ar 5.45 upper to a standard lower

[leadslinger]

what if anything must I do to put a 5.45x39 upper on a standard a2 lower?

[Thomas M-4]

what if anything must I do to put a 5.45x39 upper on a standard a2 lower?

I am not 100% positive on this subject but It seems that I remember you need complete upper with matching BCG ,5.45x39 magazines,Stronger hammer spring, and carbine buffer.
Sure some body can confirm or correct me if I am wrong.

[Iraqgunz]

You will need a complete upper receiver group, 5.45 magazines and a extra strength hammer spring. I have heard from a few people that the spring comes with the upper. Not sure about the buffer.

[Middlelength]

I am assuming you mean to say that you have the complete upper with BCG?

In that case all you need is the heavier hammer spring, although, depending on the ammo you are shooting you might be able to get away without it. I highly recommend getting actual 5.45 mags through CProducts as I've never been able to get standard mags to feed very well.

[faithmyeyes]

How about the reverse? Would changes need to be made to the lower of an M&P15R (assume purchased complete) before putting a 5.56 upper on it?

[Thomas M-4]

How about the reverse? Would changes need to be made to the lower of an M&P15R (assume purchased complete) before putting a 5.56 upper on it?

You can run the stiffer hammer spring with 5.56 if that is what you are asking.

[DRich]

You can run the stiffer hammer spring with 5.56 if that is what you are asking.

Just be careful about using some commercial .223 ammo with the stiffer hammer spring. I've seen several pierced primers with this combination over the past few months.

The harder primers on 5.56x45 ammo seem to fare just fine, in my experience.