My 18.5" NEF 20 gauge turkey gun was working great, I had shot several shells through it both 3" and 2 3/4" and they worked fine. I patterned it, and wanted to do a deep clean.

I took it apart, and polished the inside of the barrel. I used an empty hull as a chamber guide to protect the chamber.

I put it back together, and went to pattern it again. 3" shells don't fire. The primer is barely scratched.

I brought it home, sprayed brake cleaner through the firing pin channel and receiver. Took some cheap dove loads, and tried it. Shot 5 out of 5 times. Decent primer strikes. Went back to pattern my TSS loads... barely a scratch on the primer.

The shells index off of the rim, and the rims measure identically. I took it to my gunsmith, and he has been able to exactly replicate this behavior with Remington Nitro 3" hulls. All his 2 3/4" shells fire but the 3" ones are spotty at best and have light primer strikes. He's done all the measuring and it doesn't seem like there are any real differences in the shells besides overall length.

Does anyone have any idea what could be causing this? Any guesses? We're just trying to brainstorm but this doesn't make any sense at all. The only thing we can think of is that maybe the 3" hulls have a harder primer, maybe it needs a stiffer hammer spring but this showed up very suddenly.

I have a good amount of money in this project, barrel cut, remchoke installed, picatinny rail with Burris FF3 installed, etc so this isn't just some $95 throwaway gun for me. Any ideas would be GREATLY appreciated!

Any visual difference in where the action is stopping when the bolt is in its most forward position on the two different sized shells?

May not hurt to check this by removing the for end from the action bar slide.  You can make more precise notation of position from the metal to metal reference.

I might also see if you can check the space between the bolt face and the shells with a feller gauge set but that's probably nearly impossible.

Have you done anything to your trigger group?

I should have mentioned... this is a single shot.  

Nothing has been done to the action of the shotgun.

Just thinking here but here goes---

Is it possible that you have a chamber that is on the short side of what a 3" chamber is supposed to measure? When you close the action it is forcing the shell back hard against the breach face and that is somehow tweaking the action in a way that jambs or twists up the firing pin and causes light hits . No idea why this problem started when the gun used to work with 3" in the past . Any chance there is a burr or some crud down in the forcing cone that is preventing the longer shell from seating as deep as it used to?

Is there any difference in rim thickness between a 2 3/4 and a 3" shell? Take a 3 " shell and cut the end dumping shot , wad and powder (along with making it shorter) and see if the gun will fire it . If this fires it would rule out issues with rim thickness or primer hardness and point the cause towards hull length . Use caution as a shotgun primer has considerable power

We are completely baffled. He's going to order a new hammer spring and see if that fixes it... though I think there's something else going on when you compare the pics.

I agree the difference between firing pin hits is major
Sounds like your smith is thinking pretty much with me .

I believe I would consider reaming the chamber longer . This is done by some of the shotgun guys as a longer throat gives better patterns and lower recoil (sporting clays guys)
I believe they call this treatment "back boring"

Not sure if any smith can do it or if it would have to go to a shotgun specialist.

NEF is out of production/business Correct?

Does it have the transfer bar saftey or just a regular hammer? I had one with a worn transfer bar and it would light strike. This also happens if you short stroke the trigger instead of taking up all the trigger travel all the way rearward. The transfer bar will drop before the hammer hits it but it would still put a small dimple in the primer. It would be odd that it would only do it on 3" shells though.

I'm not getting any forcing cone work done. I'm already getting 235 pellets in a 10" circle at 40 yards, and that's before I polished the bore. I expect it to do even better.

This did work for several patterning shots a few weeks ago, so I don't know why the sudden failures. This one does have the transfer bar.

If it's a break action, any reason you can't pick up a second gun, box stock configuration, and swap barrel/receiver assemblies for testing?  NEF is still around and they are so cheap it would be a decent trouble shooting piece if you have quite a bit tied up in that one gun.

If it were a Thompson Center, I'd say sure, however NEF, Rossi, and older break open designs have to be fitted with a new barrel.  Each action is slightly different, so if you wanted a companion or replacement barrel, you'd either have to send it back to the manufacturer, or have a gunsmith take a barrel blank and set and weld the piece that fits it to the receiver.  Swapping random barrels doesn't guarantee proper lockup or fit.

Do the 3" shells use a thicker primer cup?

I had a problem with an NEF not firing.  The transfer bar was the culprit. The pin had worked itself loose somehow and the transfer bar would end up slightly crooked and the rifle wouldn't fire.