Advice is a form of opinion. The weight you should give an opinion, is based on the speaker's education, training and experience with the subject.
That varies a good deal among the services. Within a service, it varies a lot on the job the person did.
Anyone who served in the last several decades should have received some education and training in the use of the M16 or M4. However, depending on the service and job, they may have never received any form of follow up or have any practical experience.
An infantryman in the Army or Marine Corps gets lots of this. A personnel specialist in the Navy - not so much.
From what I've seen of various police forces, these factors vary. There are minimum requirements that must be met by all. Whether the officer goes beyond this is up to him.
I guess that's all a very fancy way of saying what the OP did. You need to know who you are taking advice from.