I have had similar problems with some FD classic 510's as used on the Pekos. Some are too large. Luckily, I had multiple attys and found one that fit (magma, love that extra long 510). There is lots of variability in the threading on those 510s. Some are just right and all atty's thread right in. I have an A7 that has very small threads and only fits into one of my FD 510s. I have some copper conductive tape that I plan to use next time I decide to change attys. I am hoping a little tiny piece will tighten things up. That looseness is very annoying. They are easy to use but so inconsistent.
The magma... That atty has destroyed more brass Reos 510s than all of the other attys combined. If you run a Magma all the way down on a brass 510'ed reo the bushing is done. I'm sure Rob still has nightmares.....