Difference in NM and OM cylinders

How can one tell if a cylinder is for an OM or NM? This one is a .357 btw

I'm a bit new to the cowboy guns but I think it is a length thing. Over all length is different.

I'm sure a member with more knowledge will be along shortly to clarify.

Which model?

Well you can't tell w/o measuring. Both had non-recessed chambers. 1st of all the OM 357 Cylinders are all made for the mid size frame. But NM mid size & NM large frame 357 Cyls. sizes are currently made.

Cyl lengths of old and new model "Mid size frames" have a similar O.A.L.; NM large frame cyl length is longer at 1.978". But the main difference is the diameter; the large frame cyls are ~ .060" larger than the mid frame cyls., old or new.

In 1973 the New Models with transfer bar were introduced. At that time, the 357 chambered Blackhawks were switched from the mid-frame to the large frame. When the Anniversary model was introduced in 2005-6 Ruger returned to the OM size mid-frame and at the same time changed the original large frame Vaqueros to the mid-frame as well. These cylinders will fit the OMs with the usual slight length fitting as always.

Hope that helps.

From what I see, the old model cylinders have a very short front snout.new model cylinders front snout are much longer.