Usually I'd cheat the hell out of it and use some tricks a certain other app has to offer.
The next approach would be to create half a capsule out of a quadball (for example subdivide a cube till it's fully rounded, cut in half, and bevel your half-capsule out of it). Already that would be a lot more easy than using a standard capsule for the boolean (I would probably use an intersect on the capsule, clean that up, and then connect the dots with a cylinder that has those polys deleted where the capsule fits in). The cylinder I'd prepare beforehand so there are enough edges to connect).
But for you and for you only, I did it the hardest way (I don't like guns, so I probably will not model one if not necessary, so I don't know what I will do with this thing): Boolean with a standard capsule, then cleaning up.
It took me a little bit more then strictly necessary, because, of course, I bungled it and didn't have the right amount of sides, could have used a few insets on the face of the cylinder and so on. I didn't even cheat with the pole ... that middle-point was outside from the cylinder (because it's not right in the middle from the pictures I looked at, and the new center was exactly on the border. That I had to change ...). Having it lined up with the border of the cylinder would already have been much easier (it wasn't lined up that properly anyways). So I had some vertices to push around and it cost me more than an hour (which is plain too much for something like this). Looking at it, I see 2 edges that aren't necessary, especially that one which creates two tris ... Well, so it's almost all-quad (I didn't bother with the poles anyway).
The next approach would be to create half a capsule out of a quadball (for example subdivide a cube till it's fully rounded, cut in half, and bevel your half-capsule out of it). Already that would be a lot more easy than using a standard capsule for the boolean (I would probably use an intersect on the capsule, clean that up, and then connect the dots with a cylinder that has those polys deleted where the capsule fits in). The cylinder I'd prepare beforehand so there are enough edges to connect).
But for you and for you only, I did it the hardest way (I don't like guns, so I probably will not model one if not necessary, so I don't know what I will do with this thing): Boolean with a standard capsule, then cleaning up.
It took me a little bit more then strictly necessary, because, of course, I bungled it and didn't have the right amount of sides, could have used a few insets on the face of the cylinder and so on. I didn't even cheat with the pole ... that middle-point was outside from the cylinder (because it's not right in the middle from the pictures I looked at, and the new center was exactly on the border. That I had to change ...). Having it lined up with the border of the cylinder would already have been much easier (it wasn't lined up that properly anyways). So I had some vertices to push around and it cost me more than an hour (which is plain too much for something like this). Looking at it, I see 2 edges that aren't necessary, especially that one which creates two tris ... Well, so it's almost all-quad (I didn't bother with the poles anyway).