failure to fire in Nelson conversion

Adam is having a problem with his Nelson conversion, has about 2 to 3 failure to fire in 5 rounds in magazine but re-cocking the hammer 100% fire. Has same problem with different brands of ammo including SK Eley and CC! plus others. I also have the Nelson conversion, I modified the firing pin after having problems with mine in my Ball gun. Later I changed the Mainspring to 16lb from EGW and still have 100% firing success. Adam modified the firing pin as I did and still has problems. What I did as I did mention in a topic quite a while ago was to make the tip the same dimensions as my AW-93 and MatchGun.
 Adam noticed that the top of the firing pin hole is about .020" below the top of the recess for the rim. I proceeded to measure quite a few dimensions on a Nelson slide and Barrel and my friends AA slide and barrel. By doing the math using the dimensions I got confirm that the top of the firing pin in both conversions is equal to the top of the chamber within a couple of thousands though the dimensions are not the same. My conversion I did so the firing pin is flush with the top of the recess and looking at my AW the pin hole is also flush with the top of the recess.
 Is this low firing pin common with Nelson and AA conversions, I don't have a Marvel but the barrel flat to top of the chamber is the same as the Nelson.
 Photo shows how I measure the height from the flat to the bore center. Gauge pin is 2" long, .217" diameter and end in quite a bit into the rifle bore. Using my special homemade height gauge. The barrel is the one piece made from 2" tight bore barrel for Adam that I will use the Lilja reamer on now that I confirmed it will work. Group testing is next. But now Adam has to determine what I have to do with his barrel, maybe lower the center of the bore?

What magazines are being used? Custom barrel or factory? New gun or new problem with a previously working gun?

Recoil spring—what weight and round count?

When my conversions start having FTF problems, it’s almost always due to rounds not seating completely (tight chambers + Eley bullets) on recoil spring force alone. All brass shows good solid rim contact. They always go off on a second pull. Swap in a brand new 9 lb spring and everything works for another 2,000-3,000 rounds.

Oil the rounds &see if the problem goes away.  Sounds like it could be a tight/dirty chamber.  Rimfire ammo is generally not very fussy about where the firing pin hits it.

New Gun, actually little over 1 year old when purchased but not fired until now, additional slide was purchased also so that another complete conversion can be assembled when one piece barrel I'm making is complete. Plastic magazines that came with conversion don't work at all. He is using German GSG magazine with brass shims as I've posted. Has no feeding problems. Conversion in on Accuracy X frame.
 I can measure the exact location of the firing pin as compared to barrel bore using a Starrett 220 Mul-T-Anvil micrometer with a .089" pin gauge in it later at my shop.  As posted I took measurements with Starrett Digital caliper and a Micrometer plus my homemade depth micrometer and computed location. Top of firing pin is within a few thousandths with top of chamber hole when using my Nelson conversion. Homemade barrel is yet un-chambered but bore height is exactly the same as Nelson barrel bore height. (Reason I made the depth mic. was to be able to measure the flat to bore height when machining)
 Adam took a photo looking down the barrel with the firing pin removed and light shinning in back of slide. You can see rifling is near top of firing pin. together we have 3 Nelson slides that the pin hole is about .020" below the rim recess top,  Dimensions confirm the location!
 To insure slide is held close the High Velocity spring was inserted simply to test ignition, brass 25 caliber brush was used to clean barrel followed by bore snake. Oily SK rounds were tried as well as others that wax was cleaned off. As mentioned 2 to 3 round of 5 in magazine fail to fire but will with second hit. Simply put I think firing pin is too low. My Nelson slide maybe slightly different in pin height, I have no ignition problems with modified pin and 16lb main spring but did have problems before modifying the pin with 19lb main spring!
 as seen in the other photo the pin hole top measures with digital caliper to be .021" below rim recess. I put pin hole in my homemade conversion flush with the top like other high end guns I have like 208, Aw-93 and MatchGun. Never had an ignition problem with 16lb main spring with my Homemade slide!
 Adam does intend to replace the unknown main spring with known 19lb spring! He did cut coils of firing pin spring but didn't help.

Just measured my Nelson and it checks the same as yours. From the top of the firing pin hole to the top surface of the firing pin holder measures .130, from the rim recess to the top measures .110.

This may prove useful.  I'm not sure where this came from originally, but I suspect it was from Eley, given the headstamps in the pictures.

https://www.snipershide.com/shooting/threads/failure-to-fire-eley-ammo-in-v22.7189342/post-11146931

+1 on playing with spring pressure. I get some FTFs on my gun when rebounding too hard on high vel ammo. Worth a test to increase spring strength.

This may sound blasphemous but have you/would you reach out to Travis at Marvel to discuss this issue?  He was extremely helpful with the Marvel I had purchased over 20 years ago.  He may very well have encountered such problems and resolved them long ago.

Mike

Swap firing pins between your and Adam's guns and see what happens.

Edit- and swap FP springs.  Heck try Adam's on a different frame just for grins.  Probably suggesting things you already thought of... apologies if so.

Edit- changed my thoughts. Not saying my suggestions will solve the mystery, but they may help diagnose the problem.  Skip the FP spring swap, but I would swap the FP's and see how they function in the other kits.  Also might be worth inspecting the FP and FP channel for burs, dirt, straightness.  Good luck.

My bet would be a spring issue, magazine height, or both.

Clean the chamber with an oversized brush, .24” or .25” and some solvent such as #9 or even Kroil after every range session and I’ll bet the problem goes away. I always use a brush with a brass core, not steel, to minimize the chance of damage. Rimfire ammo manufacturers seem to have changed lubes because it’s only been the last few years I have seen so much chamber buildup. 
Don’t swap firing pins on a Nelson, they are individually fitted to make sure the chamber doesn’t get dinged. (That’s straight from Larry himself)

Froneck wrote:
 To insure slide is held close the High Velocity spring was inserted simply to test ignition, brass 25 caliber brush was used to clean barrel followed by bore snake. 

Why use a .25 caliber brush in the bore?  Any good .22 brush will work fine on the bore. The CHAMBER needs to be cleaned with an oversized brush, but if you run that brush through the bore, you mash the bristles, and it won't work nearly as well on the chamber. 

I thought I had posted this in the past, but  I did a search & couldn't find it.

chiz1180 wrote:My bet would be a spring issue, magazine height, or both.

This. 
Call Larry Nelson and he'll walk you right through it.
I had the exact same issue, all resolved. BTW, I use the original plastic mags and also the metal GSG ones with nary an issue.

Froneck.  I have a potential solution that has not been mentioned in this thread.  I too had a Marvel by Ferking Conversion that would not fire.  THE SAME issue that you described in your opening post.  I tried everything from spring rates in recoil and hammer springs.  I tried different ammo.  I tried the oil drop.  Nothing.  
I now have a fully functional and absolutely reliable Marvel 1911 conversion.  My issue was ..... the extractor.
The outside curvature of the extractor was too big and did not allow it to fit into its cut-out housing in the barrel fixture.  This did not allow the slide to go into battery just enough to not fire.  Recocking of the hammer, or putting the round in my Xesse or Model 41, and the round would still fire.
How to check:  take a sharpie (i was able to use red or black) and cover the outside of the extractor, particularly on the inside where it would enter and potentially impact in its housing.  You can do this buy field stripping the slide and reassemble.  Next just cycle the slide in its full range of motion 5-10 times.
Results: if you see heavy wear and missing sharpie, your extractor may be to large on the outside dimensions.
If this is the case, simply flat scrape the area of missing sharpie on sandpaper and polish paper until you can conduct the experiment above without missing sharpie.  Do not sand away the point of the extractor length where it grabs the 22 round.
If the above does not wear sharpie marks, then my guess is wrong and we're all back to square 1.  Good luck. 
P.M. me if you find that the above is true and want some more details.