here's how that's done- assuming you don't have access to a usgi cleaning, ratcheting chamber brush- first you get a 45 caliber RIFLE BRUSH- measure the length of the 45 cal brush against a case to make sure that it's not longer than a 308 case- you could damage the bore otherwise- the 45/70 is perfect b/c a 308 rifle case is the same length as a 45/70 - 2.015 - the 45 is the most common- and a piece of aluminum or steel rod -if you have a section of shotgun cleaning rod , that's perfect- one WITHOUT the tapped hole for the brush- you'll be making your own later- cut a section about 2.5 inches long off - then drill and tap an 8/32 in the centre of the "fat" part-then make a small flat in the side and do the sme thing x4 if you want to get fancy- 1 if you just want something that works- screw your 45cal RIFLE brush into the fat part, and a section of cleaning rod into the side, and work it up and down( the reason for the x4 holes on the sides) - i've never used the actual chamber brush very much( the usgis have the chamber brush in the tool kit- i got a box of 10 years ago and i don't think i've actually worn out one yet- and i have 4 m14 )
what you do is put the brush with the adaptor in the chamber(after you lock the bolt back, typically with a stripper clip- i don't trust the bolt stop- seen it fail too many times) give it a quarter turn, and screw the cleaning rod section into the next hole- repeat until you get all the way round-but again, this only for HEAVY cleaning- properly cleaned using patches,and bore cleaner, it SHOULDN'T BE NECESSARY- like i say, i've still got my original box of 10- and that's over coming up on 40 years of shooting these things
btw, you mught want to check out the threading on your cleaning rod that's in the buttstock of your m14s- i've got the original and it had some goofy thread on it which i re-threaded to 8x32- no big deal with a tap set-
the original norcs did NOT come with a chamber brush- just a super small vial of oil
figure 45 in fm23-8 shows how this is done- it also shows it with the bolt forward, holding the brush in place,but that's not what i was taught, and i'm not sure whether the usgi brush has a collar so that it won't enter the chamber-i know my home made adaptor would, if slammed forward by the bolt- - also use a 45 cal mop on that same adaptor to apply cleaning liquids to the chamber