how to add a cylinder inside a cube

i want to extrude the cube to be a cylinder im kinda new to blender

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you are making a very common misstake right now. it should not all be one mesh, feel free to have multible meshes.
https://i.gyazo.com/e59f82ccd1cb1691fc061693b058833b.mp4

but to answer your question, select all the faces on the “front tip” of the gun and press I, the rearrange those new verteses into a circle.

also, going into front view and with the new verteses selected, you can do Shift Alt S.
this will turn it into a “sphere” but since ur selection has no volume it will turn it into a circle.

how do i make a circle when i press i

as i said, move it into an aproximate position, go to front view, and press Shift Alt S.

I’m not sure if I understood you, but just in case you see this (Loop Tools is an addon included in Blender that you have to enable):

Or without Loop Tools:

The latter is what finalbarrage has commented above (Shift Alt S).

still dont get it

Could you share the .blend file with the selected vertices that you want to make circle/cylinder?
http://pasteall.org/blend/

Select the faces on the front section of the slide. Hit I.

Bring it in from the edges as far as you need. From there you can use the “circle” option in looptools. Correct for any rotation offset.

The roundness of the cylinder will of course only be as smooth as the number of edges that you have to work with.

But as finalbarrage said, there’s no real need to make the barrel a part of the slide. It’s a separate part in the real world and would be easier to manage as a separate piece of geometry anyway.

Judging from the image you’re trying to model a colt 45 1911. There’s a set of tutorial videos that might be of interest to you here.