Ok so I am looking at buying an M&P 9 barrel and a couple mags due to the fact that you can straight swap them into my M&P 40 and run cheaper 9mm through it.

Will this run me afoul with the law if the caliber does not match whats on the Pistol Permit?

Ive asked this question of a few guys that I know and I always seem to get a different answer although most guys seem to err on the side of caution especially the guys I know who have 3 or 4 different calibers for their TC encores (Most of them went out of their way to have the individual caliber barrels listed on their permit individually)

The 2 different FFL's that ive used over the years told me that the most important part is the serial number on the pistol & your permit and not the caliber of the barrel in that pistol. For context, I had asked them about the 9mm/40 & 40/357 sig conversion barrels that are available for glocks as well as the 9mm/9x23/38 super barrels in 1911 slides of various calibers that are available.

As always, im sure the guys in the know here have a cut & dry answer which would be much more reliable

Whatever part of your firearm which is that which must be transferred through the FFL is what counts.  With a rifle or shotgun that's the receiver, with a pistol it's the frame.  One frame equals one pistol––barrels are irrelevant.

As with everything in NYS..........it depends.  It depends on what your particular licensing officer's view of the matter is.  I'll use the T/C Contender context.  Some licensing officers list it by make/model/serial number and list it with "multiple barrels" or "mulitiple calibers".  Other licensing officers will list the make/model and frame serial # and then list each caliber of barrel you own and plan on using on that frame.  

As always go by what made up rules your particular licensing officer wants to go by and not by what you get on this forum.

i find it hard to believe it would be illegal to install and use a .22 drop in kit from Ceiner....or Sig....etc on your pistol.

It doesnt make one hell of a difference if you take that 9mm barrel on your glock and replace it with ANOTHER 9mm barrel....as the "ballistic marks" or whatever bs would be different and unique to each barrel.

When I lost my wallet and got a new permit issued, my damn county didnt know how many handguns i had, the names of the mfgs, or the calibers.
I had to sit down and literally re-inventory my collection for them, spell out each name, model, serial, caliber and if each was an "auto" or "revolver".

Even after I inventoried, they managed to goof up and get 3-4 typos on my new permit, which they had to reprint for me.

I wouldnt feel like I was putting myself in danger by swapping a barrel out.  Pretty sure LEOs are looking for real criminals, not splitting hairs of the penal code for guys shooting at a few tin cans with a .22 conversion kit or something similar (like the 9mm vs .40 bbl you mention).

Let me repeat the above again.  ALWAYS GO BY WHAT MADE UP RULES YOUR PARTICULAR LICENSING OFFICER WANTS TO GO BY AND NOT WHAT YOU GET ON THIS FORUM.

So what you are saying is that I should not depend on strangers for advice when it relates to possible legal ramifications?
What the hell do I pay 24 bucks a year for then?

Its in the penal law, not at the discretion of the issuing authority...for whatever that's worth...

Everyone best hide those pinker kits.  

I've never heard of them being an issue.  If it takes all of arfcom to search penal code for a grey answer, confident no one will be busted.  Js. Ymmv, jmho, etc.

Your fine.....as log as you follow the law  I don't see why a problem would arise.  The "gun" is the receiver/frame.  Slide barrel etc are just misc. parts and not registered parts.  I would check with your local sheriffs office.  They may want to list it as multi caliber irregardless.  Let us know.

Exactly how is it any of their f***ing business?

isnt

you ever see a cop at the range inspecting to see if you registed that pistol with a .22 kit installed on it as a "multi caliber" firearm?

again, if it comes down to it, where they are picking things apart like that...chances are your already super duper ultra fucked.  Similar to 922R compliance.  922R compliance is easy enough to follow, just be careful when you swap any parts out on a gun you buy new/direct from a retailer or from online.  Speaking of which I just passed on a Turkish MP5 clone as I know it wasnt 922R compliant, I dont need to roll the dice and take a chance, or get caught up in that sorta crap, isnt worth it.

If your slinging crack rock by the ton, pimping 14 year old kids, exporting weapons to Nigeria, then yeah....maybe they will tack on these kinda things after they look into your stuff as extra charges.

again, just my ...opinion.     I tend to be realistic, and not live in fear to the extent where I wont have my FFL pin a stock for me, or where i wont own a post ban gun as it may be a "duplicate or copy" of a banned gun.                

I just asked this question about 2 weeks ago when I put a T/C encore pistol on my permit, now this is Nassau county so it may be different where you are, their policy is any different caliber for any given frame will be listed under the gun, so my permit now says: T/C encore, 7mm-08, 22(22-250), 6.5mm(6.5 Creedmore), I also have a Bond Arms derringer listed:
40S&W, 45, 357 & 22. As I say this is Nassau county policy, fact is I know people who don't bother to do it.