I really can't understand the argument about this. It is perfectly possible to do something for multiple reasons, and it is abundantly clear from all sources that the carbine was to provide an alternative to the pistol, or the rifle, in cases where the pistol was sub-optimal but the rifle inconvenient.
A brief memo summarizing a request does not explain anything. It does not explain the rationale for any of the requirements, or the analysis that went into developing them. People spend a lot of time on these things, building a solid case for their recommendation. Otherwise you end up in a situation like this:
Congessman: So, General, you want this light rifle. Why can't people use the service rifle?
General: It's too heavy.
Congressman: OK, it's "too heavy". So what is the magic weight for this light rifle you propose?
General: Less than five pounds.
Congressman: Hmm, so between zero and five pounds. How did you hit on five pounds?
General: Well, uh, we thought that would be about right.
Congressman: I see you have thoroughly analyzed this.
Why don't you give these men pistols?
General: Pistols satisfy the weight requirement, but have inadequate range and poor accuracy.
Congressman: But yesterday you said that this light rifle will also sacrifice range and accuracy for the sake of reduced weight, right?
General: Well, yes, but not as much.
Congressman: General, with all respect, this sounds like Goldilocks. Have you done any analysis, based on the characteristics of actual engagements, the load carried by the men would use this light rifle, the actual accuracy and effectiveness of the pistol under combat conditions? Is there a real need for this light rifle, or is this just some wild idea you guys have come up with? General: Um, let me check on that.
Congressman: Good. If your staff lacks the analytical capabilities to provide an answer, we could lend you some people from the GAO to help.