I've long had a problem finding the exact center of cylindrical shapes, such as gun barrels or exhaust tips, when trying to drill them out. I know how to find the center of large-diameter cylinders, but can't seem to come up with a way to do it for small ones (less than 1/4 inch)--which means for pretty much all of my needs, I have to eyeball things. Sometimes I get lucky, but more often I fail and the hole I drill is off-center. I usually try to correct it with the tip of a #11 blade, but keep wondering if there is a better way to do this. Any ideas?

Without resorting to some sort of machine shop gear, eyeballing is pretty much it IMHO. The way I do it is to first make a mark with a pin or scriber point, then drill a very small hole to see how close you got. You can then adjust for being off centre with a larger bit, burr, file or knife tip. The idea is that you use progressively larger bits to give you that leeway and not try to get it right in one go. Not so easy with mg barrels but even that can be done with some practice. You can also cheat a little by lightly sanding the outside of the part to better match the hole.

Kimmo

Yeah, what Kimmo said. I lightly start a pilot hole with a heated needle. Then typically, I will use 3 to 5 bits in a pin vise working up to the final hole. I don't recommend using any kind of power tool for the job, as it's just too easy to make a mistake.

The way I would do it is take a slide rule to measure the outer diameter, make an exactly fitting square out of whatever material is best suitable around it so you have an accurate way of measuring half the diameter once, use something like Tamiya tape to mark said half diameter and use a scribing tool to mark it, then at 90 degrees to this line measure half the diameter again using tape to mark it and scribe the second line and voila you have the exact center point.

I find that if you eyeball it with a small drill bit in a pin vice, cut a shallow 'dimple' first. Check it for position, then to centralise it, hold the pin vice at about 45 degrees and 'move' the dimple. When it looks central, put the correct size drill bit in and cut the hole. Works for me on all materials. Hope this helps!

Thanks, guys! I posted separately on another forum and also got some useful tips that could help every one out, so here's a link:
http://www.network54.com/Forum/149674/thread/1433686643/How+to+find+the+center+of+a+gun+barrel-

Eye ball it .

I just eyeball it as well. A pilot hole is a must, though. If you just start drilling, more often that not it's not going to end up centered. I like to use a pin to make a mark, not heating needed.
For larger diameter barrels, it's best to start with a small drill bit and go use one or two sizes before reaching the size you need the final hole. That way you can adjust the location of the hole if your pilot hole/mark wasn't perfectly centered.

I use a #11 xacto blade , if it's off center I favor the off center to correct the opening . Do we really need a fixture and jig to do this ? Now if it is a metal barrel that's a different story , but who would buy a solid metal barrel anyway . Maybe if you were doing a diorama of turning a metal barrel on a lathe ?

I usually scribe an "X" with a #11 blade for the pilot hole and start drilling in the middle of the "X". I eyeball it. It's not perfect but it gives me a better guess where the middle is.

I was looking at my Academy US Machine Gun set and thinking of this same issue. Since I pretty much use the academy guns when the kit one is lacking. I am going to take a scrap of 1/4" hardwood and Measure each barrel diameter. Then chuck up a drill bit as small as a needle in my drill press and drill a hole all the way thru. Then with out moving the piece put in the bit the size of the barrel diameter drilling halfway thru. Then I can take the gun and insert into correct hole and insert needle from other side to mark exact center of barrel.