AR Rookie with some ???'s

[Dastook]

I have had my AR for only a little over a month now. It is a S&W, M&P 15A in 5.56. I have a couple of questions. I know there is a variety of different calibers you can use by just changing the upper, so here goes:

1-What all do you have to change? The barrel assembly, upper receiver, charging handle, bolt and bolt carrier?

2-How do all of the different mag sizes (9mm thru 7.62 etc) fit in one existing mag well?

3-Who would be a good manufacturer to look at for a 6.8 that would go well with my lower.

4-What would be the best twist rate for a 6.8 barrel?

5-Would you want chrome barrel and chamber for the 6.8?

6-Does anything have to be done to the buffer spring when you go to a larger caliber?

I know this is a mouth full, but I know you'll are up to the task.

[Warrior]

I have had my AR for only a little over a month now. It is a S&W, M&P 15A in 5.56. I have a couple of questions. I know there is a variety of different calibers you can use by just changing the upper, so here goes:

1-What all do you have to change? The barrel assembly, upper receiver, charging handle, bolt and bolt carrier? Most of the time just the barrel and bolt, with a few exceptions

2-How do all of the different mag sizes (9mm thru 7.62 etc) fit in one existing mag well?if you are talking about an AR-15 lower the exterior of the magazine stays the same while it is dimesionally different internally.

3-Who would be a good manufacturer to look at for a 6.8 that would go well with my lower.Kotonics

4-What would be the best twist rate for a 6.8 barrel?1/11

5-Would you want chrome barrel and chamber for the 6.8?personal preference

6-Does anything have to be done to the buffer spring when you go to a larger caliber?only the buffer weight

I know this is a mouth full, but I know you'll are up to the task.

Check out the search feature as well alot of info about all your questions has been discussed before.

[MassMark]

7.62x51 is a no-go in an AR-15 magwell...