With all due respect to your gunsmith, firearm guys typically don't know beans about airguns. Of course you can use an aperture sight on a break-barrel rifle--I only have about a dozen of these! Below is an article I previously wrote about William-made sporter sights.
Note also that any Diana or Anschutz match sight will also fit the model 24--this is a whole different world of precision, and often you can find such sights used as cheap as a new Williams (Ebay, gunshows, etc.).
Well, on to the old article:
I was just browsing the Williams catalog online (you can view or download the whole thing in PDF at:
http://www.williamsgunsight.com/PDF/2005catalog.pdf ), and was pleasantly surprised to see there are no less than eight models that will work on airguns.
The FP series has click-adjusting elevation and windage knobs. The FP-AG is the model also sold by Beeman for many years, and features the famous lateral-sliding dovetail mount that will fit virtually any gun with a grooved receiver. It is available with either flush screw adjusters (FP-AG) or with target knobs (FP-AG TK).
The FP-GR is the same basic design and mount, but configured for a lower sight line. Also available with target knobs as FP-GR TK. These are intended more for rimfires, but if you have an airgun that the peep won't adjust low enough on, check these out.
The 5D series has simpler elevation and windage adjustments, you just slide them and then tighten a locking screw--NO click knobs (the 5D-SH for the Benjamin-Sheridan guns has been around for decades). There are NEW 5D models with the same versatile sliding dovetail mount as the FP-GR. These also come in high sight line (5D-AG) and low sight line (5D-GR) versions.
The WGRS-54 is an extremely compact sight that has a fixed mount for an 11mm grooved receiver. Specifically it is designed for Anschutz but can probably be modified for a lot of other guns--Diana, Walther, and post-1980 HW all have about the same groove spacing. It has quite a low sight line, so it does not work on many guns. You need to very carefully check the sight line and mount for your particular application.
Finally is the Target FP-Anschutz. Again this is rather specialized and not for every airgun, with a fixed 11mm dovetail mount and a long base with a low sight line. Comes only with knob adjusters.
All in all a lot of choices. Many shooters who are familiar only with the Beeman sight don't realize there are low sight-line options, and the cheaper 5D-series sights are an great new alternative.