oj how tight is it as sanding down your feed neck could make it week,have you tried some lube/oil to try to ease it on cause as time goes on the feedneck will wear down a bit..
Halo feed tubes are specifically made oversize so they can be sanded to fit your feed neck, this really stems back to a time when clamping feed necks, especially the lever-lock type we are used to today, were nos so commonplace. If you apply some care and common sense to sanding it you shouldn't weaken it at all.
On the other hand, the PM7 feed neck should fit fine, ring Heaven (it'll be most likely James that answers, or Syd at Paintball Warehouse or Jamie B at Sloth or Jon/Chris at LiPS, my guess is that the easiest option would be an upgrade feed neck for the Rail (or a similar 3rd party one), which doubtless has the same threads, would be the most cost-effective option if you prefer not to sand your Halo feed tube. Any of these guys should be able to advise and supply what you want.
Next you'll want to know how to take the old feed neck out of your PM5, they are usually Loctite'd in, there are a few options. Loctite is a bit like super glue, and you'll want to gently heat the joint to soften it before you remove the old feed neck, a hair drier is usually the best option here (some might say hot water, but lets not dip your gun in water and get the eyes/electrics wet). then the choice is yours, you can use a strap wrench, vice or Mole grips, if it is really stubborn you may have to drill a hole through it, heat it again and use a screwdriver through the hole to give extra leverage as you unscrew it. I would highly recommend that you then clean up the treads in the body and apply a little Loctite when you fit the new one to stop it coming loose and damaging the threads when you eventually bash the hopper in play. For the same reason and to stop your hopper getting damaged, don't over tighten the feed neck when you fit your hopper.
Sorry for the essay, but I thought I'd cover all the bases in one hit
If I'm teaching you to suck eggs then never mind, it's all good, it might help someone else.