Keri - is it tumas maga or reiya

Today's gemara (nidda 42a) discusses if a woman who is poletes shichvas zera is temaia because of maga or reiya. The gemara seems to assume that for the man it's definitely misum reiya and it would be tamei with just a mashehu because there is never a shiur if it's mishum reiya. Rashi (d"h metamei b'mashehu) says straight out that keri is reiya and is metamei b'mashehu and the truth is that is the pashtus of the mishna at the beginning of the perek. When I learned it, I was bothered by the gemara on 22a where Rabba asked R' Huna what the halacha is for a man who uses a kisem. R' Huna's answer seems to say that there is a shiur for keri meaning that it's not mishum reiya but maga.

So I looked up the Rambam in hilchos avos hatuma (5:1). He seems to pasken like R Huna from the gemara on 22a but seems to ignore our gemara. I think that is what the Raavad on the spot is asking on the Rambam. The Kesef Mishna attempts to answer the question for the Rambam based on the Tosefos on 22a. This is really beyond the scope as I didn't learn all of these well enough to write about them but I was just happy to see that the Rishonim did discuss this apparent stira between the gemaras. It seems that there are multiple ways to understand Rav Huna but l'halacha it comes out like our gemara that it is metamei mishum reiya and it is b'mashehu. (See the Tosafos, Kessef Mishna, Raavad for more details).

UPDATE: The beauty of daf yomi - you learn the next daf (nidda 43b) the next day and it often helps clarify things a little. From the gemara it does seems to be a machlokes tannaim/amoraim about whether there is a shiur for the reiya of the zav. It's still difficult to understand Shmuel and R' Huna who insist that there is a shiur in light of yesterday's gemara which says that there can be no shiur for a reiya. For this you (I) need to look at the kessef mishna and Tosafos. Maybe I'll look at it later or if someone else does feel free to comment.