My new 39A has been nothing but trouble and I don't want to send it back to Marlin because I gather the company is in turmoil. After many hours of work I have got the thing performing somewhat better, with only the occasional misfire or fail to extract.
When I started, the gun would fail to fire every fifth round or so, and consistently fail to extract. Here's what I've done to bring some improvement:
1. I lengthened the firing pin 0.015" by cold forging near the cartridge end of the pin. Filed away all tool marks and polished both sides of the pin on a very fine, flat stone.
2. Filed 0.015" of material off the lower hammer rebound stop and restored the original contour.
3. Removed the lever and lengthened it by welding material to the end. The purpose here was to make the bolt close completely, which it was not doing. I ground and filed the weld material to original profile, heated the reworked end to dull red and quenched in oil. Total length added - about 0.030". Your gun may be different but the bolt must close completely. This is a fitting process - You'll know it's right when the lever handle will not quite touch the underside of the receiver when fully closed.
4. Discovered the lengthened firing pin was striking the end of the chamber leaving a burr. Cartridge will not extract if the chamber is burred. Gently filed a small 45 degree ramp on the end of the pin to eliminate this problem.
5. Removed the burr at 12 o'clock position of the chamber, caused by the lengthened firing pin. A cartridge should nearly fall into the chamber of it's own weight with very little added effort and any burr will interfere.
The results are good. The firing pin now hits the cartridge more decisively - you can hear the difference. And misfires have been for what I suspect is insufficient primer in the cartridge instead of a wimpy action. I would guess the gun is 95% reliable now and the failings to fire all seem ammo related. I've been cycling the cheap Remington stuff.
UPDATE: I continued to have FTF, especially with Remington Thunderbolt, of which I have several bricks. I did some research and found that lots of folks cycle Thunderbolt with no problems so I decided to try it in my Marlin model 60. Thunderbolt worked without a hitch in the model 60. So I gathered cases from the 39A and the model 60 and compared them under a 20X stereoscope. The firing pin impression was definitely longer and deeper on the model 60 casings so I went back to work on the 39A firing pin and action. I reshaped the end of the firing pin, making it slightly longer in footprint while keeping the same overall firing pin length. My intent was to match the firing pin impression of the model 60. Then I removed the rebound stop and took another 0.015" off the lower end of it. When I reassembled I put a 1/16" thick washer between the hammer spring and the retainer bracket, to add just a touch more hammer power. I can now cycle through a box of Thunderbolt without any FTF. I hope this helps someone out there who's having 39A problems.
Also, on the aesthetic side, the lever comes from Marlin with uncomfortably sharp edges. If you do a lot of shooting these will cut your hands up. I took a file and some emery paper and put a nice radius all the way around. This really improves the fun factor. Chemical bluing is next.
Still had the occasional fail to extract and I did write to Marlin for a new extract spring in hopes that will take care of it. In the mean time I took an old file and shaped it to fit in the extractor recess right next to the barrel. I gently worked about .010" of material out of the recess so the extractor spring has a better grab at the spent brass. This has completely solved the extraction problem. Marlin, you can keep your new spring
Maybe these steps will be helpful to someone with similar trouble. Do yourself a favor: measure and photograph everything before you make changes. You must know your starting point so you can tell where you've gone. Take notes. Make your changes one at a time and test each result. It's unfortunate that Marlin's quality has sunk to this. You are essentially buying raw material if you buy one of the newer 39As. I've invested about 20 hours in troubleshooting and reworking this gun.
2nd UPDATE: After 1 week of shooting (at least 500 rounds) I'm upgrading reliability to 100%. I have had NO failures to extract, No failures to eject, NO failures to fire, and NO failures to feed.
3rd UPDATE: Marlin did send me a new extractor spring this week and threw in a chamber guide spring to boot. I have no plans at this time to install the new springs as the rifle is now functioning flawlessly.