different price points. The 870P is subject to a bit higher manufacturing standards (materials, buffing, coatings, etc) than the eXpress, so it costs a few bucks more. (Actually, about $150 more last I looked.)
While both are very good, servicable weapons, the P is produced for professionals who's life is on the line everyday, who want it to rack smoothly from day one, and not just at the range.
And (I confess), I have a habit of buying the best I can even if I don't really "need" the best. (If I had a new baby like you do, I'd be perfectly happy with an eXpress.)
If you search the shotgun forum (keyword: 870P), you'll find lots of discussion about this topic, about what is the real difference between eXpress and 870P.
Here's
an informative one that I bookmarked. Read post #8 by dfariswheel. (I'm starting to agree guys: maybe that thread should be a shotgun sticky.)
For me, after a lot of reading first, I started going to gun shops. Most, including Costco, carried the express. When I first picked up the 870P, shouldered it, and racked it, I immediately
felt the difference. For example, at least on the 870P with synthetic Speedfeed stocks, the stock is shorter than the eXpress by some large fraction of an inch. That translates to faster shouldering.
The fore-end of the 870P doesn't have quite as much lateral play as the eXpress. Not that's it's a lot on the X, but it's just less on the P. Tighter tolerances in the machining.
Plus, there are more options available with the P than with the Xpress model: various combinations of sights (bead, GR, rifle); stock (Speedfeed 1, II, III, IV, or wooden); magazine extension (standard; +2); barrel length (14", 18", 20"). {I've got my combo already picked out. Hold the mayo, extra tomato & onion, please.}
Still, again, I agree with many, including Dave McC: an 870 of any flavor - long-barreled Wingmaster, eXpress HD, 870P, marine magnum, ... - is a great shotgun.
AND, regardless of which one you own, THE most important thing you can do with it is BA/UU/R ("Buy Ammo/Use Up/Repeat"). That is, practice, practice, practice until it's use deployment from deep sleep to awake to rack is fast and as second nature as brushing your teeth.
Otherwise, it's just a dangerous paperweight standing near (or under) your bed.
Hope this helps.
NemA~