Want to thank those who responded to my request for the screw size for the Rossi 15 but would like to report what really is probably the correct size.  In setting up a few Rossi 15's with radial mounts I needed longer screws to pass through the backplate/radial mount .  The stock pan head Rossi screws measure right at 0.100 inch ( I apologize for mixing metric and English dimensions) which is  a couple thousands more than 2.2.5 mm.  Screws are generally a little smaller than the nominal diameter. Also the M2.5 x 0.45 screws I have from Micro Fasteners measured 0.0940 inches in diameter and screwed in just fine but wobble quite a bit. At that point I asked the question published in the news letter. 

I continued to look what is really the proper screw size and discovered that a socket head M2.6 screw is available although not at many places or in many different lengths.  Found that Grainger, an industrial supplier located in most major cities,  carries the M2.6 x 0.45 in 8, 10 and 12 mm lengths.  The stock Rossi head screws are 8 mm long and the backplate screws are 6 mm long.  Ordered 50 each of the 8 mm and 10 mm lengths  and they arrived yesterday.  They measure 0.0995 inches and fit much better than the M2.5 screws.  If you want some they can be obtained at Grainger.  Part numbers are 6CE44 and 6CE45 for the 8 and 10 mm lengths and cost about $12 for 50.

Although the strength difference between a M2.5 and M2.6 screw is not all that much using and undersize screw in a tapped hole can significantly reduce the torque required to strip out the threads in the aluminum casting.  Thread depth on a 0.45 pitch screw is only about 0.25 mm or 0.010 inches.  Use of the M2.5 screw in a M2.6 hole would therefore reduce radial thread engagement by about 0.003 inches out of 0.010 inches or 30% which should be directly proportional to pull-out strength.

Thanks again,

Bob