Alright boys and girls, gather round and let me tell you the tale of how to model Rifling in solidworks without losing your mind. This is one of two ways to achieve this, and if there is interest, I'll do a follow on writeup on the second way.
So, we start with an empty tube. For clarity, I'm using a tube with a 25mm ID, 35mm OD, that is 200mm long.
We're going to sketch on one of the end faces of the tube.
Now we're going to draw a circle from the center to the ID edge.
Now we're going to create a Helix that will have the diameter of the circle we just made.
Now we're getting somewhere. You have several options here. You want constant pitch, as we're doing basic rifling, and there are very few reasons to use a variable rate. Your pitch and revolutions are how you set the angle of your rifling. Let's say you want to model an 18 inch barrel with 1:9 pitch rifling. (Yes, I'm using those numbers because they're easy, sue me.) The pitch dictates how far down the barrel something would have to go before the rifling made one full revolution. In the case of 1:9, it means that it does one revolution in 9 inches. For an 18 inch barrel, we will do two full revolutions (18" barrel divided by 9" pitch). Thus we would set our revolutions dialog to 2. The pitch would be set to 9 inch. Start angle is where the origin of the helix will be. Usually use 0 or 180 degrees so that everything can be lined up in the next step. For our example, remember that I am using a 200mm long tube, but I want a LOOOOONG pitch, so we set the Pitch to 400mm, and revolutions to 0.5, giving us a 200mm long helix.
Keep in mind that your helix will probably be protruding the wrong direction, so click that little box next to "Reverse" and it will go the other way. You can also change whether it is generated clockwise or counterclockwise with the radio buttons. Once you hit the check mark, you end up with this.
This is the path our rifling profile will follow. Now we need to make a profile to actually follow that path. For the sake of simplicity here, I'm using a circle. Usually you'd use a square tooth, but have fun with it.
Now, close your sketch. The profile you just drew will gray out. You're now going to click the Swept Boss/Base button at the top, or go to Insert - Boss/Base - Sweep. The top little box should already have your profile sketch selected, but if it does not, click the grayed out sketch. A little dialog bubble should pop up next to it saying "Profile(YourSketch)". In the second little box, you'll select your helix. Click the second box, then click the Helix to select it. A little dialog bubble should pop up saying "Path(Helix/Spiral)"
Hit the check mark and we have one of the lands made.
But WarFairy! Surely we need more lands than that. Commercial barrels have three or more for symetry!
And to that I say, shush, we're getting there. And stop calling me Shirley. We're going to do a circular pattern of that land you just made.
You can also get there via the menu by going to Insert - Pattern/Mirror - Circular Pattern. Now this pops up.
In the first little box, you want the circular guide you're going to use. In this case, it is the bore. Click in the box, then click the inside surface of your bore. It will also work with the Outside face. Then you set how many instances you want to spawn. In this case, I did five. Now, you click in the box just under "Features to Pattern" and select the helical land you just made. A preview should pop up of the instances as they will be generated in yellow as shown. Once you are happy, click the green check.
And now we're done. You have rifling in a barrel.
Keep in mind that there is more than one way to skin a cat. You can use this same helix generation to do internal cam grooves, external cam grooves, anything that really needs to be a helix. The helix tool is little used, but very powerful when used correctly.