The
Aurora suppressor is the most compact one I've seen, but last I checked, they were only available in
9mm. That said, you could just as easily claim in your story that they, or someone else, cranked out one of similar dimensions for a .45. (Realistic? No. Probably not. That big hole in the .45 ACP tube creates more suppression difficulty.)
The Aurora specifications:
O.A. Length: 3.1 inches
Diameter: 1.125 inches (this is in 9mm, remember)
Weight: 2.7 oz.
Degree of suppression: In excess of 24 dB
Barrel Threads: 1/2x28, 3/8 inch long
This silencer was designed specifically for use with the Glock 26, and the design criteria called for operational usefulness exceeding two magazines. The manufacturer claims to exceed those requirements. The suppressor uses urethane wipes and “artificial environment technology to achieve extreme performance.” In other words, it's a wet suppressor, not a dry one (using oil, gel, or water to help achieve noise reduction levels greater than that seen in otherwise identical, but “dry,” suppressors). As of 2002, this gadget was listed at $545.00, and their advertising stated that the sales were restricted to governmental “and other” users. No mention of who those “others” might be.
On average, many of the .45 ACP suppressors I usually see run right around seven to eight inches in length.
Gemtech, however, makes this one...
http://www.gem-tech.com/BLACKSIDE.html
...which is among the shorter I've seen in .45 ACP, at 6.1 inches in length. At the shooter's ear, these gadgets produce somewhere in the ballpark of 140-141 dBs when run dry. Get them wet, they produce about 128-129 dBs at the shooter's ear (or about 130dBs measured 1 meter from the barrel).
Some folks compare the sound of a suppressed .45 to that of an airnailler or paintball gun. The pain threshold for impulse noise, by the way, is around 141 dBs, and the European Risk Limit for impulse noise is 140 dbs. For reference, consider that a
Crossman American Classic Model 1377 .177 caliber air pistol generates 120-123 dBs.