Elongating the voice is exactly what I mean by an allophonic difference. As for 'hamster', no one (that I know of) pronounces the /m/ as a glottal stop, althou

Elongating the voice is exactly what I mean by an allophonic difference. As for 'hamster', no one (that I know of) pronounces the /m/ as a glottal stop, although people sometimes epenthesize a voiceless consonant [p] (by devoicing the end of the /m/). Where some English dialects get a glottal stop is for an intervocalic or word-final /t/ (in addition, of course, to the glottal stop in the middle of 'oh-oh' and 'uh-uh'). I've also heard American English speakers glottalize word-final /k/ and sometimes /t/, but a glottalized stop is not the same as a glottal stop.