When drilling all the way through square tubing, I often snap my drill bits. If it’s not possible to drill from the other side, is it better to drill a slightly bigger hole first followed by a smaller hole.
Feeds and speeds
First, there is one correct feed and speed that will make even a $1 drill cut like through butter. This is an eye-opener for those new to metal work; they think they can buy effective cutting with expensive bits. Nope, it is not for sale. What you're thinking is a waterknife; you can definitely buy those! I work in metal a lot; and I don't buy the top-shelf drill bits; I bottom-shelf cobalt's and use them correctly, insofar as that is possible in the field; hence I rarely make it to 1x.)Why not buy the "3x longer" top shelf? I never make it tot 1x before they break.
So you need to have your feeds and speeds dead nuts perfect, or as close as possible. Speed means the RPM of the drill bit. Feeds means how fast the drill bit advances (to be more precise, how far per revolution). Feeds are very difficult to get on a hand drill, since you only have pressure, and most proper feed rates require high pressure and a lot of intuition. You will know it when you've found it because the drill makes lots of pretty, long chips. Speed is a matter of getting a drill that's fast enough, which drill-drivers are not. I found a dust covered $35 drill in my notoriously overpriced local hardware store; it has a 1500 RPM speed, far better than the sea of 400 RPM drill-drivers for the ideal 2700 RPM of a 1/8" drill into mild steel.
But second, if you have access to the backside, you need to jig this. You need to make a jig that will let you place the holes on opposite sides in exactly the right place.
The simplest jig is an Ikea cloth tape measure. Wrap it around the pipe. Either it will spiral, or each wrap will land directly over the last. Make the latter case happen. Now you have fixed one dimension. Then get the hole centered (or not but matching on both sides, if that's what you‘re into) and golly gosh, that's a tape measure right there!
Even better, make a more permanent jig out of a 2x4 notched to embrace the channel. Holes in the 2x4 are positioned so you drill through them to start the holes in the channel.
Or, use a center punch
You should always use a punch to make a divot in the metal to give the drill a place to start. However, once the first hole is drilled, you can get a special type of punch called a center punch, which is a specific diameter shaft that has a punch at the end. This goes through the made hole, and correctly positions the punch for the second hole. Now the drill isn't wandering on the other side, a sure way to break it.
If I have to drill square tubing with a hand operated electric drill I will use a square and mark a line all the way around the four sides of the tubing at the location along the length where the hole is to be located. Then mark the center locations of the holes on the two faces of the tubing on opposite sides.
Then use a center punch to dimple the center mark for the holes on both sides. Next step is to drill a small diameter pilot hole, one from each side, at the center punch locations.
Now you are ready to drill the finished hole size with the appropriate sized drill bit. The pilot hole helps to guide the larger sized drill bit. Drill each hole separately from its side.
Using careful marking and measurements this technique is the best way to get the two holes aligned nicely across from each other. It removes the worry of the angled drill bit not making the hole in the second side in the right place. It eliminates the possibility of an angled drill bit binding in the first drilled hole and breaking. And finally it gives you the benefit of the pilot hole on the second side which you do not get when trying to drill the second hole from the inside of the tubing.
One dirty hack is an old optical disk, a CDROM or a DVD or similar.
Lay this on the stock, or tape it on if the stock is not flat. Make sure the silver reflective side is facing your drill.
Put your drill through the hole in the middle, and its really easy to see when the drill is not square by the reflections. Check in two different directions separated by about 90 degrees and it will be square-on.
See how the reflection is in-line with the real drill bit?
Second option is to use a mag-drill. This is either a holder for your electric drill, or a complete unit with a motor and chuck embedded.
These are essentially drill presses, but have a switchable magnet in the base. Turn the magnet on and they will clamp down on a ferrous surface with enough tenacity to resist the pushback of the drilling.
If this is beyond your budget, consider making something similar that will fit your standard square tube size. Consider that the drill's handle could point sideways or even upward perfectly well while drilling horizontally, and you could strap the drill to some kind of moving sled on a wooden frame that clamps to your stock with ratchet-straps.
I usually drill a pilot all the way through and "drill craft" is paramount...
If you let the drill change angle from upright as you go through, then that sideways load on the bit will snap it.
If you've got a drill that has a removable front handle, your best bet is to buy a drill press accessory for it. The cheapest are under £/$/€20 and you'll save that in drill bits over a few years. I keep my mains drill in my press as I've got a good cordless for other jobs.
Screw/clamp the press to your workbench, and clamp down the workpiece. These presses are more versatile than mag drills if you don't have steel around for the magnet to attach to (I work in wood, plastic, and aluminium more than steel, and have a wooden workbench).