In the newer 3-digit model numbering system, if the first number is a:
- "3", then it is USUALLY a Scandium-frame gun (see model 317, below, for exception).
- "4", then it is an aluminum-alloy frame gun.
- "5", then it is a carbon-steel (blued steel) frame gun.
- "6", then it is a stainless-steel frame gun.
J-frames: most of your 5-shot .38s and .357s, 6-shot .327s and short-barrel 6-shot .22s. Some will have visible hammers, some shrouded hammers (can still be thumb-cocked for single-action firing), and some will be internal hammers (double-action only guns). The model 317 is a J-frame 8-shot .22 in the AirLite series, but it has an aluminum-alloy frame, not Scandium. J-frames are considered small frame guns, and usually will have short (3" or less) barrels -- but not always.
K-frames: Mostly 6-shot 38s and .357s, and longer-barreled (4" or longer) 6- and 10-shot .22s. Medium frame, some with plain barrels, some with solid-ribbed barrels, some with lugged/weighted barrels.
L-frames: 6-shot .357s, 7-shot .357s, and a few 5-shot .44 Specials with short (3" or less) barrels. L-frames usually have barrels with integral lugs/underweights. Medium frame, beefier than a K-frame in some areas, but designed to use K-frame grips due to their popularity.
N-frames: 6-shot .357s, .41 magnums, .44 Specials, .44 magnums, .45 Colt, .45 ACP, and 8-shot .357s. Large frame, barrels usually solid-ribbed, some lugged/weighted.
X-frames: the newer honkin' huge 5-shot .460 and .500 magnums, with lugged/weighted barrels.
More info at these sites: