Without risking a weird-ass website, perhaps he means the rifle Mike was using in Season 6? It was a Remington 700, IIRC. .308, might be a stand-in for an M40A1 since he bought one in Season 2 and I think he said he used an M40 in Vietnam.
Hmm so we get Mike with his "M40A1" in... three seasons I know of but IMFDB isn't fully updated yet. Does it seem to be a reasonable facsimile of one, or is it just all the way wrong except for the stock?