Grain weight of bullet is related to twist rate of your barrel. Certain bullets stabilize at certain twist rates, some will go out of control and tumble or ,if jacket is to thin, spin fast enough to tear off the jacket.

I am not sure about BoatTail bullets. I have heard it is done to shift the center of gravity as needed to stabilize and also to "center/stabilize" the projectile upon exiting the barrel. this somehow relates to the gasses interacting with the rifling and crown of the barrel. At least that is what I have been told. I never ran it down and it could be because some designer thought it was sexy. But probley not.

You might want to try a google search of "boat tail vs flat bottom bullets" and see what shows up.

 

At CQB distance it doesn't matter what the tail of the bullet looks.like. it does matter how its constructed though. Some are better than others and that's the research you wanna do.

 

I guess first of all what is CQB distance ? I do know that the 55gr boat tail fmj bullet is put out by all of the ammo manufactors. but as said befor the twist rate does come in too play I built a match rifle for the local 600yd match with 1in 8 twist to shoot a 69 gr boat tail bullet,

 

The boat tail bullet is easier to get to start into the case during reloading. That's the big "plus" on Boat Tail for short range. At shorter ranges (say less than 100yards) you probably won't notice much accuracy difference...

 

The "boat tail" on the bullet increases it's ballistic coeffiicient which represents it's ability to over come the resistance of the air while in flight. In essence, more streamlined which produces better results at range.

In your "CQB" situation (which I assume is less than 50 yds??) There would be negligable improvement over a flat based bullet.

 

FB almost ALWAYS shoots better within 300yds...this is universal. you don't see 30br guys running BT's....just as you don't see PPC guys running them either...or pointblank br shooters. 300-600yds it's a toss up in MOST calibers, but i've seen some damn fine groups with FB's out past 300yds...on very calm days. Beyond 600yds, run a bt for sure...the extra BC will buck the wind better.

Reloading bt's is easier, and if you're not loading for group ammo....then it won't make a difference either way

 

Here's some light reading on the subject
http://www.rifleshootermag.com/2011/01/04/ammunition_rs_boattails_093009wo/