A question for our military members..

Hi guys and gals. First off thank you for your service. I have attempted to ask my father this question but him being a smartass and getting on in years as well as being in the Seabees over 50 years ago, I might not get an accurate answer...lol. I have always wondered how, during bootcamp, do the DI's or whoever makes this decision decides what role in an infantry squad and what weapon you will be issued. Is it marksmanship alone or is it bodytype and other factors. Essentially how do they decide who is to be the support gunner, who gets the 203 attachment. I have always thought about this and just never thought to throw it out there. Also is it frowned upon to carry a backup pistol other than an assigned pistol?

You should probably try U2Uing ADVISOR since he's supposed to be in Combat Arms. He's one of the mods who might know, I'm just pre-BCT Army.
We've got a few Marines here on the board, they might give an answer.

It's not an absolute "MUST KNOW", so U2U might be a little much, but most active or ex members might beable to give me some insight. I was dropped from MEPS for having had knee surgery within 2 years. So I never got to learn first -hand.

You learn all of the basics in basic. The position you eventually get assigned to depends upon your unit of assignment.
As a private in the infantry back in the eighties I started as a mule. I carried a rifle and extra Dragon tubes. (Because they were heavy and the lousiest jobs go to the newest people) I then was a dragon gunner, then an assistant m60 gunner then gunner then 203 grenadier then RTO then team leader, squad leader and so on. Think heaviest to lightest.
In an ideal situation each infantryman will rotate through many weapons positions to gain the best experience.