It's not, I just did it because I liked it, that rifle fit for me, but it's not remotely correct. I just beat DNA to it for the 'part he never had but we put it in anyway!" bit (I saved a couple of the long guns from that pack for Onslaught too but that's probably a bit closer to right). It is odd how that one gun from the pack didn't actually fit ANYTHING. The axe and roller were clearly for Earthrise Prime, but the gun is too big for it and most all of the mainline Primes released (and my nephew has probably had it attached to almost all of them at some point trying). I wonder if it was originally planned for some Optimus we never saw? The closest might be the TR Powermaster, but his hands can't use it.
Then again, I'm the guy who smokestack extenders on RID Prime in a comic once as a joke...they HAVE to be long! Even if they don't exist! And his hands must be blue! The internets tellz me things!!!