I have found that such a large pin as you describe will sometimes cause problems with smokeless and the small rifle primers, small pistol also. The symptom is that the internal cartridge pressure upon firing pushes the primer back against the firing pin with enough force to actually cause the firing pin to retreat slightly. If this pressure is high enough, then the primer can crater enough to cause sticky operation or even blow a plug out of the primer.

I have noticed that this occurs more frequently and to a much greater degree with the small size primers rather than the large rifle or large pistol types. Also seems to occur much more with smokeless and seldom with black, at least in my very limited experience with black.

My conjecture is that, since the flash holes are the same size for all US cartridges, then they would flow the same volume  of gas regardless of whether the cartridge used a small or large primer. Well, the same volume of gas flowing into a large space (the large rifle size primer cavity) as opposed to a small space (the small rifle cavity) would naturally cause a higher pressure in the smaller cavity. I believe that this is why I've noticed that some rifles will need bushing to a smaller size pin in order to shoot a cartridge with a small size primer but will not need bushing for a large rifle size cartridge.

A prime example of this is the Martini Cadet. Almost all will need bushing to a smaller size unless chambered for such as the 30-30 or 44 Mag or used with pressures under about 30K psi. My own 256 Win Mag Cadet is no exception.

I've made several low wall Hornets and 25-20 WCFs with firing pins somewhat smaller than yours and still had trouble with even light smokeless loads at times, depending upon the strength of the mainspring. You may get lucky & not have any trouble but if I were you then I'd expect the primers to back up into the firing pin hole to some degree, at least with smokeless.

My friend Ben the Knifemaker says that the CCI primers are enough harder than most others so that they back up & crater less, causing less trouble. I figure he knows of what he speaks, so you might try that approach if you have problems.
Good luck, Joe

Bob, one easy way to bush the hole is to simply tap it for a fine-thread screw & then solder or glue the screw in place after turning it in tightly. If soldering, then remove the blued surface on the threads so the solder will stick better. Then face the end(s) off smooth & drill the new firing pin hole using a bench-made bushing for a guide.

Make the bushing guide O.D. a sliding fit into the firing pin hole from the rear, and drill a concentric hole in it using a drill to match the new firing pin nose size. Afterward you'll turn down the f/p nose to match, and polish. If you don't have the tools, most any competent machinist can do this little job in short order.

Now affix the bushing guide firmly in the block and drill through the block face from the rear, making sure the bushing doesn't move. Deburr all edges inside & out, blue it, and you're done with the block.

Make sure the firing pin is polished very smooth on the sides as well as the nose, to avoid stress risers that will encourage breakage. I also make sure that the f/p is annealed thoroughly, down to a dead soft state if necessary, to avoid breakage. F/P protrusion should be between 0.030" & 0.055", I like to keep it on the short side (0.030"-0.040") in single shots for easier operation and fewer problems. My favorite nose dia is 5/64" but smaller dias are OK too, I don't use anything larger unless unavoidable.

This is only one way, there are a number of other ways & the results will depend more upon the expertise of the operator than upon the exact method used. The most unobtrusive way would be to have the block face welded up using the TIG or heli-arc process and then bore the new f/p hole as described above. No re-heat-treating of the block (after welding) is necessary IMO when using such a cartridge as the 25-20, or in fact for any cartridge developing less than 40-45K psi. That is, IMO.
Email me for more if needed, good luck, Joe

While I do not pretend to be a gunsmith, I have a breech block that needed bushing and had a local man, recently deceased, do the job for me back in the day.  He described the bushing he used as a little hat, with the brim of the hat out flush to the breech face and a smaller cylindrical portion countersunk into the breech block.  I THINK he silver soldered this bushing in, but I don't remember for sure and of course can no longer ask him.  I do know that it has stood up to several years shooting in my .45-70 high-wall, so I think it is pretty strong and durable.  Maybe somebody else has experience with this method?  BTW, due to the shape and dynamics of the various 'wall breech blocks, I don't think the bar bushing is very practical, JMHO.

Froggie

Guys I have been following the discussion on the threaded insert approach with interest and have a couple of questions.  I think I understand the argument for saving the last thread and using it as a jam to inhibit the screw insert from sinking further into the breechblock but-

How does the screw move if it is soldered in, or does this problem only seem to occur with a glued screw?

With any of the approaches is there a necessity to anneal the area around the old firing pin hole before drilling for the new bushing?   

If so is there a good method for doing the spot annealing?

Thanks 

Bob J

Joe, the way that I did it was to counterbore the breech face for an 8-40 Weaver oval screw head just deep enough that the slot was above the breech face when tightened hard, with Loctite on the threads to prevent screw movement.  I'm guessing that either the steel of the screw or the breechblock compressed over time (about 3,500 + rounds) to allow the primers to back out, and I had not noticed the compression when cleaning the action...this was on a Martini 12/15, so the "works in a drawer" were easy to pull for cleaning.   

David 
Montezuma, IA