nvv Posted February 22, 2012 Report Share Posted February 22, 2012 So I get tempted at the Melbourne gun show and buy an AO Worcester M1, semiauto. Looks pretty good, but when I performed the close inspection I should have done before, I noticed a firing pin protrusion that is just absurd, must be .120" at least. I'm not firing this, obviously. The question: I'm sure this is way out of spec; all the guns I've worked ended up at .050" protrusion or so. A 1/8" protrusion will surely rupture a primer. The most obvious (and probably oversimplified) approach IMHO is to adjust the firing pin length until I get that .050".....there's a protrusion gauge available somewhere for these but it's easy enough to measure. The FP is retained by a pin, but it appears to stop on the front of the pin itself and _not_ on the pin. The pin hole in the front of the bolt is large, about .115 to clear the .110" pin. That seems like a pretty large pin and hole, but then I'm more experienced in higher pressure applications where you'd not dare run such a monstrosity of a pin. Comments will be appreciated. - nvv Link to comment Share on other sites More sharing options...
mnshooter Posted February 22, 2012 Report Share Posted February 22, 2012 .005" clearance in the FP hole is not excessive. Maybe someone added some clearance to the firing pin as their idea of a solution to light strikes. As you said, it's a simple enough fix. Actually, with the low pressures of .45, and depending on the strength of the striker spring, you might not even notice a difference. But, why take a chance? Fix it first. Did you buy it? Link to comment Share on other sites More sharing options...
nvv Posted February 22, 2012 Author Report Share Posted February 22, 2012 Thanks for the response, it's appreciated. The key issue is the protrusion, the length the FP sticks out of the bolt face with the FP fully forward. This dimension is easy enuff to correct, pull the pin and shorten/round off/polish the tip until you get to the .045' - .055" protrusion. Measure with a depth micrometer. The tip is accurately hemispherical, but just sticks out over double the amont I believe it should. Yes, I bought it, and if a long pin is all that's wrong I'll be very pleased. The recoil springs are STOUT and the cocking handle is a little bitty thing (the past owner tried to give me a piece of 1/2" dowel he called a 'chicken handle'!), and I'll probably fit a set of the EZ pull springs (which are expensive) and make a cocking handle that is longer and has external knurling. It's an interesting gun. Again, appreciate the response. - nvv Link to comment Share on other sites More sharing options...
PK. Posted February 22, 2012 Report Share Posted February 22, 2012 Stop! The ATF approved design of the semi guns requires the extended FP to prevent full auto function. If you shorten the firing pin as you describe you will be in violation and subject to “making” a machinegun. The idea is simple; if the sear/FP were to fail to catch while cycling, the cartridge being fed will jam against the bottom of the protruding FP and the feed ramp, preventing FA fire. The low pressure 45acp will exhibit deformed primers, but rarely pierced or blown. It’s in the design and you need to leave it alone, or risk a stay in the hotel fed. 1 Link to comment Share on other sites More sharing options...
mnshooter Posted February 22, 2012 Report Share Posted February 22, 2012 What Paul Said!!! This is what makes this board so great.There is no other source in the world to get this kind of information.I just took a look at a semi, -yep; maybe even more than .125" protrusion.Never knew that had to do with the semi design.I hope this information gets added to the pinned section. Link to comment Share on other sites More sharing options...
dalbert Posted February 22, 2012 Report Share Posted February 22, 2012 What Paul Said!!! This is what makes this board so great.There is no other source in the world to get this kind of information.I just took a look at a semi, -yep; maybe even more than .125" protrusion.Never knew that had to do with the semi design.I hope this information gets added to the pinned section. mnshooter, Yes, I will add this thread to the reference pinned post in a few minutes. PK, Thank you very much for your reply. nvv, Welcome to the board! Hope you enjoy it here, and also hope you will heed PK's wise words. Thanks! David Albertdalbert@sturmgewehr.com Link to comment Share on other sites More sharing options...
nvv Posted February 23, 2012 Author Report Share Posted February 23, 2012 Heeded. My thanks to all that took the time to post. You're right, there is no better way to get otherwise obscure information out. My question does not sound so dumb at this stage. - nvv Link to comment Share on other sites More sharing options...
Piep Posted February 23, 2012 Report Share Posted February 23, 2012 PK Thank you for your input. I never knew this and resisted " I'm glad " making that bad mod On my last 27....Bob Link to comment Share on other sites More sharing options...
Merry Ploughboy Posted February 25, 2012 Report Share Posted February 25, 2012 I was going to venture a guess that gun camme from the same source as the gun in this thread: http://www.machinegunboards.com/forums/index.php?showtopic=13764 Link to comment Share on other sites More sharing options...
T Hound Posted February 26, 2012 Report Share Posted February 26, 2012 (edited) Stop! The ATF approved design of the semi guns requires the extended FP to prevent full auto function. If you shorten the firing pin as you describe you will be in violation and subject to “making” a machinegun. The idea is simple; if the sear/FP were to fail to catch while cycling, the cartridge being fed will jam against the bottom of the protruding FP and the feed ramp, preventing FA fire. The low pressure 45acp will exhibit deformed primers, but rarely pierced or blown. It’s in the design and you need to leave it alone, or risk a stay in the hotel fed.Mindful and respectful that you are a Tech Expert here, I am confused about your answer and especially since I do not want to be in violation of Fed Law. I don't understand how shortening the primer end of the FP can cause a full auto function. Have I misunderstood something here? It is conceivable that changes to the part of the firing pin underneath the bolt and that catches the sear can cause auto function, but the very tip of the firing pin eludes me. Can you explain? Never mind I just understood the primary idea but would appreciate some clarification. So the protruding pin on the FP, if the sear fails to connect, then travels forward with the bolt and, because of its length/protrusion, stops the cartridge against the ramp before it is chambered. Is that essentially correct? Edited February 26, 2012 by T Hound Link to comment Share on other sites More sharing options...
2ndArmored Posted February 26, 2012 Report Share Posted February 26, 2012 T Hound -Yup.The BATFE requires several "fail-safes" on semi-auto versions of full-auto guns (e.g. AR-15). They're not impossible to defeat, but they're implemented to keep the average Joe from having an unregistered FA weapon with ease. Given enough time, a knowledgeable well-equipped machinist can defeat them to make the gun rock & roll. Of course the same guy can make a full-auto gun from scratch too, so the point is moot. The salient deterrent is legality, not physics. As PK pointed out, doing so is possible, but a really really bad idea. The freakishly long oval shaped firing pin contraption on the SA Thompson is like that for a reason. Leave it alone. Link to comment Share on other sites More sharing options...
Joe H Posted February 26, 2012 Report Share Posted February 26, 2012 (edited) I've researched this subject to build a semi Thompson and 2ndArmored couldn't have said it better and just as PK noted don't change it. When you are looking at the face of the bolt, when it is retracted and held by the pawl, after the last shot in the clip, the firing pin should extend past the feedlips. If the sear is disabled or the disconnector isn't functioning it will stay in that position as the bolt pushes the next cartridge forward and will jam on the extended firing pin. If caught by the sear the firing pin will retract into the bolt and feed the next round. When the bolt is in battery the firing pin will be below the bolt face. When fired, a lip on the firing pin assembly will hit the face of the receiver and limit the protrusion of the firing pin past the bolt face and into the primer to the required .040- .050". The ATF requires that if the sear is disabled or the dis-connector doesn't function that the rifle cannot fire more than one round. The Kahr fire control mechanism meets this requirement. Joe Edited February 26, 2012 by Joe H Link to comment Share on other sites More sharing options...
T Hound Posted February 27, 2012 Report Share Posted February 27, 2012 (edited) I definitely did not know that. I am very glad I joined this site. Thank you gentlemen. I should add that in this day and age, given the possibility of anti-2nd Amendment atf agents trying to make a case for an illegal full auto simply for a malfunctioning gun, that you really cannot afford to be ignorant of such things. You better know when your Thompson is malfunctioning and why or you risk significant legal jeopardy, especially if you live in a liberal state like mine. In California, while the Thompson is certainly NOT on the banned "assualt weapons" list, it does come with all the suppsedly liberal defined "evil" features such as rear grips. Here it is a crime to have both a detachable magazine AND a rear grip, or a forward grip or both. And it is a felony. So we long suffering Cali SA Thompson owners really have to walk a thin line. So knowing these things is very important. John Edited February 27, 2012 by T Hound Link to comment Share on other sites More sharing options...
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