In general, it has been noted that when estimating the range of firing from discharge residue dispersion, the results from non-toxic primers do not tally with results from lead based primers (Gundry and Rockoff). This is probably due to the quantity of partially combusted materials present as well as the higher organic content of non-toxic primer compositions. The fact that lead has a high specific gravity might also be a contributing factor.
In very general terms, the spread of discharge residues from non-toxic primers will be less than that from lead-based primers, and the range at which they can be detected will be correspondingly shorter. As an example, if a GSR spread of 3 in. is witnessed around the bullet entry hole, with a lead- based primer, this will indicate a range (depending upon the barrel length, calibre and ammunition used) of 16 in., whilst a Sintox round under the same conditions will indicate a range of 13 in.
As there are so many different non-toxic compositions in use, range of firing estimations must be made with exactly the same ammunition as that used in the case under review.
A complicating factor is that non- lead-based primer compositions can give what appears to be a positive reaction with sodium rhodizonate. This is probably due to the presence of barium which, in a mildly alkaline (pH 8) solution, gives a red/brown colouration, whilst lead gives a purple colouration in mildly acid (pH 2.8) conditions. If the correct pH is not selected, the test results can, to the inexperienced, be confusing.
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