[Lion]

Anyone know if the BCG for 5.45x39 and 5.56x45 are the same, one can replace the other? Any difference in component parts?

 

[Lion]

Oops, forgot to mention for AR15 milspec upper. Specifically regards S&W MP-15R.

 

[Big Bore]

Bolt carrier, cam pin, firing pin retaining pin, gas key, gas key bolts, and staking are the same. Gas rings on the bolt itself are the same.
Bolts are different. The 5.45 x 39 needs the larger faced bolt but does not use the same bolt as the 7.62 x 39
The 5.56 bolt face is smaller than the 5.45 x 39 bolt face.

 

[Combat Diver]

You'll need a new bolt

5.56x45 Base Dia .377
5.45x39 Base Dia .394
7.62x39 Base Dia .447

CD

 

[Lion]

Thanks! That's the level of detail I was looking for. Hadn't found it in my internet searching. S&W finally replied, but not specifically, only that there were differences, and I could order after warranty parts from them. I was a bit concerned that if their 5.45 version didn't take-off as they expected, that I might be left partless. Looks like I need to latch onto a replacement bolt just in case, and the other parts are readily available.