To clarify the position of Rav Moshe Heinemann shlita regarding this issue:
In a Hebrew language letter dated 19 Menachem Av 5769 which was widely publicized at the time (and posted in many shuls and yeshivos), Rav Heinemann wrote that since there is a dispute between the poskim whether any shaving machines are permissible and those who prohibit their use consider them Osur Min HaTorah, therefore those who do not want to grow their beards should instead use a trimmer such as the Outliner which does not remove the hair entirely and leaves stubble. The letter is co-signed by the Roshei Yeshiva of BMG in Lakewood (Rav Malkiel Kotler, Rav Yeruchim Olshin, Rav Dovid Tzvi Shustal and Rav Yisroel Tzvi Newman) who add that regular shavers involved a “sefeika d’oraysa.”
It seems clear from this letter that Rav Heinemann does NOT take the position that, based on the Igros Moshe being Posek Hador, the opinions of the Poskim who prohibit shavers should be dismissed or discounted. On the contrary, he advises that l’maasah, practically speaking, one who wants to shave should instead use a trimming machine (and not a regular shaver).
In another Hebrew language letter dated Parshas Re’eh, 5736, Rav Heinemann confirms that “emes nachon hadavar” that his Rebbe Rav Aharon Kotler zt’l did not permit the use of shaving machines. Rav Heinemann added that Rav Aharon Kotler did not want to issue a heter even for the Outliner trimmer, out of deference for the Chofetz Chaim.